Transition-metal-catalyzed
cascade sequences combine the novelty and power of organometallic chemistry
with the synthetic utility and economy of cascade reactions, providing an even
more ecologically and economically desirable approach to organic synthesis.
For instance, rhodium catalysis was used to convert acyclic monoterpenes of the
type 59 to 4H-chromen products in a hydroformylation cascade (Scheme 12). First, selective rhodium-catalyzed
hydroformylation of the less sterically hindered olefin bond in 59 yielded unsaturated aldehyde 60, which under the same conditions was
then converted to intermediate 61
via a carbonyl-ene reaction. A second rhodium-catalyzed hydroformylation to
species 62 was followed by
condensation to form 4H-chromen
products of the type 63 in 40%
overall yield. 
Rhodium catalysis was also employed to initiate a cyclization/cycloaddition cascade in
the synthesis of a tigliane reported by the Dauben group (Scheme 13). Treatment of diazoimide 64 with rhodium(II) acetate dimer generated
a carbenoid that yielded reactive ylide 65
after an intramolecular cyclization with the neighboring carbonyl group. An
intramolecular [3+2] cycloaddition then spontaneously occurred to afford the
target tigliane 66.
The formal intramolecular [4+2]
cycloaddition of 1,6-enynes of the type 67
mediated by gold catalysis is another example of a transition-metal-catalyzed
cascade (Scheme 14). A variety of
1,6-enynes reacted under mild conditions in the presence of Au(I) complexes 68a–b to yield the tricyclic products 69 in moderate to excellent yields.
This formal cycloaddition was proposed
to proceed via the cascade process shown in Scheme 15. Complexation of the 1,6-enyne
67 with the cationic form of the
catalyst yields intermediate 70, in
which the activated triple bond is attacked by the olefin functionality to
yield substituted cyclopropane 71. Electrophilic
opening of the three-membered ring forms cationic species 72, which undergoes a Friedel-Crafts-type reaction and then
rearomatizes to give tricyclic product 69. Due to the nature of
the interaction of gold complexes with unsaturated systems, this process could
also be considered an electrophilic cascade.
An example of palladium-catalyzed
cascades is represented by the asymmetric polyene Heck cyclization used in the preparation
of (+)-xestoquinone from triflate substrate 75 (Scheme 16). Oxidative addition of the aryl–triflate bond
into the palladium(0) complex in the presence of chiral diphosphine ligand (S)-binap yields chiral palladium(II)
complex 77. This step is followed by
the dissociation of the triflate anion, association of the neighboring olefin
and 1,2-insertion of the naphthyl group into the olefin to yield intermediate 79. A second migratory insertion into
the remaining olefin group followed by a β-elimination
then occurs to afford product 81 in
82% overall yield and with moderate enantioselectivity. The palladium(0)
catalyst is also regenerated in this step, thus allowing the cascade to be reinitiated.