MECHANISTIC ASPECTS ON THE FORMATION OF CHIRAL ALLENES FROM PROPARGYLIC ETHERS AND ORGANOCOPPER REAGENTS

Propargylic ethers react with organocopper reagents to afford alienes 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 aliene with 96% optical yield. The same reaction, run with a Grignard reagent RMgX and a catalytic amount of a Cu1 salt, affords alienes 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 RMgl and of syn type with RMgCl. Propargylic acetates, which also afford alienes in this reaction, but through a CuIII intermediate, are not sensitive to this “halogen effect”. © 1990, American Chemical Society. All rights reserved.