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					Molecules 2010, 15, 2211-2245; doi:10.3390/molecules15042211

                                                                                        OPEN ACCESS

                                                                                   ISSN 1420-3049

The Heck Reaction in Ionic Liquids: Progress and Challenges
Fabio Bellina * and Cinzia Chiappe *

Dipartimento di Chimica e Chimica Industriale, via Risorgimento 35, 56126 Pisa, Italy

∗ Authors to whom correspondence should be addressed; E-Mails: (F.B.); (C.C.)

Received: 3 March 2010; in revised form: 22 March 2010 / Accepted: 26 March 2010 /
Published: 30 March 2010

     Abstract: As the interest for environmental increases and environmental laws become
     more stringent, the need to replace existing processes with new more sustainable
     technologies becomes a primary objective. The use of ionic liquids to replace organic
     solvents in metal catalyzed reactions has recently gained much attention and great progress
     has been accomplished in this area in the last years. This paper reviews the recent
     developments in the application of ionic liquids and related systems (supported ionic
     liquids, ionic polymers, and so on) in the Heck reaction. Merits and achievements of ionic
     liquids were analyzed and discussed considering the possibility of increasing the
     effectiveness of industrial processes.

     Keywords: ionic liquids; heck reaction; Pd catalyst; selectivity

1. Introduction

   Over the past decade, ionic liquids (ILs), salts having melting points lower than 100 ºC, have
become one of the fastest growing research areas. The application of these remarkable salts as solvents
in organic reactions and catalytic processes has been extensively investigated and reviewed [1–9]. The
term “ionic liquid” identifies a really large class of compounds, constituted exclusively by ions, which
are liquid at/or near room temperature and present unique physicochemical properties. As first, the
ionic nature determines the lack of a measurable vapor pressure and this feature, associated with a high
thermal and chemical stability, makes these compounds intrinsically excellent candidates for industrial
applications compared to common organic solvents. This is particularly true considering that ILs can
Molecules 2010, 15                                                                                   2212

nicely complement, and even sometimes work better, than organic solvents in a number of industrial
processes [1–3].
   The Heck reaction represents not only the most extensively investigated Pd-catalyzed process in
ILs, but, the related studies practically cover all the development of ILs chemistry up today; they start
with the use of onium salts as reaction media, continue with the pyridinium and imidazolium ones to
arrive, more recently, to the supported ILs passing through the so-called task-specific ionic liquids,
TSILs. This intense activity in this field is probably due to the fact that the Heck coupling is an
extremely popular reaction, of industrial interest, which is very versatile but presents a number of well-
known problems and it is generally considered the classical example of palladium catalyzed carbon-
carbon bond formation process. Heck arylation of electron-deficient olefins have been investigated in
ILs mainly to verify the effect of medium on the catalytic system; activity, selectivity and stability of
the catalyst under ligand free-conditions, as well as the possibility of catalyst and solvent recovery and
recycle have been tested under different conditions. In the case of arylation of electron-rich olefins also
the effect on the medium on regioselectivity has been investigated, whereas the possibility of use
alternative arylating agents has been rarely evaluated; many of the reported data is related to the
reaction of aryl halides (iodides and bromides) with styrenes and acrylates.

2. Heck Reaction in Common ILs

   Simple palladium salts such as PdCl2 or Pd(OAc)2 in the absence of stabilizing phosphine ligands
have been widely used in the Heck reaction in ILs [10–14], and preparatively significant examples
have been performed.
   In 1984, in a pioneering work Jeffery reported the palladium catalyzed vinylation of organic iodides
under solid liquids phase transfer conditions, around room temperature, using tetrabutylammonium
chloride [15]. However, the first well-documented application of ILs in the Heck reaction, that is also
the first palladium-catalyzed reaction performed in ILs, was reported in 1996 by Kauffmann and co-
workers [16]. In particular, they surprisingly found that Pd(OAc)2 and PdCl2 are both appropriate
catalyst precursors for Heck reactions without additive ligands (Scheme 1) in [C16PBu3]Br.

                   Scheme 1. Heck reaction without additive ligands in [C16PBu3]Br.

                                           Pd cat (1 mol%)
                                           Et3N (1.5 equiv)           Ph
              Ph-Br +        COOBu                                             COOBu
                                       [Bu3PC16H33]Br (1 equiv)
                        (1.4 equiv)                               PdCl2 : 81% after 16h
                                               100 °C
                                                                  Pd(OAc)2 : >99% after 43h

   The addition of 1.5 equiv. of NaOAc when Pd(OAc)2 was used as the catalyst precursor, improved
the rate of the coupling, but a concomitant decrease of the selectivity was observed, leading to the
formation of 5% of the (Z)-isomer. The precipitation of Pd clusters slowly started after some hours
when PdCl2 was used, but with Pd(OAc)2 no precipitation was observed. In this last case the catalyst
remained in the melt even at complete conversion of the aryl halide, and after the recover of the
product by distillation it resulted active in the following two runs. The authors explained these results
by supposing that the phosphonium salt exerted an efficient stabilizing effect on the Pd(0) species
Molecules 2010, 15                                                                                         2213

obtained in situ by reduction of the Pd(II) catalyst precursors. The results reported by the group of
Kauffmann attracted the attention of several research groups on the important challenge of developing
new protocols for a ligand-free Heck reaction. In fact, ligand-free conditions in metal-mediated
reactions should be always preferred, especially on large scale productions, not only for saving the cost
of the ligand but also because the purifications are simplified if no ligand is present [17].
   In 1999 the Earle group reported the use of hexylpyridium chloride, [C6Py]Cl, as solvent in the
Heck reaction of iodobenzene with ethyl acrylate using Pd(OAc)2 as the catalyst precursor and Et3N or
NaHCO3 as the base (Scheme 2) [18].

                                     Scheme 2. Heck reaction in [C6Py]Cl.

                                              Pd(OAc)2 (2 mol%)
                                               base (1.5 equiv)         Ph
                  Ph-I +           COOEt                                           COOEt
                           (1.25 equiv)                                 99% (base = Et3N)
                                                 40 °C, 24h
                                                                        98% (base = NaHCO3)

   Interestingly, the use of the corresponding salts containing the non-coordinating anions
hexafluorophosphate and tetrafluoroborate required longer reaction times and higher temperature to go
to completion (Table 1).

             Table 1. Influence of the counteranion on the efficiency of the Heck reaction.
                                                 Pd(OAc)2 (2 mol%)           Ph
                     Ph-I +           COOEt                                            COOEt
                                                 NaHCO3(1.5 equiv)
                               (1.25 equiv)
                                                IL (additive, 4 mol%)

        IL                 Additive           Reaction temp. (ºC)       Reaction time (h)      Yield (%)

    [C6Py][PF6]                -                      80                          72              42

    [C6Py][BF4]                                       80                          72              99

     [C6Py]Cl                 Ph3P                    40                          24              82

     [C6Py]Cl                                        100                          24              99

   In contrast to the chloride-based systems, the addition of Ph3P as ligand also promoted the reaction
when an imidazolium-based IL, [bmim][PF6], was used. The authors explained the experimental
results, and in particular the hindering on the efficiency of the reaction of a phosphine, suggesting that
in a chloride- (or halide-) reach environment the Heck reaction involves a Pd(II)/Pd(IV) redox cycle,
whereas in the case where the phosphine ligand is present a Pd(0)/Pd(II) catalyst is operative.
However, today a Pd(II)/Pd(IV) mechanism for the Heck reaction is no more accepted. On the
contrary, direct NMR measurements and a great deal of experimental data indicate that the Heck
reaction proceeds via a Pd(0)/Pd(II) catalytic cycle [19]. It is then plausible that the chloride anion
contributes to the dissolution of Pd(0) species which come from the reduction of the Pd(II) precatalyst
better than BF4- or PF6-.
Molecules 2010, 15                                                                                                    2214

   Related to this work, it must be remarked that benzoic anhydride, in the absence of a base, has been
used as a source of the aryl moiety, although higher temperatures and longer times were required to
obtain high conversions. The use of hydrophobic ILs gave the possibility to separate products and salt
by-products from the catalysts immobilized in IL (for example, in [bmim][PF6]), simply by extraction
with ethyl ether and water, respectively [18].
   Starting from the results reported by Kauffmann, and taking into account the observations made by
Jeffery on the activation performed by salt additives and on the ability of stabilizing the catalytically
active Pd species [20], Herrmann and co-workers reported in 1999 their results on ligandless Heck
reaction performed in [NBu4]Br [21]. As illustrated in Table 2, where the results of these studies have
been summarized, PdCl2 resulted highly active in promoting the reaction of olefins with iodobenzene
(TON = 10000, entry 1), bromoarenes (entries 2-9) and also activated chloroarenes (entries 10–12).
However, this catalyst precursor is not capable of coupling 4-chloroanisole efficiently (entry 13).

                                Table 2. Ligand-free Heck arylation in [Bu4N]Br.[21]

                                X                                 PdCl2 (cat)
                                     +         R1                                                     R1
                   R                                           NaOAc (1.2 equiv)
                   (X = I, Br, Cl)       (1.5 equiv)        T = 130 °C (for X = Br, I)
                                                             T = 150 °C (for X = Cl)

Entry          R                     R1                X       PdCl2 (mol%)       t (h)   Yield GLC (%)(a)     TON

  1           Me                    Ph                  I           0.01           14           100            10000
  2          COMe                 O-n-Bu               Br           0.2            14           98             490(b)
  3            H               4-MeOC6H4               Br           0.1            14           50              500
  4            H                4-MeC6H4               Br           0.1            14           68              680
  5            H                4-CF3C6H4              Br           1.0            14           91              910
  6            H                  O-n-Bu               Br           0.2            14           69             345(c)
  7            H            n-butylmethacrylate        Br           1.0            14           100             100
  8          OMe                  O-n-Bu               Br           0.2            13           52             260(d)
  9          OMe                    Ph                 Br           0.1            14           69              690
  10          NO2                   Ph                 Cl           0.1            19           100            1000
  11         COMe                   Ph                 Cl           0.1            14           97              970
  12           H                    Ph                 Cl            5             45           89              18
  13         OMe                    Ph                 Cl            3             40            5               2
        (a) The yield is referred to the (E). (b) 42% (E), 37% (Z), 20% (1/1). (c) 17% (E), 24% (Z), 29% (1/1). (d)
        10% (E), 14% (Z), 27% (1/1).

  It is noteworthy that PdCl2 displayed an increased reactivity in [Bu4N]Br when compared with
DMF, a “classic” solvent for Heck reactions (Scheme 3).
Molecules 2010, 15                                                                                    2215

                     Scheme 3. Ligand-free Heck arylation in [Bu4N]Br and DMF.

                                                      PdCl2 (2 mol%)
                         Ph-X +            Ph                                 Ph
                                                     NaOAc(1.2 equiv)                Ph
                                  (1.5 equiv)
                                                         150 °C

                          t = 2h, X = Cl         t = 18h, X = Cl        t = 2h, X = Br
                         DMF      [Bu4N]Br      DMF     [Bu4N]Br    DMF        [Bu4N]Br
                          0          7           0         50        16           94

   The unusual long-term thermal stability of the catalyst in [Bu4N]Br most probably accounts to the
significant differences observed when the reaction was performed at 150 ºC. The catalyst decomposes
later in the ammonium salt - if it does at all - than it does in DMF under the same conditions. Indeed,
the catalyst seems to be more activated; in fact, PdCl2 runs even about five time faster in [Bu4N]Br
than in DMF.
   The catalyst and the rather expensive solvent could also be recycled. In fact, in the reaction of
bromobenzene and styrene using 1 mol % PdCl2 and NaOAc as the base the authors were able to
recycle the system several times, nevertheless heavy Pd black formation was observed already during
the first run, simply distilling off the reactants and the products in vacuo. It was also possible to filter
off the NaBr and the Pd black precipitate after 8 recycling runs by dissolving the mixture in acetone.
After evaporation of the acetone the filtrate can be used again and stilbene was obtained in 60%
yield [22].
   The high activity displayed by PdCl2 in [Bu4N]Br was ultimately attributed by the authors to the
thermal reduction of the Pd(II) precatalyst to extremely active colloids. However, it is noteworthy that
neither Pd2(dba)3 nor preformed Pd colloids resulted to actively promote the Heck reaction in
[Bu4N]Br, confirming that active species have to form in situ by reduction. The same ammonium salt,
in a mixture with [Bu4N]OAc, was employed three years later by Cacchi and co-workers in an efficient
stereoselective synthesis of (E)- and (Z)-3,3-diarylacrylates [23].
   In particular, the Pd-catalyzed reaction of neutral, slightly electron-rich and slightly electron-poor
aryl iodides with methyl cinnamate in a molten [Bu4N]OAc/[Bu4N]Br (2:1.5) mixture provided with
high stereoselectivity (E)-3,3-diarylacrylates (E/Z = 98/2 to > 99/1) (Scheme 4). Phosphine-free
Pd(OAc)2 was employed as the catalyst precursor.

                          Scheme 4. Heck arylation in [Bu4N]OAc/[Bu4N]Br.

                                       Y               Pd(OAc)2 (5 mol%)
                          COOMe                       [Bu4N]OAc/[Bu4N]Br
                                   +                                                      COOMe
                                                         100 °C, 3 - 9h
                                                 I        (38 - 91%)
                                       (1.5 equiv)

                                   (Y = 4-EtOOC, 4-Cl, 4-MeO, 3-Me, 3-MeCONH, 3-MeO, 4-F)

   Moreover, the reaction of a variety of methyl 3-arylacrylates with iodobenzene under the same
reaction conditions afforded selectively the corresponding (Z)-isomers (E/Z = 4/96 to <1/>99) (Scheme
5). It was also observed that the catalyst system might be recycled without any loss of activity.
Molecules 2010, 15                                                                                                      2216

                  Scheme 5. Heck arylation of methyl 3-arylacrylates with iodobenzene.

                                                             Pd(OAc)2 (5 mol%)                 Ph
              Y                                                                      Y
                                 COOMe                      [Bu4N]OAc/[Bu4N]Br                              COOMe
                                                +   Ph-I
                                                               100 °C, 3 - 6h
                                                                (70 - 84%)

                     (Y = 4-EtOOC, 4-Cl, 4-MeO, 4-Me, 3-Me, 4-MeCONH, 4-NO2, 3-MeO, 4-F)

   The authors, in order to justify the observed elevated stereoselectivity, attributed to the acetate
anion a key role; in particular, they supposed that this anion could favour the irreversible displacement
of Pd from σ−Pd adducts thus suppressing any possible isomerization mechanism. They also attributed
the high reactivity of Pd(OAc)2 in the used molten salt mixture to the formation of palladium
nanoparticles stabilized by the ammonium salts.
   A similar stabilization of the active Pd catalytic species was invoked by Muzart and co-workers,
who employed NaHCO3 as the base and PdCl2 as the catalyst precursor in a Heck arylation of allylic
alcohols in [Bu4N]Br, affording β-arylated carbonyl compounds (Scheme 6) [24].

                            Scheme 6. Heck arylation of allylic alcohols in [Bu4N]Br.

                                                                 PdCl2 (10 mol%)
                            R2                                                                     R2
                                                                NaHCO3 (1.2 equiv)
                   R1              R3       +          Ar-X                              R1                 R3
                                                    (X = Br, I)
                                 OH                                120 °C, 24h                Ar        O
                             1              :           1           (33 - 71%)

  This protocol, which let possible to recycle the active catalyst up to four times, was also applied to a
one-step synthesis of the nonsteroidal antiinflammatory drug nabumethone (Scheme 7) [24].

                                        Scheme 7. Synthesis of nabumethone.

                                                             PdCl2 (0.05 equiv)                                     O
                                                            NaHCO3 (1.2 equiv)
              OH                                                 [Bu4N]Br
                                                                120 °C, 6h      MeO
              1         :               1

    During the past decade, in a series of important papers [25–27] Calo` et al. have evidenced the
superiority of tetraalkylammonium halides over common pyridinium and imidazolium salts in terms of
catalysts stability, reaction rates and regio- and stereoselectivity in Pd nanoparticles catalyzed coupling
reactions. Bromoarenes were coupled with less reactive 1,2-disubstituted alkenes, such as trans-
cinnamates, in a stereospecific manner under ligand-free conditions in 1:1.5 molar mixtures of
tetrabutylammonium bromide and tetrabutylammonium acetate, TBAB-TBAA (Scheme 8) [26]. The
observed stereoselectivity was ascribed not only to a better solubility of TBAA in TBAB, but also to
an intramolecular neutralization of PdH, still ligated to the olefin, by an acetate ion in the metal
coordination shell through a five-membered transition state.
Molecules 2010, 15                                                                                 2217

                           Scheme 8. Heck arylation of ethyl trans-cynnamate.

                            X                CO2Et      Catalyst
                                                       [NBu4]Br                 CO2Et
                       R                             [NBu4][OAc]

   The absence of acetate in the coordination shell would allow the PdH isomerization, leading to a
thermodynamic mixture of isomeric olefins. This palladium nanoparticles-catalyzed Heck arylation
was extended to butyl methacrylate and α-methylstyrene. In this case, however, a 3:1 mixture of
regioisomers was obtained in favour of the terminal olefins, together with variable amounts of double-
arylated products [27].
   In this contest, recently Nacci et al. have recently reported [28] a general method for coupling of
aryl chlorides, including deactivated and electron-rich aryl chlorides, using ligand-free Pd(OAc)2 in the
molten mixture of tetraalkylammonium ionic liquids (TBAB-TBAA) under aerobic and relatively mild
conditions (Scheme 9). Interestingly, using this system it was possible to couple 1-bromo-4-
chlorobenzene with two different olefins in a one-pot sequential manner by activating the C-Br and C-
Cl bonds on the aromatic ring at two different temperatures of 100 and 120 ºC. Unsymmetrical
substituted arenes were produced with high reaction rates and high overall yield.

                  Scheme 9. One-pot sequential coupling of 1-bromo-4-chlorobenzene.

                                                      R                             CO2Bu
                           Br       CO2Bu
                                  Pd-NP     120 ¡C
             Cl                 TBAB/TBAA
                                  100 ¡C
                                  30 min

   An extensive investigation on Pd-catalyzed Heck reactions in imidazolium based ILs has been
performed by the research group of Xiao. In 2000, they published [29] a detailed study on reaction of
iodobenzene with styrene and acrylates catalyzed by Pd(OAc)2, showing for the first time that the
imidazolium cation can react with a catalyst precursor to form N-heterocyclic carbene complexes via
deprotonation in the imidazolium-based ionic liquids under catalytic conditions, and that the carbene
complexes so generated are active for C-C bond coupling reactions. Subsequently, they focused on the
palladium catalyzed arylation of the electron-rich olefins, such as butyl vinyl ether in [bmim][BF4],
using aryl iodides and bromides as the arylating agents. The results of these studies, reviewed in 2008
[30], showed that imidazolium ILs in combination with the readily available Pd(OAc)2 and 1,3-bis-
(diphenylphosphino)propane (DPPP) form an excellent catalytic system, in which highly
regioselective and yielding arylation of electron reach olefins can be performed with a wide range of
aryl halides with no need for any halide scavengers (Scheme 10).
Molecules 2010, 15                                                                                     2218

                  Scheme 10. Highly regioselective arylation of electron rich olefins.

                  OBu                Br
                                          Pd(OAc)2                                               OBu
                        +                                            OBu +
                            R             IL, Base    R                       R

    Generally, the arylation gives only the branched olefins; the unique stereocontrol has been
attributed to the ionic environment, which alters the reaction mechanism favoring the ionic pathway. It
is however noteworthy that for this kind of reactions little conversions (<1%) were observed [31] in
[bmim]Br and [bmim]Cl and these results were attributed to the coordination of halides with palladium
and/or the formation of inactive imidazolium carbene complexes of Pd. The inhibitory effect of
bromide on this reaction has been subsequently more extensively investigated [32] by the same
authors, who on the basis of the dramatic decreases in the arylation rate with increasing bromide
concentration, hypothesized the presence of an equilibrium (eq. 1) before the rate determining step.

                            P        Ar           R [HNEt3]+    P        Ar
                                Pd            +                     Pd            +   -Br   H NEt3+     (1)
                            P        Br                         P


   HBr, generated from each arylation, must be effectively scavengered by the NEt3, which may be
able to trap also the bromide anions by hydrogen bonding between [HNEt3]+ and Br-. A remarkable
accelerating effect exerted by the potential hydrogen bond donor [HNEt3]+ was evidenced in the
same paper.
   In conclusion, whereas halides ([NBu4]Cl) are able to accelerate the Heck reaction employing
palladium catalysts containing monodentate olefins or no ligands, they have an inhibitory effect on Pd-
dppp catalyzed reactions. To a different mechanism, molecular in the first case and ionic in the second
one, was attributed the different behavior (for a detailed discussion see below).
   Despite of the extensive work performed on Heck reactions in the last ten years, it is however
noteworthy that only few data have been reported on the intramolecular process. The sole paper on this
topic evidenced that substituted benzofurans can be obtained [33] in modest to good yields by
palladium-catalysed intramolecular Heck reaction in [bmim][BF4] (Scheme 11). In general, the yields
of the substrates with a substituent on the aryl group were lower, although the character of the
substituent was not able to affect significantly the reaction yield. The catalyst in the IL phase could be
re-used by the addition of another portion of substrate, Bu3N and HCO2NH4; the catalytic activity
varied from 71 to 57% after four cycles.

                                 Scheme 11. Intramolecular Heck reaction.

                                                     5% PdCl2, (Bu)3N                  O
                                                     [bmim][BF4], 60 °C
                                          I                24h
Molecules 2010, 15                                                                                 2219

   Finally, it must be remarked that although generally aryl halides have been used in most of the
investigated reaction in ILs, in 2004 Kabalka et al. reported [34] the palladium catalysed reaction of
methyl acrylate and methyl acrylonitrile with arenediazonium salts in [bmim][PF6]. The reactions
could be performed at room temperature for methyl acrylate and at 50 ºC for acrylonitrile in the
absence of base and in relatively short reaction times (Scheme 12). The catalytic system was recycled
at least four times without loss of activity, although electron-rich olefins did not react and styrenes
produced dimerisation products.

                         Scheme 12. Heck reaction with arenediazonium salts.
                               N2BF4                                              X
                                                    Pd(OAc)2 2mol%
                           R                                              R

3. Task-Specific Ionic Liquids (TSILs) in the Heck Reaction

   On the base of the data reported in the previous section it is possible to conclude that the Heck
reaction based on the use of ILs is clearly advantageous from the point of view of product separation
and re-use of the catalytic system. In addition, most of the reactions can be performed in the absence of
added phosphorous containing ligands and, selecting ionic liquid and reaction conditions, it is possible
to use also aryl bromides and chlorides or benzoic anhydrides. However, common ILs still have a
tendency to leach dissolved catalysts into the co-solvent used to extract the product(s). To avoid metal
catalysts leaching out of the IL system, significant efforts have been made to enhance the solubility of
the catalysts in ILs and practically two approaches have been followed: i) functional groups able to
coordinate with metal centers have been inserted into the ILs, ii) imidazolium/pyridinium tags have
been introduced into a metal complex.
   In this context, in 2004 Shreeve et al. reported [35] the synthesis of the monoquaternary product
arising from the reaction of 2,2’-diimidazole with iodobutane and the application of the obtained IL as
solvent and ligand for the Heck reaction. The new palladium complex, reported in Scheme 13, was
isolated by adding PdCl2 to this ionic liquid in methanol. The same 2-imidazole functionalized IL was
used for the Heck coupling of iodobenzene with methyl acrylate in the presence of PdCl2 (2 mol%)
and a Na2CO3 as a base. After product recovery (92% yield) the system was washed with water and re-
used another four times without significant activity loss. The recovered system was used also with the
non-reactive chlorobenzene, leading to the same product in 74% yield after two runs.

                 Scheme 13. Palladium complex from 2-imidazole fuctionalized ILs.

                                                      PF6-         Bu N
                                Bu                            N
                               Bu N                      Bu           N
                                            PdCl2                        Cl
                          +N                                       Cl Pd
                                           CH3OH                      N Bu
                        Bu PF -                                             N
                                                                             N PF6-
Molecules 2010, 15                                                                                 2220

   An analogous functionalized palladium(II) complex was obtained by reaction 1-butyl-2-(2-pyridyl)-
3-methylimidazolium bistriflimide with PdCl2 in methanol. The coupling reaction of iodobenzene with
butyl acrylate in the corresponding IL, 1-butyl-2-(2-pyridyl)-3-methylimidazolium bistriflimide, or in
the analogous 1-butyl-2-(phenyl)-3-methylimidazolium bistriflimide could be performed successfully
10 times without detectable loss of catalytic activity (Scheme 14).

                 Scheme 14. Palladium complex from 2-(2-pyridyl) fuctionalized ILs.
                                                       Tf2N-        Bu
                                                        Me             N
                               Bu N
                                               PdCl2                   Pd Cl
                          +N                                        Cl
                          N                   CH3OH                    N Bu
                       Me Tf N-                  rt                          N +
                            2                                                 N

   However, when the Heck reaction was carried out using PdCl2 under similar conditions, the
formation of palladium black was evidenced and after two cycles the system became practically
inactive. Moreover, whereas no appreciable difference in yields between activated and deactivated aryl
iodides was found, the electronic nature of aryl bromides had a clear effect on the coupling reaction.
   But the functionalization of the imidazolium ring not necessarily have to involve the C(2)
imidazolium carbon. Substituents able to interact with palladium(II) have been introduced also on the
imidazolium nitrogen. Pyrazolyl-functionalized 2-methylimidazolium based ILs have been therefore
synthesized and the activity and recyclability of the palladium complexes have been evaluated [36]
(Figure 1).

            Figure 1. Palladium complex from N-pyrazolyl 2-methyl imidazolium based IL.
                                              Bu           +
                                          + N
                                      N                   N
                                  N        Tf2N           N N Pd N N

   Initially, the Heck reaction was performed using as catalyst the functionalized palladium complex
dissolved in the pyrazolyl-functionalized IL. However, interestingly, this catalyst could be used also in
a unfunctionalized IL (1-butyl-2,3-dimethylimidazolium bistriflimide) obtaining at the 9th recycle a
complete conversion of iodobenzene. The efficiency of the process was attributed to the presence on
the catalyst of a pendant 2-methylimidazolium tag which is similar to the unfunctionalized IL and
favour immobilization. Of course, when the Heck reaction was examined using PdCl2 as the catalyst
precursor in the pyrazolyl-functionalyzed IL under identical conditions similar results were obtained.
In this reaction, reactivity decreased using bromobenzenes whereas only traces of the product were
obtained with 4-chlorobenzene.
Molecules 2010, 15                                                                                 2221

   Starting from N-alkylimidazoles another series of pyrazolyl-functionalized imidazolium based ILs
was subsequently synthesized and characterized [37]. Through a (carbene)silver complex, a palladium
complex was prepared, isolated and characterized. In this case, as a consequence of the absence of the
methyl group at C(2) on imidazolium ring, the functionalized IL acted as a chelating ligand
coordinating the palladium(II) center through its carbene carbon atom and the pyrazolyl nitrogen atom
to give a six-membered metallacycle with boat-like conformation (Figure 2).

                 Figure 2. Palladium complex from N-pyrazolyl imidazolium based IL.

                                            Bu                 N Bu
                                        + N
                                    N                      N     Cl
                                          Cl                  Pd
                                    N N                    N N Cl

   The catalytic activity as a pre-catalyst was investigated in the corresponding IL using the above
reported procedure. Aryl halides gave with butyl acrylate the expected product in > 90 % yield
independently of the substituents on the phenyl ring. No detectable loss of catalytic activity was
observed after five recycles. Contemporaneously, also the activity of a hemilabile pyrazolyl-
functionalized N-heterocyclic carbene complex of Pd(II) in [bmim][PF6] was evaluated by the same
group [38]. The catalyst could be recovered and recycled at least three times without significant loss of
activity (Figure 3).

            Figure 3. Palladium complex from N-pyrazolyl-N-aryl imidazolium based IL.

                                                   + N
                                               N N Cl

   On the other hand, considering that some chelate or pincer N-heterocyclic Pd-carbene complexes
exhibit high catalytic activity for Heck reactions [39], the application of an IL containing a pincer
dication substituted with two alkyl chains has been investigated in the Heck reaction of aryl halides
with butyl acrylate at 120 ºC [40] by Shreeve et al. (Scheme 15). Iodobenzene was found to have the
highest reactivity, giving trans-butylcinnamate in 92% yield using both PdCl2 or the isolated
palladium complex as catalyst and NaOAc as base, whereas bromobenzene resulted in 71% yield and
chlorobenzene was practically unreactive (3% yield). In this case, the recovered IL could be reused
three-times without significant loss of activity.
Molecules 2010, 15                                                                                  2222

               Scheme 15. Palladium complex from an IL containing a pincer dication.

                                                  N Bu
                                                                                    N Bu
                                                          PdCl2, NaCl           N
                                                2Tf2N                               Pd
                                        N                NaOAc, DMSO            N
                                            N                                       N

   Since the introduction of a third nitrogen atom on the five member heteroaromatic system could
increase the acidity of the C-2 position favoring Pd-carbene complex formation, unsymmetrical
dicationic salts incorporating imidazolium and triazolium functionalities were also synthesized and
tested [41].
   It is noteworthy that, instead of a chelating palladium(II) dicarbene complex, the reaction with
Pd(OAc)2 in DMSO gave a dinuclear complex, which analogously to the above reported complexes
could be used as pre-catalyst in the Heck reaction (Scheme 16). Comparable results were obtained
using the same procedure.

                       Scheme 16. Synthesis of dinuclear palladium complex.

                                        N Bu
                                N                                  Bu N      N      N        N Bu
                                                 Pd(OAc)2, DMSO       I             I
                                                                           Pd           Pd
                            N                                                   I             I
                        N                                          Bu N
                                                                                N   N        N Bu
                                        Bu                                           N

   Contemporaneously, the strategy to introduce specific functional groups on the alkyl chain of
imidazolium cation to increase the catalyst immobilization has been followed by other research groups.
   In this contest, it has been shown [42] that the use of an imidazolium based phosphinite IL
(Figure 4) as solvent and as ligand in the coupling reaction of aryl halides (including chlorobenzene)
with styrene and butyl acrylate allows the synthesis of the expected product in high yield (generally,
around 90% also for ArCl) assuring also a high recyclability (six runs without losing its efficiency).
Although the crystal structure of the catalyst was not reported, the formation of a carbene-type
complex was excluded on the basis of the 1H NMR spectrum, and a ML2 complex was hypothesized.

                                    Figure 4. Imidazolium based phosphinite IL.



   Significant results have been obtained also by Dyson et al. introducing a nitrile group on the alkyl
chain(s) of pyridinium and imidazolium based-ILs (Figure 5) [43–45].
Molecules 2010, 15                                                                                                               2223

                                        Figure 5. Nitrile functionalized ILs.
                            N             C                                                      N     C
                                              N                                                          N
                                R                                   N                            N X
                          N         X                                     X
                                                                   (CH2)n                     (H2C) C
                          CH3                                  C                             n

   Nitrile-functionalized ILs have been used with success in several palladium catalyzed reactions,
including the Heck process and their usefulness became apparent upon catalyst recycling; while in
conventional ILs activity rapidly decreased to zero, in nitrile-functionalized ILs little change was
observed after several recycles. Functionalization of the alkyl chain facilitates the solubility of the
Pd(II) precatalyst via weak coordination of the nitrile groups to the palladium center, as evidenced
through IR measurements (Figure 6). Moreover, when palladium nanoparticles are involved nitrile
group may form a protective sheath around the nanoparticles, preventing aggregation.

                  Figure 6. Coordination of the nitrile group to the palladium center

                                                  N                C
                                                                       N PdCl2
                                              N       R
                                              CH3                       2

    It is noteworthy that, at variance with other palladium catalyzed reactions, reuse of the IL system in
the Heck reaction was not very encouraging, due to observed progressive loss of activity. The decrease
in reactivity of the catalyst could be however ascribed to the consumption of the base; the addition of 1
equiv of cholinium acetate after the fourth cycle restored the reactivity of the system.
    To overcome the problems arising from the depletion of the base during the recycles, more recently
basic IL have been employed as both solvents and proton scavenger in Heck coupling reactions. The
first example, reported [46] by Shreeve et al., evidenced that IL bearing tertiary aliphatic amines are
effective media for the Heck reactions (Figure 7).

                                        Figure 7. Basic ionic liquid cations.

         +                                                 +
     N       N                                        N        N                                                         +
                  N                       N                                 N                                        N       N
                                                                                                              (CH2)n             Bu
  PF6 or Tf2N                                         Tf2N                                                      Tf2N

                                                                                N       N
                          + N                                                                                    N
                                                                            N           2Tf2N
                   N Tf N       N                                                                                    Tf2N
                                                                                N   +   N
Molecules 2010, 15                                                                                2224

   Under reflux conditions, iodobenzene reacted with butyl acrylate to give the cinnamic ester in 100%
yield. The catalytic system could be reused simply by washing with NaHCO3; after five recycles no
activity loss was observed. Also in this case the formation of palladium nanoparticles was observed; all
attempts to synthesize carbene-palladium complexes resulted in stable palladium colloids. The
hypothesis that the tertiary amine might act as reducing agents in the redox process, leading to the
formation of nanoparticles, was advanced.
   Subsequently, 1,4-diazabicyclo[2,2,2]octane (dabco)-based ILs (Figure 8) having as counteranion
the basic dicyanamide or tetrafluoroborate have also been tested in the Heck reaction [47]. Although
after two hours the yields of the recovered product for the model reaction (iodobenzene-ethyl acrylate)
were lower than in the comparative reaction performed in [bmim][BF4], the dabco-based ILs could be
reused without re-addition of base and reducing agent.

                     Figure 8. 1,4-Diazabicyclo[2,2,2]octane (dabco)-based ILs.

                                            R        (CN)2N- or BF4-

                                     R = -C4H9; -C6H13; -C8H17

   Finally, also hydroxyl or diol functionalized ILs (Figure 9) have been employed has reaction media
and catalyst ligands in the Heck reaction. In 2003, Handy et al. prepared a new class of hydroxymethyl
substituted imidazolium based ILs using fructose as the starting material [48.].

                                Figure 9. Hydroxyl functionalized ILs.


   The ionic liquid having bistriflimide as counteranion was employed as solvent for the model Heck
reactions of methyl acrylate with simple aryl iodides in the presence of Pd(OAc)2. The coupling
products were isolated in almost quantitative yields simply by extracting the crude mixtures with
cyclohexane. Moreover, the solvent and catalyst were recycled 4-5 times without affecting the
efficiency of the reaction. Also in this medium, however, the Heck coupling was slower than those
performed in the standard, non-protic [bmim] series ionic liquids, although not at a great extent. An
interesting accelerating effect of catalytic amounts of certain halide ions on the coupling of
iodobenzene and methyl acrylate was observed.
   More recently, a series of imidazolium based ILs N-functionalized with a 2,3-dihydroxypropyl
group (Figure 10) has been used as ligands and solvents for palladium(II)-catalyzed reactions; these
compounds present, besides the N-heterocyclic donor moiety arising from the imidazolium skeleton,
the ethylene glycol group able to act as an excellent chelating ligand [49].
Molecules 2010, 15                                                                                 2225

                            Figure 10. 2,3-Dihydroxyl functionalized ILs.

   The Heck coupling of iodobenzene and ethyl acrylate in glycerylimidazolium-based ionic liquids at
100 ºC has been performed in the presence of 5 mol-% PdCl2 as the catalyst precursor and AcONa (1.1
equiv) as the base. Although reuse of the IL systems was not very encouraging, due to the progressive
loss of activity, a high reaction rate was observed in particular in 1-glycerol-3-octylimidazolium
chloride, [GLYOCTIM]Cl. The reaction yields increased on increasing the alkyl chain on cation
([GLYOCTIM]Cl > [GLYBIM]Cl > [GLYMIM]Cl, and [GLYOCTIM]Br > [GLYBIM]Br) and were
affected by anion nature and by the presence of a methyl group at C(2). The very low yield obtained by
using 1,2-dimethyl-3-glycerylimidazolium chloride as solvent suggested that imidazolium carbenes,
formed by reaction of the base with the acidic C(2)-H bond of the [GLYIM+] cations, might be
actively implicated in these reactions (vide infra). Since the formation of carbenes in imidazolium-
based ILs should be favoured by the basicity of the counteranion [50], the involvement of these species
is also in agreement with the anion effect: bromides give lower conversions than chlorides and
practically no reaction was observed in [GLYBIM]OMs.
   Finally, 1-glycerol-3-methylimidazolium hexafluorophosphate has been successfully used [51,52]
also as the active and recyclable catalytic system in the reaction of aryl bromides with methyl ethyl
acrylate under aerobic condition in DMF.
   Recently, a novel IL able to act as base, ligand and activator (Figure 11) has been synthesized by
Wang et al. [53] The olefination process of alkenes (styrene, acrylates and acrylonitrile) with activated
and deactivated iodo, bromo and chloroarenes has generated the corresponding products in good to
excellent yields in 4-(dihydroxyethyl)aminobutylammonium bromide). Transmission electron
microscopy (TEM) has evidenced the formation of particles having an average diameter of ca. 4 nm
suggesting that the ethanolamine functionalized IL are able to stabilize palladium colloids. It is
noteworthy that the system is characterized by a high recyclability; palladium and IL were recycled six
times with no loss of their activity and the inductively coupled plasma (ICP) analyses of the extracted
organic solvents indicated that Pd content was < 0.2 ppm.
Molecules 2010, 15                                                                                 2226

                                       Figure 11. Multifunctionalized IL.

                                                           Organic Base


                                                            Effective Anion

   An alternative approach to the use of multifunctionalized ILs may be the use of mixtures of
differently substituted ILs which can exert the functions of ligand, base and reaction medium as one
unit. In this contest, the ternary mixture of 1-butyl-2-diphenylphosphino-3-methylimidazolium
hexafluorophosphate,            [bdppmim][PF6],           1-(2-piperid-1-yl-ethyl)-3-methylimidazolium
hexafluorophosphate, [pemim]PF6] and 1-butyl-3-methylimidazolium hexafluorophosphate,
[bmim][PF6] represents an efficient and recyclable system for the PdCl2 catalyzed coupling of aryl
halides (including hindered and electron-rich aryl iodide and activated aryl bromides) with ethyl
acrylate (Figure 12) [54].

                                  Figure 12. Mixture of monofunctionalized ILs.

                                                                      N           N   N
                      N                               N     N

                          [PF6]                                 [PF6]                     [PF6]

   Experiments performed using the single ILs or their binary mixtures evidence a synergistic effect
when used as one unit: due to the presence of more coordinating sites the palladium catalyst combined
the advantageous activity and stability of N,P-containing ligand coordinated Pd complexes and Pd-
carbene complexes, although the exact nature of the species present in solution was unknown.
Furthermore, the strong hydrophobicity of the resulting system allowed the recovery of [pemim][PF6]
during the washing phase between two subsequent cycles and the recycling uses of palladium catalyst.
   More recently, the catalytic activity of 1-butyl-2-diphenylphosphino-3-methylimidazolium
hexafluorophosphate,        [bdppmim][PF6],       or        1-(2-piperid-1-yl-ethyl)-3-methylimidazolium
hexafluorophosphate, [pemim][PF6] in the model Heck reaction of aryl halides with methyl or ethyl
acrylates in [bmim][PF6] has been compared [55] with that of a hybrid P,N ligand functionalized IL, 1-(2-
piperid-1-yl-ethyl)-2-diphenylphosphino-3-methylimidazolium hexafluorophosphate, [pedppmim][PF6],
combining the structural peculiarities of the other two ligands in a sole molecule. In this case an
organic base was always added.
Molecules 2010, 15                                                                                          2227

                                    Figure 13. Mixture of monofunctionalized ILs.

                P                                            P
                                N                                                                       N
                    N                                    N       N                      N   N

                        [PF6]                                    [PF6]                          [PF6]

   With the involvement of the hybrid P,N-ligand, PdCl2 exhibited a very good activity and stability
also after seven runs without any precipitation of black palladium. In each run pure ethyl cinnamate
was obtained in >96% yield, whereas the ICP analysis indicated that leaching of Pd in the organic
phase was below the detection limit (< 0.1 mg/g). When the sole P-containing ligand was used lower
yields were obtained although good stability was observed. At variance, in the N-containing ligand the
palladium catalyst deactivated gradually during the recycle procedure with the obvious precipitation of
Pd black, though its fresh activity was better than the corresponding hybrid P,N-ligand. Therefore, as
suggested by the authors, the results seem to show that as a bidentate ligand, the P,N-functionalized IL,
with a hemilabile ligation to metal center could offer the necessary insaturation site for the substrate (at
N-coordinating site) simultaneously holding the Pd by the P-ligand arm. Moreover, the bonding of the
phosphine moiety to C-2 position of the imidazolium, having an electron-withdrawing nature, results
in a decreased electron density at the phosphorous center, leading to an improved oxidation tolerance
of its self with consequent stability of the catalyst. This feature was evidenced also for the
P-containing ligand in the above discussed ternary mixture.
   Finally, although many of the works on Heck reaction are related to imidazolium based ILs, in 2006
an interesting paper highlighting the possibility to the use of a Brønsted guanidine acid-base ionic
liquid as media and catalyst was published (Figure 14) [56].

                                Figure 14. Brønsted guanidine acid-base ionic liquid.

                                              H   AcO-
                                          N                              N

                                                                     N       N   +   AcOH
                                      N       N

   The basic butyltetramethylguanidinium acetate, prepared by neutralization of the corresponding
guanidine with acetic acid, was used successful as solvent, ligand and base in the reactions of aryl
halides with styrene and butyl acrylate at 140 ºC. High yields of the expected products were obtained
in subsequent runs (the system was reused five times) until all acetate was converted to bromide.
Determination of the palladium that remained in IL evidenced only a slight decrease. It is noteworthy
that quantitative conversion was achieved within 0.25 h when 0.16 mol % of PdCl2 was used;
decreasing the catalyst loading to 0.01% still resulted in quantitative conversion although the reaction
time was prolonged at 20 h.
   Considering that at elevated temperature the guanidinium based IL may dissociate into guanidine
and acetic acid, the author suggested that the generated guanidine acts as a base and a ligand for active
Molecules 2010, 15                                                                                  2228

Pd species. The molecular structure of the corresponding L2PdCl2 complex was reported (Figure 15)
and its activity as catalyst for the reaction between bromobenzene and styrene was tested in DMA.

                 Figure 15. The molecular structure of the Guanidine2PdCl2 complex.

                                                              N        N

                                                    Cl             N
                                                N             Cl

                                            N       N

   In conclusion, this simple IL is able to play multiple roles in this process: i) it acts as ligand
stabilizing the activated Pd(0) during the reaction; ii) it offers a strong base to favor the β-hydride
elimination; iii) it acts as a polar solvent increasing the reaction rate. Unfortunately, no other data or
example of application of this kind of IL has been subsequently reported.

4. Heterogeneous Heck Reaction in ILs

   To simplify processes and ensure reproducibility, process chemists generally gravitate toward
reaction chemistry that can be run in a single common liquid phase. However, significant productivity
advantages can frequently be achieved by employing multiple phases (i.e., liquid-liquid, gas-liquid,
solid-liquid). As reported in the previous sections, liquid-liquid biphasic Heck processes involving
ionic liquids and organic product phases have been the subject of intensive investigation since the
ionic catalyst phase often induces excellent catalyst immobilization and also low miscibility with the
organic products. However, application of these procedures in industrial processes is still hampered by
the fact that these methodologies use large amounts of ILs. Moreover, in many cases liquid-liquid
biphasic catalysis uses only fraction of the IL and of the dissolved catalyst. Finally, catalyst leaching
cannot be completely avoided.
   To increase catalyst performance and develop more sustainable and environmental benign reaction
conditions, in the last decade several attempts have been done to perform the Heck reaction under
heterogeneous conditions. The concept of confining a catalyst on porous supports is far from new, and
several palladium catalyst reservoirs are commercially available; these systems have tested also in ILs.
In 2001, the use of Pd/C as a stable and inexpensive catalyst in [bmim][PF6] in the presence of Et3N
was reported [57]; the procedure assured the re-cycle of the catalytic system without loss of activity.
Moreover, ICP analysis of the ionic liquid performed before and after reaction revealed that the
concentration of Pd in the ionic medium was negligible, suggesting that the Pd/C catalyzed reaction
occurs on the surface of Pd held on the carbon. More recently, the same research group has reported
[58] the very facile and economically friendly immobilization of Pd(OAc)2 in silica gel pores, with the
aid of [bmim][PF6], and the application of this catalytic system in recyclable Heck reactions (figure
16). The catalyst immobilization on the silica gel was highly effective in dodecane at 150 ºC giving a
95% average yield and a TON of 68,000 up to 6th use. The use of a hydrocarbon solvent was however
necessary to prevent removal of the IL layer from the silica.
Molecules 2010, 15                                                                                   2229

               Figure 16. Immobilized Pd(OAc)2 in silica gel pores using [bmim][PF6].

   Promising results in terms of both yield and sustainability, were obtained through the
immobilization of Pd(OAc)2 on reversed phase amorphous silica gels, always with the aid of
[bmim]PF6. In this case, a water compatible catalytic system was obtained [59]. The Heck reaction of
iodobenzene with cyclohexyl acrylate using this catalyst, carried out in water at 100 ºC, was
characterized up to the sixth re-use by an average 95% yield with TON and TOF 1,600,000 and 71,000
(h-1), respectively. Moreover, different by the reaction in dodecane, there is no need, for recycle use, to
wash the recovered catalyst with alkaline solution to remove ammonium halide. It is noteworthy that
when silica supported palladium complex catalysts were used for the reaction of iodobenzene with
olefin also in [bmim][PF6], although the system exhibited higher activity that in DMF, a dissolution of
the catalyst from the silica support to the medium during the reaction was evidenced [60]. This latter
process, therefore, although mediated by supported metal Pd may be considered to proceed as a quasi-
homogeneous process.
   Support, solvent and form of palladium catalyst determine the intrinsic nature of the process and the
possibility to have or not the direct involvement of Pd atoms located onto the support into the catalytic
cycle. In this contest, a recent study of the Heck reaction between bromobenzene and styrene
performed in tributylhexylammonium bistriflimide, using ultrafine particles of palladium metal
supported onto hydrophilized mesoporous soot as the catalyst, has confirmed the fundamental role of
the IL in the catalyst stabilization, also evidencing that the reaction occurs through a “true
heterogeneous” mechanism [61].
   Highly efficient and stable supported palladium nanoparticles have been obtained also by
contacting a solution of Pd(OAc)2 with an ionic liquid- modified xerogel [62], containing a
phosphinite functionalized IL (Figure 17) attached to the silica matrix able to act as both as
complexing and reducing agent.
Molecules 2010, 15                                                                                   2230

                               Figure 17. Phosphinite functionalized IL.

                                           N     N        OPPh2


    It is noteworthy that Pd(OAc)2 solution gives by interaction with this matrix highly dispersed
uniformly sized Pd nanocatalysts, tightly supported on the surface of the silica and not embedded in
the bulk of xerogel.
    However, sol-gel processing has been also used to encapsulate Pd(OAc)2 catalyst into a ionic
phase-confined silica gels [63]. Pd-doped ionogel pellets have been employed in the coupling reaction
of ethyl acrylate with iodobenzene using toluene as solvent. No significant difference in the reaction
rate, when compared to homogeneous system, was observed, whereas leaching tests showed that
catalysts actually took place in the IL phase confined within the silica matrix.
    But, silica and carbon are not the sole supports used. Palladium nanocolloids supported on chitosan
have been found to be an efficient and recyclable catalyst for Heck arylation of alkenes using
tetrabutylammonium bromide (TBAB) as solvent and tetrabutylammonium acetate (TBAA) as base
[64]. The efficiency of this catalyst has been attributed to the stabilization of palladium colloids by the
solvent and to a very fast PdH neutralization by the base. It is noteworthy that the conversion rate
decreases significantly in imidazolium based IL (including [bmim]Br) whereas in TBAB recyclability
is possible only using aryl iodides; when bromobenzene was used the efficiency of the catalyst
decreased with a concomitant leaching of the palladium from the complex, attributed to a
decomposition of TBAB at 130 ºC (temperature necessary for reaction of bromobenzene) which
destabilizes the nanocomposite.
    Recently, also Pd nanoparticles immobilized on molecular sieves SBA-15 (thiol-modified
mesoporous materials) using 1,1,3,3-tetramethylguanidinium lactate have been synthesized [65]. The
Heck arylation of olefins using this catalytic system was carried out under solvent-free conditions; the
model reaction, iodobenze- methyl acrylate proceeded smoothly and the yield of trans-methyl
cinnamate was 93%, even when the amount of Pd was 0.001 mol%. Moreover, no deactivation of the
catalyst was observed after six repeated catalytic coupling reactions. It is noteworthy, that the
reusability of the Pd catalyst supported on SBA-15 in the absence of the IL was significantly lower; it
became an inactive white power after two recycles. The different behavior, with and without IL, has
been attributed on the basis of transmission electronic microscope (TEM) images to the different size
and distribution of the catalyst. In the presence of the IL, most of the Pd nanoparticles had before use a
diameter in the range of 3-6 nm and they existed in the channels of the support. At variance, in the
absence of the IL, the diameters of most Pd nanoparticles were in the range of 9-12 nm, bigger that the
pore diameter of SBA-15, resulting in their existence on the outside surface of SBA-15. After, six
recycles, most Pd nanoparticles on IL containing SBA-15 still exist in the channels, although some
bigger Pd particles were formed, whereas in the absence of IL already after 2 recycles, most of the Pd
disappeared and few Pd particles existed on the surface of the support. But, interesting results related
to the catalytic mechanism of these systems have been obtained through filtration tests. The solid-free
filtrate, obtained after separation of the catalyst before the end of the reaction, was able to continue to
Molecules 2010, 15                                                                                 2231

react, suggesting that the leaching of the active palladium species from the solid support occurred
during the coupling. However, Pd re-deposited back onto the support after the end of the reaction.
   Among the possible supports for Pd nanoparticles ionic polymers also have to be mentioned. A
number of imidazolium-based polymers have been indeed synthesized (Figure 18), including poly(3-
(4-vinylbenzyl)-1-methylimidazolium chloride and bistriflimide [66], and their ability to stabilize
metal nanoparticles has been tested in coupling processes, including the Heck reaction [67].

                               Figure 18. Imidazolium-based polymers.

   In particular, it has been shown that highly stable Pd nanoparticles, protected by an imidazolium
based ionic polymer in a functionalized IL, can be easily prepared by reduction with NaBH4. These
viscous systems, containing Pd nanoparticles, are excellent pre-catalysts for Suzuki, Heck and Stille
coupling reactions and can be stored without undergoing degradation for at least two years; compared
to commercial Pd/C systems that typically required Pd loading of 2–4 mol % loading of 0.5–1.0 mol %
can be employed using these systems.
   In all the above discussed systems the Pd catalyst is held in/on a more or less porous solid by physic
absorption, sometimes using the ability of the IL to coordinate the active catalytic species. However,
immobilization can also be obtained through the covalent anchoring of the catalysts to the support
surface [68]. Trimethoxysilylpropyl functionalized ILs may been anchored to silica, via the reaction of
the appended alkyl–silyl side branch tethered into the anion or cation. Using this approach to graft
imidazolium cations onto pre-dried silica, the subsequent treatment with PdCl2, gave non-acidic
supported palladium-based ILs to use in Suzuki-Miyaura coupling reactions [69]. But the same
approach was used also to tether a N-heterocyclic carbene palladium/ionic liquid matrix on the silica
surface (Scheme 17).
   The latter system, constituted by grafted N-heterocyclic carbene palladium complexes in grafted
ILs, was effectively applied to the Heck reaction of a wide variety of iodo and bromoarenes. The
catalyst showed high thermal stability (up to 280 ºC) and could be recovered and reused for four
cycles, giving a total TON = 36,600. It is noteworthy that, TEM coupled with EDX analysis indicated
the formation of Pd nanoparticles with irregular shape and wide-range size distribution confined inside
the irregular pores of amorphous SiO2.
Molecules 2010, 15                                                                                         2232

        Scheme 17. N-heterocyclic carbene palladium/IL matrix tethered on the silica surface.

                 (EtO)3Si        N
                                                               (EtO)3Si            N        N
                                                                                       Pd        2
                                                                               Cl           Cl

                                Cl-                        Cl                                        Cl-
                                          N          N        N                             N
                                          N          N     Cl   N                       N

                                        Si          Si              Si                 Si

                                O         O   O O    O     O    O   O      O   O       O         O

                                      SiO2          SiO2            SiO2               SiO2

   Finally, it is worth mention that polymeric beads of supported ILs (Figure 19), prepared via the
covalent anchoring of an imidazolium salt to a PEG support, have also been used as a recyclable
(100% yield after five cycles in the reaction of bromobenzene with styrene) catalytic system for the
Heck reaction of aryl bromides and activated aryl chlorides [70]. Also in this case the drop in
conversion starting from the sixth cycle was attributed to the accumulation of inorganic salts.

                                        Figure 19. PEG supported ILs.

5. The Role of ILs in the Heck Reaction: the “Ionic Liquid Effect”

   Ionic liquids constitute not only good solvents for the Heck reaction, due to their properties, but
their unique ionic environment may change the course of the reaction, activating and/or stabilizing
intermediates or transition states in the reaction mechanisms. As a consequence, performing the Heck
reaction in these solvents may lead to an increasing in the reaction rate in comparison with “classical”
solvents, to a stabilization of the catalytically active species and, in favorable cases, also to a control
on the regio- and stereoselectivity of the coupling products. The nature of the ionic liquid however
affects not only the kinetic constants of the single steps of this reaction but, probably, determines also
the nature of the catalysts or pre-catalysts. The involvement of carbene complex, palladium
nanoparticles and palladium anionic complexes seems to depend on the ionic liquid anion-cation
Molecules 2010, 15                                                                                    2233

5.1. ILs in ligand-free Heck reactions and their role in the stabilization of metal nanoparticles

   As reported above, the formation of palladium nanoparticles in the Heck reaction performed in ILs
has been more times evidenced. Calò and co-workers described the use of Pd-NPs generated in situ by
reaction of the carbene Pd precatalyst A in [Bu4N]Br using [Bu4N][OAc] as the base (Figure 20)
[26,28]. They found that Pd-NPs are rapidly formed when [Bu4N][OAc] is added to A dissolved in
[Bu4N]Br at 130 ºC. In addition to the formation of a black suspension, which is typical when NPs are
involved, the authors isolated 2-oxo-3-methylbenzothiazole (B) arising from deligation of the carbene
ligands in A (Figure 20).

         Figure 20. Carbene palladium precatalyst (A) and 2-oxo-3-methylbenzothiazole (B).

                                S      I   S                                 S
                                      Pd                                           O
                                N      I   N                                 N
                                 Me                                           Me
                                      A                                  B

   Pd-NPs where confirmed by TEM analysis of a reaction sample [26]. Moreover, as already
observed by Reetz and Westermann [71], Pd-NPs were also formed simply dissolving Pd(OAc)2 in
[Bu4N]Br and adding [Bu4N]OAc, obtaining rapidly a dark dispersion [26].
   Pd-NPs dispersed in [BMIM][PF6] have been prepared also by Dupont and co-workers and tested in
the coupling of aryl iodides with n-butylacrylate using NEt(i-Pr)2 as the base in a temperature range of
30–130 ºC [72]. The Pd-NPs before and after the coupling were analyzed by TEM, and the palladium
content of the organic phase during the arylation reaction was checked by ICP-AS. The results
obtained in this study were interpreted as strong indications that Pd-NPs dispersed in the ionic liquid
act as a reservoir of catalytically active Pd species, and that very probably the reaction proceeds
through the oxidative addition of PhI on the NPs surface, and the oxidized Pd species thus formed are
detached from the surface and enter in the main catalytic cycle [72]. However, more recently this
definition has been considered not formally correct. In fact, when the Pd(0) agglomeration rate is quite
high there is an increase in the contribution of a second catalytic cycle to substrate conversion, and this
make impossible a definition of metallic palladium as a catalyst reservoir [19].
   The non-linear dependence of reaction yield on catalyst concentration has also been associated with
the formation of Pd-NPs as the reactive form of the catalyst. In fact, initially the Pd(0), formed by a
rapid reduction in situ of the Pd(II) precatalyst, is in the form of Pd clusters that are stabilized to some
extent by the ionic liquid (for example, it has been suggested that in ammonium halides an
“electrosteric” stabilization of metal NPs is operative: the halide anions provide electrostatic
stabilization, and the ammonium cations a steric stabilization [73]). If the oxidative addition reaction is
slow, the clusters may undergo ripening to form large crystals that precipitate as Pd black. Hence, to
prevent this it may be better to increase the substrate/catalyst ratio [74]. As an example, in a recently
published protocol for the base-free Pd-catalyzed Heck reaction in [Bu4N]Br, the yield of
butylcinnamate formed by reaction of bromobenzene and butylacrylate strongly depends on the
concentration of palladium. In particular, the maximum yield was obtained for PdCl2(PhCN)2 at
Molecules 2010, 15                                                                                2234

0.09 mol%, and for PdCl2 and Pd(OAc)2 at ca. 0.4 mol%, while lower yields were obtained below and
over that concentrations (Scheme 18) [75].

                     Scheme 18. Base-free Pd-catalyzed Heck reaction in [Bu4N]Br.

                                                 Pd cat)       Ph
                        Ph-Br +      COOBu                              COOBu
                                               140 °C, 4h

   As stated, a similar behavior has been interpretated as the effect of Pd-NPs formation, but the
authors suggested that also a concomitant formation of less active dimeric or polymeric Pd(II)
complexes could also be considered another explanation of the observed concentration effect [75].
   Palladium nanoparticles have been evidenced also in functionalized ILs [43]. TEM images evidence
that nanoparticles isolated from [bmim][BF4] and [C3CNmim][BF4] have a similar diameter of
ca. 5 nm, but they show different morphologies. The nanoparticles obtained from nitrile functionalized
ILs were well-separated, whereas those from [bmim][BF4] formed nanoclusters up to ca. 30 nm,
suggesting that the nitrile group exerts a stabilizing effect and prevents aggregation. Nevertheless, in
nitrile functionalized ILs the involvement of carbene complexes of the type, [(C3CN)2im]2[PdCl2] as
catalysts or precatalysts has been excluded: Heck reactions performed using the pre-formed
[(C3CN)2im]2[PdCl2] carbene complex were characterized by low conversions (ca. 2–8%), when
carried out under conditions giving > 95% yields with PdCl2. It is noteworthy that not only PdCl2 but
also [(C3Cdmim][PdCl4] can act as an active pre-catalyst in the Heck reaction. [(C3Cdmim][PdCl4] is
formed by addition of PdCl2 to [C3CNmim]Cl; also in nitrile functionalized ILs if the anion is a strong
nucleophile, such as chloride, the preferential coordination of the chloride anion with the metal takes
place. The resulting complex is however less catalytically active with respect PdCl2 or, more likely,
less able to transform to the active species (Figure 21).

                   Figure 21. Palladium complexes with nitrile functionalized ILs.

5.2. ILs as precursors of carbene ligands

   It is noteworthy that, despite the vast majority of the research in the area of ILs is focused on
compounds derived from imidazole, due to the simplicity in preparing these derivatives and the
generally lower melting points and viscosities as compared to other classes (such as ammonium-,
phosphonium- or pyridinium-based ILs), at the beginning of the development of Heck protocols in
ionic liquids they proved to be significantly less effective when compared to tetraalkylammonium salts
in the absence of phosphine ligands [18,21]. However, it has been reported that the addition of a
Molecules 2010, 15                                                                                                   2235

phosphine ligand when the reaction was carried out in [BMIM][PF6] resulted in a dramatic increase in
the reaction rate (Table 3) [18].

                                   Table 3. Heck reaction in imidazole-based ILs.
                             I                                                R
                                                        Pd(OAc)2 (cat)
                                  +        COOEt
                                                       Et3N, IL (additive)
               R                                                                                COOEt

   1             [C6Py]Cl                   -                   40                         24                 99
   2            [pmim]Cl                    -                   80                         72                 10
   3           [bmim][PF6]                  -                   100                        20                  7
   4           [bmim][PF6]                  -                   140                        18                 94
   5           [bmim][PF6]                 Ph3P                 100                        1                 95-99

   It is also interesting to note that, under phosphine-free conditions, the reaction was not effective at
100 ºC (entry 3), but when the temperature was raised to 140 ºC the required cinnamate was obtained
in 94% yield (entry 4). This result was attributed, once again, to the formation of Pd-NPs, which
became active as catalyst at T > 100 ºC [6].
   Similarly, Herrmann and co-workers found that imidazole-based ILs gave less satisfactory results
compared to tetraalkylammonium salts (Table 4) [21].

                                 Table 4. Palladacycle-mediate Heck reaction in ILs.
                                                      Palladacycle (0.5 mol%)         Ph
                    Ph-Cl +                 Ph                                                   Ph
                                                       IL, [Ph4P]Cl (6 mol%)
                                      (1.5 equiv)    NaOAc (1.2 equiv), 150 °C

         Entry                               IL                  Reaction time (h)              GLC yield (%)

           1                             [Bu4N]Br                        18                           51
           2                             [pmim]Br                        19                           22
           3                              [bpim]Br                       16                           11
           4                             [bbim]PF6                       15                           5

   It has to be noticed that a particularly low GLC yield was observed when the noncoordinating [PF6]
anion was used as counterion (entry 4). All these results clearly suggested that either different
catalytically active species are involved in relation with the used ionic liquids, or that the reaction
proceeds through different mechanistic pathways.
   It is well established that the α-position of imidazolium salts can be deprotonated to form stabilized
carbenes [76],and that these Arduengo-type carbene are employed as ligands in the area of transition
metal-catalyzed reactions [77]. In 2000, as already stated, Xiao and co-workers obtained the first
convincing evidence that carbene species are involved in Heck reactions carried out in imidazolium-
based ionic liquids (Scheme 19) [29].
Molecules 2010, 15                                                                                    2236

                            Scheme 19. Heck reaction in imidazolium based ILs.

                        X                                PdCl2 (1 mol%)
                                                      [bmim]Y (Y = BF4, Br)
                               +          R1
             R                                          NaOAc (1.1 equiv)
                                                          90 - 125 °C            R
              (X = I, Br)           (1.4 equiv)
        (R = H, CHO, COMe)      (R1 = COOnBu, Ph)

   They found that the Heck reaction proceeded markedly more efficiently in the ionic liquid
containing bromide as the counterion than in the analogous tetrafluoroborate salt. In conjunction with
these observations, carbene complexes have been isolated by reaction of [bmim]Br with Pd(OAc)2, but
non-carbene species were detected when [bmim][BF4] was used. The authors attributed the stability of
the isolated carbene complex C (Figure 22) to the bromide ions (X = Br), while the presence of [BF4]-
probably converts C into an inactive complex [29].

                              Figure 22. Isolated carbene palladium complex.
                                                  Me    Bu
                                                 N   X    N
                                                 N   X    N
                                                  Bu    Me

   Subsequent studies proved that the acidity of the α-position in imidazolium salts depends not only
on the structure of the heterocyclic moiety, but also on the nature of the anionic partner [50,79]. These
studies revealed that more basic anions (such as halides) resulted in imidazolium salts that undergo an
easy deprotonation even in the absence of any added base (thus generating carbenes), while weakly
coordinating anions (such as tetrafluoroborate or hexafluorophosphate anions) resulted in salts which
necessitate strong external bases for the C-2 deprotonation [79]. As a consequence, it may be possible
to conclude that imidazolium halides, or imidazolium-based ionic liquids containing coordinating
(basic) anions, give rise to carbene Pd complexes which resulted stabilized by the same anions, while
imidazolium salts containing non-coordinating anions furnished inactive Pd species, which can convert
to Pd-NPs at elevated temperatures [73] or into active Pd catalysts in the presence of phosphine
ligands [18].
   These conclusions are in agreement with the negative result observed when a 2-methyl substituted
glycerol-containing imidazolium salt, which cannot generate carbenes in the reaction medium, was
used as the solvent in the Heck arylation of ethyl acrylate. In contrast, the analogous Cl or Br salts, that
may be converted into Arduengo-type carbene ligands, resulted good solvents for the Heck reaction
promoted by PdCl2 in the presence of NaOAc as the base [49].
   Moreover, Lauth-de Viguerie and co-workers reported the use of palladate salts, generated from
PdCl2 and [BMIM]Cl, as catalysts for the Heck reaction in [BMIM][PF6], which proved to be more
effective than traditional precatalysts [80]. These results were attributed to the presence of NPs which
served as reservoir of Pd species , while the formation of carbenes was excluded.
Molecules 2010, 15                                                                                                  2237

5.3. ILs and regioselectivity in the Heck reaction

   The regioselectivity of the Heck reaction is considered to be affected by the two generally accepted
pathways: ionic versus neutral, as illustrated in Scheme 20 [81].

                   Scheme 20. Proposed neutral and ionic pathways for the Heck reaction.

                                        Ar-X                             Ar-X

                       L                              L        L                            L    L
                  L                                       Pd                                  Pd
                    Pd                                                                     Ar    X
                 Ar    X

                                  R                                base•HX                                      R

         L         L    X                                                            L    L                 X
                     Pd                                                                Pd
                  Ar      R                                                         Ar              R

                          β-arylation                                                           α-arylation

                                                      L    L
                                                      X    H
             L        L      Ar                                                        L        L
                 Pd                                                                                     Ar
             X                                                                             Pd
                      R                                                                                 R

                 "neutral" pathway               Ar                             "ionic" pathway

   As depicted in Scheme 20, the neutral pathway, which is more sensitive to steric factors than to
electronic factors, leads mainly to the formation of linear or β-substituted alkenes, while the ionic
pathway, which depends on electronic factors than on steric factors, prevalently yields branched or α-
products. The neutral pathway is characterized by dissociation of one of the coordinating neutral
ligand L, while ionic pathway generates a vacant coordination site through the dissociation of the
(pseudo)halide anion. As a consequence, with monodentate ligands and aryl halides the neutral
pathway should dominate due to the easy dissociation of the ligand in comparison with the relatively
strong Pd-X bond. In contrast, the liability of Pd-Y bond (where Y = OTf, OMs, OTs) means that the
ionic route should be preferred when using low-coordinating anions, particularly in the presence of
chelating bidentate ligands [31,83].
   For reactions involving electron-deficient olefins (R = EWG) under “normal” Heck conditions, that
imply the use of monodentate phosphines and aryl/vinyl halides, the products usually result from
arylation/vinylation at the terminal β-position through the neutral pathway. However, under similar
Molecules 2010, 15                                                                                         2238

conditions the observed regioselectivity when electron-rich olefins (R = EDG) are reacted with halides
might be poor, and a mixture of products is often obtained [11,83]. In contrast, the promotion of the
ionic pathway by using bidentate ligands and pseudohalides such as triflates, tosylates or mesylates as
labile counterions, as showed by Hallberg [11] and by Cabri [83], generated a highly reactive
cationic palladium complex and resulted in an efficient regioselective α-arylation/vinylation of
electron-rich olefins.
   Unfortunately, problems such as poor availability, high cost and thermal liability limited the
synthetic use of this approach, and now aryl/vinyl halides are used in the presence of a halide
scavenger such as silver or thallium salts. However, the cost of silver and the toxicity of thallium
severely prevent their use on a large scale.
   To overcome these problems, Xiao and co-workers reported the use of imidazolium-based ionic
liquids in conjunction with Pd-dppp precatalyst for highly regioselective Heck arylations of electron-
rich olefins [31,84,85]. As previously discussed, they supposed that the ionic pathway might be
promoted by using ionic liquids as solvents, producing branched olefins without halide scavengers,
according to the recent kinetic studies made by Amatore, Jutand and co-workers [86,87]. The strong
electrostatic interactions existing in an ionic liquid would favour the generation of a Pd-olefin cation
and a halide anion from two neutral precursors over the formation of a neutral Pd-olefin intermediate.
In fact, they were able to obtain regioselectively and with high chemical yields α-arylated
olefins starting from electron-rich olefins such as vinyl ethers, enamides, allyltrimethylsilane
(Scheme 21) [31,84].

                        Scheme 21. α-Arylated olefins from electron-rich olefins.

                                       Pd(OAc)2 (2.5 mol%)
                                          dppp (5 mol%)          Ar
                   OR       + Ar-Br                                         +   Ar                   (a)
                                         Et3N (1.2 equiv)                                       OR
                                       [bmim][BF4], 115 °C       α                          β
                                             24 - 36h                (α : β = > 99 : < 1)
                                            (80 - 97%)
                   O                    Pd(OAc)2 (4 mol%)                           O
                                          dppp (8 mol%)                                              (b)
               N       R1   + Ar-Br                                   Ar        N       R1
               R                         Et3N, 115 °C, 36h                      R
                                            (71 - 86%)               (α : β = > 99 : < 1)
                                         Pd(OAc)2 (4 mol%)
                                           dppp (8 mol%)
                   SiMe3 + Ar-Br                                                    SiMe3            (c)
                                          Et3N (1.2 equiv)
                                      [bmim][BF4], 115 °C, 36h
                                                                     (α : β = > 99 : < 1)
                                             (65 - 95%)

   In addition, the group of Xiao observed that hydrogen-donating salts such as [HNEt3][BF4] could
increase both the rate and the selectivity in ionic as well in molecular solvents, presumably by
facilitating dissociation of bromide from Pd(II) complex (see eq.1) [85].
Molecules 2010, 15                                                                                                               2239

   The selectivity of the Heck reaction is influenced not only by the reaction pathway, ionic or neutral,
but also by other parameters including, among others, the choice of additives and the structure of the
coupling partners.
   The influence of the base and the chosen ionic liquid on the selectivity was evidenced by Calò and
co-workers [88]. In particular, they found that using [Bu4N][OAc] dually as solvent and base, the Heck
arylation of allylic alcohols under phosphine-free conditions resulted highly selective towards the
formation of aromatic conjugated alcohols, while with [Bu4N]Br as solvent and NaHCO3 as the base
the aromatic carbonyl compounds were selectively obtained (Scheme 22) [88].

                                  Scheme 22. Heck arylation of allylic alcohols.
                                                       [Bu4N]Br            Ar               R1
                                                                                                  +   Ar                R1
                                                       NaHCO3                   R       O

          R              R1             Pd(OAc)2
                              + Ar-X
                        OH                                                                                 Ar
                                                                           Ar               R1
                                                      [Bu4N]OAc                                   +                 R1
                                                                                R       OH
                                                                                                       R        OH

   The authors, in order to explain the obtained results, supposed that the reaction can follow two
different pathways, depending on the nature of the ligand (X) bonded to palladium before the
migratory insertion of the aryl group on the olefinic double bond (Scheme 23) [88].

                                       Scheme 23. Possible reaction pathways.

                                     (a)        (X = I, Br) Pd(0)      AcO          X
     X                        (base = NaHCO3)
         Pd                                          Ar Pd X                                      Ar Pd OAc
    Ar              OH                                                      (b)
                                         OH                         (base = [Bu4N]OAc
               R                                                                                                        OH

                                                                                                  Ar Pd
                                                                                                                    OH       AcO
    Ar              R

                                                                  OH                        AcO             OH
                                                                                                      Pd      Hb
                                                       Ar              R                                     R

                                                            (mainly)                                  Ha    Ar
Molecules 2010, 15                                                                                  2240

   In particular, when [Bu4N]Br is used as solvent, and a conventional base such as NaHCO3 is
employed, ligand X will be bromide or iodide anion (path a) and the reaction follows the neutral
pathway (path a). For steric reasons, the migratory insertion of the aryl group predominantly occurs to
the β-position of the alkenol, giving rise to β-arylated carbonyl products.
   In contrast, when [Bu4N][OAc] is used as the reaction medium, an anionic metathesis probably
occurs, and the acetate anion readily dissociates to give a cationic complex. This may explain the
higher catalytic activity displayed by Pd(OAc)2 in [Bu4N][OAc], which allows the metal to activate
iodo- and bromoarenes at low reaction temperatures (r.t. and 60 ºC, respectively, in contrast with the
100 ºC and 130 ºC required in [Bu4N]Br). In this case, the migratory insertion on the double bond
occurs prevalently on the β-position due to the formation of a chelate structure, which impedes also the
hydrogen atom (HB) to adopt the syn-relationship with Pd, necessary for the β-elimination. Hence, the
abstraction of the benzylic hydrogen atom (HA) remains the only possible pathway.

6. Conclusions

   The research of more sustainable procedures prompted organic chemists to look for more eco-
compatible and more economical transition metal-catalyzed reactions. In the last ten years, a growing
number of palladium catalyzed Heck reactions in ILs have been proposed as greener alternative to the
classical cross-coupling procedure. Homogeneous and heterogeneous conditions have been explored
showing that ILs are not only suitable solvents for the Heck reaction, but their unique physico-
chemical properties are able to change the course of the reaction, activating and/or stabilizing
intermediates or transition states in the reaction mechanisms. Consequently, Heck reactions performed
in ILs, or ILs containing environments, may have higher reaction rates, and may be characterized by a
higher control on the regio- and stereoselectivity of the coupling products. The involvement of carbene
complex, palladium nanoparticles and palladium anionic complexes has been evidenced depending on
ILs structure and reaction conditions. In spite of the numerous interesting reports mentioned in this
review, the study of the Heck reaction in ILs is still strongly related to the development of the IL; only
simple reactions such as the coupling of aryl halides with cinnamates or styrene have been
investigated. Thus, in the future, the use of ILs in Heck reactions involving more complicated
substrates could give important indications about the possibility of application of these alternative
media on large scale reactions.


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