Allenyne reductive elimination

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)). 

Due to the higher reactivity of the allene moiety toward hydropalladation in 1,6-allenynes, the reaction may proceed via a hydropalladation of the allene moiety of 146 affording a vinylic palladium intermediate 147. Subsequent intramolecular carbopalladation of the C-C triple bond moiety would lead to the 1,3-dienyl palladium formate 148. Releasing of CO2 and reductive elimination afford the final product 149 and Pd(0). Pd(0) would react with HCO2H to afford HC02PdH, which is the catalytically active species (Scheme 61) [36]. 

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... 

Mukai et al. have found scission of a cyclopentane ring of allenyne 34 in the rhodium-catalyzed cycloisomerization (Scheme 7.11) [14]. When 34 was treated with a rhodium catalyst, the bicyclo [7.4.0]tridecatriene 37 was formed. Mechanistically, initial coordination of 34 to rhodium(I) would occur between an allenic distal double bond and an alkyne to form the intermediary ir-coordinating complex, which undergoes oxidative cyclization to form the rhodabicyclo[4.3.0]nonadiene intermediate 35. Subsequent P-carbon elimination, presumably assisted by release of the ring strain of the cyclopentane (6.3 kcal mol ), results in the formation of the 10-membered rhodacycle 36. Reductive elimination ensues to give the final product 37. 

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