Mechanistic aspects of the formation of chiral allenes from propargylic ethers and organocopper reagents

Abstract

Propargylic ethers react with organocopper reagents to afford allenes by a syn addition to the triple bond followed by a β-elimination of the resulting alkenyl copper species. With use of chiral propargylic ethers and stoichiometric organocopper reagent, it was shown that the β-elimination step is purely anti, resulting in the formation of a chiral allene with 96% optical yield. The same reaction, run with a Grignard reagent RMgX and a catalytic amount of a Cu I salt, affords allenes through an anti or syn overall process. The crucial step is the β-elimination of the intermediate alkenyl organometallic species, which is of anti type with RMgI and of syn type whith RMgCl. Propargylic acetates, wich also afford allenes in this reaction, but through a Cu III intermediate, are not sensitive to this «halogen effect»