Allenyne mechanism

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

The mechanism proposed for this allenic Alder-ene reaction is shown in Scheme 8.3. The rhodium] I) catalyst coordinates with the allenyne V forming intermediate VI, which undergoes an oxidative addition to form the metaUocycle VII. The metaUocycle then undergoes a /9-hydride elimination producing triene intermediate VIII, which... 

Evidence for this mechanism includes (1) an experiment showing a 4.5% nOe between and of the triene in Eq. (14), validating the -stereochemistry of the exo-cyclic olefin and (2) preparation of a hexadeuterated allenyne that was subjected to the reaction conditions which gave complete transfer of a deuterium atom to the exocyclic double bond of the triene (Eq. 15). 

Murakami et al. reported a ring-closing metathesis reaction of allenynes using Schrock s molybdenum alkylidene complex [37]. Treatment of allenynes ISl with a catalytic amount of the complex 15 2 in toluene at rt gave cyclopentene derivatives 1 S3 in good yield. Two possible reaction mechanisms were proposed, one through a vinylidene complex 154 and the other through a carbene complex, but based on several mechanistic studies, they favored the vinylidene complex pathway, which is shown here (Scheme 5.42). 

The proposed mechanism based on product observation is depicted in (Scheme 16). Regioselective insertion of the proximal double bond of allene 31 into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, HlRhJSiRs, affords intermediate D-L Subsequent allqme insertion forming the cyclic vinyl-Rh complex D-II followed by silane promoted reductive elimination provides the product 32 and the active Rh-Si complex. When allenynes that did not have substitutions on the alkyne terminus or on the distal allene double bonds were subjected to the optimized conditions, complex product mixtures were obtained. These... 

Five years ago, Houk, Toste, and coworkers [31] suggested a fascinating new reaction mechanism. The cycloisomerization of 1,5-allenynes 23 by the [(Ph3PAu)30]Bp4 catalyst delivers cross-conjugated trienes 24 (Scheme 4.8). 

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