homogeneous gold catalysis 
 is a relatively new field of research 
 that became popular in the early years of this century .
although there 
 were a few scattered papers describing reactions of aromatic compounds 
 with gold salts , the first signals of the synthetic potential of gold 
 in homogeneous catalysis came with the work of teles and tanaka on the addition 
 of alcohols and water to alkynes .
we focus on the developments of synthetic methods that were 
 inspired on our initial studies of platinum(ii)-catalyzed reactions 
 and , in particular , on the multifaceted nature of cyclopropyl metal 
 carbenes , key electrophilic intermediates formed by the activation 
 of alkynes in functionalized 1,n - enynes and related 
 substrates .
inspired by the seminal work of the group of trost 
 on the electrophilic 
 activation of enynes using palladium(ii ) complexes as catalysts , which was followed by related work by murai 
 and chatani using ruthenium(ii ) and by the synthesis 
 of phenols by intramolecular reactions of furans with alkynes developed 
 by hashmi , we initially examined the 
 intramolecular reaction of allylsilanes and allylstannanes with alkynes , 
 discovering that ptcl2 was the best catalyst , although 
 palladium(ii ) , ruthenium(ii ) , and silver(i ) salts and complexes could 
 also be used .
several other groups had almost 
 concurrently found that platinum(ii ) also catalyzed the skeletal rearrangement 
 of 1,6-enynes and other cycloisomerizations under relatively mild 
 conditions .
we also found that ptcl2 was an excellent catalyst for 
 the addition of water and alcohols ( hydroxy- and alkoxycyclization ) 
 to 1,6-enynes such as 1a  d , a process 
 that proceeds stereospecifically ( scheme 1 ) .
although these reactions proceed by an exo - dig pathway , the alternative endo - dig cyclization mode was also observed in other cases . the formation of five- 
 or six - membered ring compounds 2a  c or 3 , respectively , was rationalized 
 by the competitive opening of a common intermediate 4 by attack of the nucleophile at the most electrophilic site , which 
 was dictated by the substitution of the alkene following the markovnikov 
 rule , a hypothesis that was supported by dft calculations ( scheme 2 ) .
intermediates 4 can be described as cyclopropyl platinum(ii ) carbenes , although 
 the contribution of a charge - separated resonance form corresponding 
 to metal stabilized homoallyl carbocations is also significant .
it 
 is important to note that the involvement of very similar reactive 
 intermediates was already suggested in the palladium(ii)- and ruthenium(ii)-catalyzed reactions 
 of 1,6-enynes . in a parallel work
, 
 we also examined the synthesis of phenols by 
 cyclization of furans with alkynes using platinum(ii ) catalysts instead of gold(iii ) , as originally reported by hashmi , concluding that this transformation is mechanistically 
 related to metal - catalyzed cycloisomerization of enynes in which the 
 furan ring acts as an electron - rich alkene ( scheme 3 ) .
according to the dft studies performed for the platinum(ii)-catalyzed 
 reaction , the attack of the furan to the -alkyne 
 metal complex 5 leads to a cyclopropyl metal carbene 6 , which opens up to form 7 .
cyclization to 8 , followed by elimination of the metal generates oxepine 9 , in tautomeric equilibrium with arene oxide 10 , which leads to phenols 11 .
although almost all the examples reported concerned with the intramolecular reaction of 
 furanynes , we recently developed an intermolecular version using cationic 
 gold(i ) catalysts .
during our initial studies focused on 
 the use of platinum(ii ) complexes as 
 catalysts , we also found that highly lewis acidic aucl3 could be used as a catalyst in some cases .
however , we soon realized that cationic phosphine gold(i ) complexes 
 were more convenient catalysts , actually being the most active and 
 selective for reactions of alkynes , outperforming platinum(ii ) and 
 all the other metal catalysts that we had examined for the addition 
 of heteronucleophiles , cycloisomerizations of 1,6-enynes , and similar 
 transformations .
first , we generated in situ the 
 reactive cationic gold(i ) complexes by protonolysis of neutral complex 
 ph3paume , which leads to the formation of methane .
however , this method introduces a strong brnsted 
 acid into the reaction medium , which may promote unwanted side reactions . 

more convenient proved to be the use of ph3paucl and a 
 silver(i ) salt with a noncoordinating anion such as agsbf6 .
recently , we found that the mode of activation 
 of the laucl precatalysts with silver(i ) salts has an important effect 
 in catalysis .
thus , if complexes laucl 
 are allowed to react with agx in poorly coordinating solvents and 
 in the absence of the substrate , dinuclear chloride - bridged species 
 [ lauclaul]x are readily formed , regardless 
 of the counterion .
these very robust complexes are poor catalysts , 
 so that their formation significantly reduces the reaction rate .
the 
 formation of [ lauclaul]x can be minimized 
 by premixing laucl with the substrate , followed by addition of silver 
 salts , allowing formation of complexes [ lau(substrate)]x , which can directly enter the catalytic cycles . 
 of course , the formation of dinuclear chloride - bridged complexes can 
 be entirely circumvented by the use of silver - free , cationic [ laul]x , where l is a weakly coordinating 
 ligand , which are often the alternative of choice in gold(i ) catalysis . along with our initial study in 2004 on the cycloisomerization 
 of 1,6-enynes such as 1e to form 1,3-diene 12a by skeletal rearrangement , the groups 
 of frstner and toste reported related examples of cycloisomerization 
 of 1,5-enynes of type 13a , b to give bicyclo[3.1.0]hex-2-ene 
 derivatives 14a , b , respectively , under almost 
 identical conditions using ph3paucl and a silver salt ( scheme 4 ) .
a gold(i)-catalyzed conia  ene reaction 
 of -ketoesters with alkynes was also reported by toste . in general ,
cationic gold(i ) complexes activate selectively 
 alkynes even in the presence of other potentially 
 coordinating functional 
 groups as alkenes .
it is important to 
 remark that , unlike other transition metals , gold(i ) does not promote 
 alder  ene cycloisomerizations of 1,n - enynes , 
 which require the unfavorable coordination of gold(i ) to both the 
 alkyne and the alkene .
the oxidative cyclometalation of gold(i ) to 
 form a gold(iii ) metallacycle , is also a very unlikely process . in the case of 1,6-enynes ,
gold(i ) forms ( -alkyne)metal 
 complexes 15(40 ) that react 
 intramolecularly with the alkene to form cyclopropyl gold(i ) carbene - like 
 intermediates 16 and/or 17 by 5-exo - dig or 6-endo - dig cyclization , respectively ( scheme 5 ) .
similar pathways are followed 
 by 1,5- , 1,7- , and higher enynes in the presence of gold(i ) catalysts . according to dft calculations , 
 intermediates 16 and 17 are even more delocalized 
 cationic structures than their pt(ii ) counterparts ,
although for convenience , 
 we prefer to represent these complexes as cyclopropyl gold(i ) carbenes 
 to highlight their propensity to undergo cyclopropanation reactions . 

however , although the back - donation from gold(i ) to the cationic centers 
 in these metal carbene - like structures is low , it still provides 
 sufficient stabilization as has recently been 
 shown in a few well - characterized gold carbenes [ lau = cr2 ] and [ l2au = cr2 ] .
the bond between au and c in these gold(i ) 
 carbenes has been described as a half - double bond .
terminologically , we recommend using the term gold carbenes 
 rather than gold carbenoids for these species since genuine gold(i ) carbenoids lau  ch2x 
 ( x = cl , sph ) have been structurally characterized .
these neutral complexes can be considered as formal precursors of 
 gold methylidene complex [ lau = ch2 ] by 
 an -elimination of the x group .
intermediates 17 usually lead to 
 bicyclo[4.1.0]hept-2-ene 
 derivatives 18 by -proton elimination ( scheme 5 ) . on the other hand , intermediates 16 of 
 5-exo - dig cyclization afford 1,3-dienes 19 by a skeletal rearrangement ( single - cleavage ) in an interesting 
 process that gives rise directly the -coordinated 
 gold(i ) diene complex and involves a 1,3-suprafacial migration of 
 the terminal carbon of the alkene via transition state ts1619 .
intermediates 16 can also undergo a slightly 
 different migration ( double - cleavage rearrangement ) by the formal 
 insertion of the terminal alkene carbon into the alkyne carbons that 
 forms a new gold(i ) carbene 20 .
compounds 21 ( r = h ) with z configuration usually predominate , although both e- and z - configured products have been observed .
the scenario shown in scheme 5 is a 
 simplification , 
 as other similar pathways may also be possible depending on the substitution 
 pattern of the substrate .
thus , intermediates 17 of endo - dig cyclization can undergo ring expansion to give 
 ( -cyclobutene)gold(i ) complexes 22 , which can evolve to form bicyclo[3.2.0]hept-2-ene derivatives 23 ( scheme 6 ) .
intermediates 22 can also open up to form 24 and then dienes 19 in an overall single - cleavage process .
isomeric 1,6-enynes 1f and 1 g , which 
 differ in the substitution pattern of the alkyne , react with gold(i ) 
 to form the same 1,3-diene 12b by two different mechanisms , 
 single- and double - cleavage rearrangement , respectively ( scheme 7 ) .
other 1,6-enynes bearing terminal alkynes such as 1d and 1h give 1,3-dienes 12c , d by single - cleavage rearrangement .
l , with strongly electron - donating 
 substituents at the terminal alkene carbon , react with cationic gold(i ) 
 catalyst 25a to give z - configured dienes 12f  h ( scheme 7 ) .
this z - selectivity 
 was also found using different electrophilic gold(i ) or platinum(ii ) 
 catalysts .
this is rather mechanistically puzzling , since both the 
 platinum(ii)- and the gold(i)-catalyzed single - cleavage rearrangements 
 are in the vast majority of cases stereospecific processes in which 
 the configuration of the alkene is retained . complexes 
 of gold(i ) bearing commercially available bulky dialkyl biphenyl phosphines 
 such as 25a , proved to be 
 particularly useful in homogeneous gold(i ) catalysis .
experimental 
 and theoretical studies confirmed that gold(i ) does not interact significantly 
 with the closest arene ring of the ligand .
we examined the intermolecular reaction 
 between alkynes and electron - rich 
 alkenes with many different gold(i ) catalysts , although only complex 
 mixtures of oligomers were obtained .
in addition , electron - rich alkenes 
 are very good ligands for gold(i ) , thus 
 competing with the alkyne for the coordination to the metal . however , 
 by using cationic gold(i ) complex 25b with a very bulky 
 phosphine , cyclobutenes 26 were finally obtained regioselectively 
 for a variety of electron - rich alkenes ( scheme 8) .
interestingly , the isolated yields 
 were improved in most cases by changing the anion from sbf6 to softer bar4 , presumably by minimizing the formation of unproductive  , 
 -digold(i ) alkyne digold(i ) complexes , an undesired side reaction 
 in many gold - catalyzed reactions of terminal alkynes .
the same improvement was observed in the intramolecular 
 [ 2 + 2 ] cycloaddition of alkynes with alkynes in substrates of type 27 to form macrocycle 28 ( scheme 9 ) .
the regioselective 
 formation of cyclobutenes by [ 2 + 2 ] cycloaddition 
 probably proceeds by electrophilic addition of the ( -alkyne)gold(i ) complex to the alkene via ts2930 to form a cyclopropyl gold(i ) 
 carbene 30 very similar to intermediate 16 involved in the cyclization of enynes ( scheme 10 ) .
intermediate 30 was also trapped intramolecularly 
 with an alkene to form the corresponding cyclopropane .
the intermolecular reaction of propiolic acid and related 
 alkynes 
 with strongly electron - withdrawing groups gives dihydropyrones 32 or 1,3-dienes 33 via a similar mechanism , 
 although the initial attack of the alkene to the ( -alkyne)gold(i ) complex occurs with the opposite regiochemistry ( scheme 10 ) .
thus , cyclopropyl 
 gold(i ) carbene intermediate 34 , with gold bonded to 
 the internal carbon of the alkyne , evolves by intramolecular opening 
 to form 32 by attack of the carboxylic acid to the most 
 substituted carbon of the alkene . on the other hand , in the reaction 
 of alkenes with two very similar substituents , intermediate 34 undergoes a 1,3-migration similar to that found in the 
 single - cleavage rearrangement to form stereospecifically 1,3-dienes 33 . a transition state similar to ts3433 has been proposed in the reaction 
 of cyclopropyl gold(i ) carbenes generated by retro - buchner reaction 
 of 7-cyclopropyl 1,3,5-cycloheptatrienes .
gold(i ) 
 complexes are able to catalyze the addition of amines , alcohols , or 
 water to 1,n - enynes affording products of amino- , 
 alkoxy- , or hydroxycyclization under much milder conditions than other 
 metal catalysts .
these additions are stereospecific , as illustrated by the alkoxycyclization 
 of diastereomeric enynes 1 m and 1h , and 
 proceed via opening of cyclopropyl gold(i ) carbene intermediates 35 by attack of the nucleophile to the cyclopropane ring ( scheme 11 ) .
gold(i ) complex and the heteronucleophile 
 to an alkene following the markovnikov regiochemistry , which is further 
 demonstrated by the reaction of enyne 1d with gold(i ) 
 in the presence of meoh to form six - membered ring 3 via 
 intermediate 36 .
1,6-enynes such as 1n bearing an aryl substituent 
 at the alkyne terminus undergo an analogous 5-exo methoxycyclization to afford 2f in the presence of 
 gold(i ) and meoh ( scheme 12 )
. however , similar 1,6-enyne 1o tethered by 
 a benzene ring undergoes gold(i)-catalyzed hydroxy- or alkoxycyclizations 
 preferentially via 6-endo - dig pathway 
 through intermediates 37 giving rise to dihydronaphthalenes 38 .
amino- and alkoxycyclizations can also take place intramolecularly 
 starting from amino- or hydroxy-1,n - enynes .
thus , 1,6-enyne 1p containing a propargylic alcohol gave 40 quantitatively 
 upon treatment with gold(i ) as a result of an intramolecular attack 
 of the hydroxyl group to cyclopropyl gold carbene intermediate 39 ( scheme 13 ) . in a mechanistically related transformation , amino- or 
 hydroxy-1,5-enynes 41 afforded in the presence of gold(i ) 
 spirofused heterobicyclic compounds 43 through intermediates 
 of type 42 . in a similar 
 vein ,
1,n - enynes bearing nucleophilic moieties such 
 as carboxylic acids , carbamates , or carbonates are also prone 
 to undergo gold - catalyzed tandem cyclizations , giving rise to a variety 
 of complex molecular architectures .
oxo-1,6-enynes 44 also react in 
 the presence of gold(i ) 
 by a formal [ 2 + 2 + 2 ] alkyne / alkene / carbonyl cycloaddition to afford 
 predominantly oxatricyclic compounds 48 , together with 
 minor amounts of dienes 49 ( scheme 14 ) .
these reactions proceed via 
 intramolecular attack of the carbonyl to cyclopropyl gold carbenes 45 to form oxonium cations 46 , which undergo 
 a prins - type cyclization to give intermediates 47 .
demetalation 
 from 47 forms oxatricycles 48 , whereas an 
 alternative elimination with fragmentation of the seven - membered ring 
 leads to dienes 49 .
we exploited gold(i)-catalyzed 
 intramolecular [ 2 + 2 + 2 ] alkyne / alkene / carbonyl 
 cycloadditions in the synthesis of several oxygen - bridged sesquiterpenoids . 

thus , ketoenynes 44d and 44e were respectively 
 converted into oxatricycles 48d and 48e , 
 which are key intermediates in the total syntheses of ( + ) -orientalol 
 f ( 50 ) and pubinernoid b ( 51 ) ( scheme 15 ) .
oxo-1,6-enyne 44f was analogously cyclized to form 48f , which 
 was subsequently converted into ( )-englerin a ( 52 ) . another total synthesis of 52 included a very similar gold(i)-catalyzed cyclization as 
 the key step .
remarkably , an unprotected 
 aldol subunit could be used in both syntheses as the substrate for 
 the gold - catalyzed reaction .
1,n - enynes also react intermolecularly with carbonyl 
 compounds giving rise to a wide range of products depending on the 
 substitution pattern of the alkene and the nature of the carbonyl 
 compound .
1,6-enynes such as 1q react with aromatic aldehydes 
 in a formal [ 2 + 2 + 2 ] cycloaddition to give oxabicyclic adducts 
 of type 55 together with dienes 56 , which 
 result from a metathesis - type reaction of the enyne with the aldehyde 
 ( scheme 16 ) .
the 
 formation of these products can be explained by attack of the aldehyde 
 to the cyclopropyl gold carbene intermediate to form 53a , which undergoes a prins - type cyclization followed by either demetalation 
 to form 55 or fragmentation to give 56 . 

this mechanism is analogous to that proposed for the intramolecular 
 [ 2 + 2 + 2 ] cycloaddition of oxo-1,6-enynes ( scheme 14 ) .
1,7-enynes also undergo [ 2 
 + 2 + 2 ] cycloadditions with carbonyl compounds to give analogous 
 products . on the other hand , following 
 a related mechanism , 1,6-enyne 1q reacts with cyclopropenones 
 in a ring - expanding spiroannulation to afford spirocyclic cyclopentenones 58 after incorporation of a molecule of water to rearranged 
 intermediate 57 .
1,6-enynes 1r bearing a monosubstituted alkene react with aldehydes and 
 ketones in a different way to form tricyclic compounds 61 .
this transformation presumably proceeds 
 via trapping of the rearranged carbene that results from the enyne 
 to form oxonium cation 59 , followed by prins cyclization 
 and demetalation .
the intermolecular 
 reaction of terminal alkynes with oxoalkenes 62 leads 
 to 8-oxabicyclo[3.2.1]oct-3-enes 64 as 
 a result of a formal [ 2 + 2 + 2 ] alkyne / alkene / carbonyl cycloaddition 
 via cyclopropyl gold carbene intermediates 63 ( scheme 17 ) .
electron - rich 
 aromatic and heteroaromatic compounds can act as nucleophiles reacting 
 with 1,6-enynes .
cyclopropyl gold(i ) carbenes that result as intermediates in the 
 cyclization of 1,6-enynes can act as bifunctional electrophiles , reacting 
 with arenes either at the carbene or at the cyclopropane .
hence , indole reacted with 1,6-enyne 1s in the presence of different gold(i ) complexes to give 
 two different adducts 66 and 67 ( scheme 18 ) .
we found that the more electrophilic gold(i ) 
 complexes with less donating phosphite or phosphine ligands favored 
 the attack to the cyclopropane ring to give 66 , whereas 
 complexes with more donating nhc ligands favored the formation of 67 by attack at the metal carbene .
an analogous reaction in 
 the presence of a gold(i ) catalyst generated in situ from ph3paucl / agsbf6 was reported to give exclusive formation 
 of products of type 66 by attack to the cyclopropane 
 ring .
1,3-dicarbonyl compounds can also add to 1,6-enynes such 
 as 1s acting as carbonucleophiles through their enol 
 tautomer 
 to give compounds 68 ( scheme 19 ) .
however , some dicarbonyl compounds 
 such as 1,3-cyclohexadione may act as o - nucleophiles 
 to form adduct 69 . as in the addition of heteronucleophiles 
 to 1,6-enynes ,
products derived from the attack at the carbene carbon 
 are favored with more donating ligands , whereas more electrophilic 
 complexes favor the attack to the cyclopropane .
gold(i ) also catalyzes 
 the addition of allyl silanes to 1,6-enynes , as well as the addition 
 of a variety of carbonucleophiles to 1,5-enynes .
intramolecular attack of aryl 
 groups in 1,6-enynes of type 1 t leads stereospecifically 
 to tricyclic products such as 74 of intramolecular formal 
 [ 4 + 2 ] cycloaddition ( scheme 20 ) .
this is a very general reaction 
 that proceeds via initial exo - cyclization of the 
 enyne followed by opening of the 
 cyclopropyl gold(i ) carbene 70 by a friedel  crafts - type 
 reaction .
these cycloadditions can be performed enantioselectively 
 in the presence of chiral phosphine or 
 phosphite gold(i ) complexes .
the endo - cyclization of the 1,6-enyne also takes place in certain 
 cases , being the preferred pathway for arylalkynes bearing enesulfonamides 
 or enamines such as 72 to afford tricycles 74 .
benzyl - substituted 1,5-enynes such as 13c also undergo 
 a formal [ 4 + 2 ] cycloaddition via 5-endo - dig cyclization to afford tricyclic product 75 .
in contrast , 1,5-enynes 13d with an aryl substituent at the alkyne react with gold(i ) through 
 a different mechanism to form dihydrobenzofluorenes 78 in a formal [ 3 + 3 ] cycloaddition .
this transformation can be explained by a 1,2-h shift in intermediate 77 , followed by a friedel  crafts alkylation ( scheme 21 ) .
the carbene - like character of the intermediates formed in metal - catalyzed 
 cycloisomerizations of enynes is more clearly exhibited in intra- 
 and intermolecular cyclopropanations of alkenes .
formation of tetracyclic compounds 81a , b by cyclization of dienynes 1u , v at room 
 temperature nicely illustrates the high reactivity and stereoselectivity 
 usually exerted by gold catalysts ( scheme 22 ) .
this reaction , which was first reported 
 using ruthenium(ii ) complexes under more forcing conditions ( 80 c , 
 toluene ) , proceeds by a 5-exo - dig cyclization followed by cyclopropanation of 
 the resulting cyclopropyl gold(i ) carbene 80 . when the 
 starting dienynes are cyclic substrates these intramolecular cyclopropanations 
 lead to very complex ring systems .
1,5-dienynes 
 such as 13e react similarly to afford pentacyclic product 83 through endo - carbene 82 .
dienynes bearing a methoxy or other or group at the propargylic 
 position such as 87a undergo an intramolecular 1,5-or 
 migration in the presence of gold(i ) to form tricyclic compounds 88a ( scheme 23 ) .
this reaction presumably takes place via cyclopropyl gold(i ) 
 carbene intermediate 85 , which leads to bridged system 86 by attack of the or group at the cyclopropane .
opening 
 of 86 results in the formation of ,-unsaturated 
 gold(i ) carbene 87 , which is prone to undergo intramolecular 
 cyclopropanation with the alkene at the side chain to form 88a .
this cascade cyclization process 
 is ideally suited for the total 
 syntheses of sesquiterpenes ( )-4,7-aromadendranediol 
 ( 92 ) , ( )-4,7-aromadendranediol 
 ( 93 ) , and ( )-epiglobulol ( 94 ) from 
 a single dienyne 84b by using a stereodivergent gold(i)-catalyzed 
 reaction as the key step ( scheme 24 ) . in the absence of any external nucleophile , 
 cyclopropyl gold(i )
carbene intermediate 89 undergoes 
 a 1,5-obn migration followed by intramolecular cyclopropanation to 
 form tricyclic compound 88b , which is the precursor of 93 and 94 . on the other hand , by adding an external 
 nucleophile such as allyl alcohol , intermediate 89 leads 
 to 91 and then to 88c after intramolecular 
 cyclopropanation .
intramolecular cyclopropanation 
 of a gold(i ) carbene can lead to 
 very complex structures starting from 7-alkynylcyclohepta-1,3,5-trienes 95 ( scheme 25 ) .
this reaction generates highly fluxional barbaralyl gold(i ) 
 cations 96 that can be trapped by the pending alkene 
 to form 97a , b , which are in equilibrium 
 by cope rearrangement both in the solid state and in solution .
intermediate cyclopropyl gold(i ) 
 carbenes such as 65 resulting from the cyclization of 
 1,6-enynes can also be trapped 
 intermolecularly by cyclic or acyclic alkenes to afford adducts 98 stereoselectively ( scheme 26 ) .
1,6-enynes such as 1d bearing a terminal alkene react 
 differently yielding 100 , probably by rearrangement of 
 cyclopropyl gold(i ) carbene 36 to give carbene 99 .
the involvement of carbene 99 in the trapping 
 by the alkene also supports the involvement of this type of species 
 as intermediates in the double - cleavage rearrangement ( see scheme 5 ) . in the presence of gold(i ) , 
 1,6-enyne 84c undergoes 
 a cyclization followed by 1,5-acetoxy migration to give ,-unsaturated 
 carbene 103 , which reacts intermolecularly with alkene 104 giving rise to 105 with only 5% loss of enantiomeric 
 excess ( scheme 27 ) .
this transformation has been used as the key step in the total synthesis 
 of antiviral sesquiterpene ( + ) -schisanwilsonene a ( 106 ) .
it is remarkable that the cyclization/1,5-acetoxy migration is 
 faster than the alternative 1,2-acyloxy migration , which would lead 
 to racemization .
gold(i ) catalysts trigger complex reactions of substituted 1,n - enynes by stabilizing the key reactive cationic intermediates , 
 which can be viewed in a simplified form as cyclopropyl gold(i ) carbenes . 

these intermediates react with diverse nucleophiles inter- or intramolecularly 
 and cyclopropanate alkenes leading to complex structures with total 
 atom economy .
this work , along with that of many other groups , has 
 promoted gold from a mere curiosity to the metal of choice for the 
 activation of alkynes in complex molecular settings under homogeneous 
 conditions .
the same basic principles uncovered during the mechanistic 
 study of gold(i)-catalyzed reactions of 1,n - enynes 
 can also been extended for reactions of more complex systems as well 
 as for the development of more challenging intermolecular transformations 
 between alkynes and alkenes . in this regard , in order to fulfill the 
 synthetic potential of intermolecular transformations , new types of gold(i ) 
 catalysts that selectively activate alkynes in reactions with highly functionalized alkenes are still required , particularly in the 
 arena of enantioselective synthesis .
 cycloisomerizations 
 of 1,n - enynes catalyzed by 
 gold(i ) proceed via electrophilic species with a highly distorted 
 cyclopropyl gold(i ) carbene - like structure , which can react with different 
 nucleophiles to form a wide variety of products by attack at the cyclopropane 
 or the carbene carbons . particularly important are reactions in which 
 the gold(i ) carbene reacts with alkenes to form cyclopropanes either 
 intra- or intermolecularly . in the absence of nucleophiles , 
 1,n - enynes lead to a variety of cycloisomerized products including 
 those resulting from skeletal rearrangements . 
 reactions proceeding 
 through cyclopropyl gold(i ) carbene - like intermediates are ideally 
 suited for the bioinspired synthesis of terpenoid natural products 
 by the selective activation of the alkyne in highly functionalized 
 enynes or polyenynes . 
Introduction
Gold(I)-Catalyzed Cyclizations of 1,
Gold(I)-Catalyzed Intermolecular Reactions of
Alkynes with Alkenes
Gold(I)-Catalyzed Nucleophilic Additions to
Enynes
Cyclopropanation Reactions
Concluding Remarks