Stereochemical aspects on the formation of chiral allenes from propargylic ethers and epoxides

Abstract

Chiral propargylic ethers react with organocopper reagents to afford optically active allenes by a syn addition to the triple bond followed by an anti β-elimination of the resulting alkenyl copper species. However, the same reaction, run with a Grignard reagent and a catalytic amount of copper (I) salt affords allenes through an anti or syn process. The crucial step is the β-elimination of the intermediate alkenyl metal species, which is of anti type with RMgI and of syn type with RMgCl. Propargylic acetates, which also afford allenes in this reaction, but through a Cu(III) intermediate, are not sensitive to this “halogen effect”. By close analogy to ethers, propargylic epoxides react with Grignard reagents and catalytic amount of Cu(I) salt, leading to a-allenic alcohols. The reaction is highly diastereoselective and its stereochemical outcome can be fully controlled.The syn diastereomer, probably arising through an addition-elimination mechanism, is better obtained with RMgCl and copper (I) bromide, whereas the anti diastereomer is best obtained with RMgBr and a complexed copper (I) salt. © 1992 IUPAC