Allenic copper intermediate reductive elimination

Similar models explain the 1,8-, 1,10- and 1,12-addition reactions to the extended Michael acceptors 91, 93 and 95, respectively (Schemes 2.32 and 2.33). Again, these transformations start with the formation of a cuprate Jt-complex at the double bond neighbouring the acceptor group [61a]. Subsequently, an equilibrating mixture of a-copper(III) intermediates is presumably formed and the regioselectivity of the reaction may then be governed by the different relative rates of the reductive elimination step of these intermediates. Consequently, the exclusive formation of allenic prod-... 

These additives also serve to suppress the formation of a common side-product, that is, an allene containing a hydrogen atom instead of the carbon substituent which should have been delivered by the cuprate. The occurrence of such reduction products is also in accordance with the generally accepted mechanistic model (Scheme 44), in which the copper(m) intermediate 186 resulting from the epoxide 185 may be sufficiently stable to survive until work-up of the reaction mixture (or undergo reductive elimination of R-R to give an allenic copper(i) compound), so that protonation leads to the reduction product 188, besides the desired substitution product 187 (Scheme 45).65,74... 

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