Abstract
Chiral propargylic ethers react with organocopper reagents to afford optically active allenes by a u addition to the triple bond followed by an anti p- 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 svn process. The crucial step is the p-elimination of the intermediate alkenyl metal species, which is of & type with FWa and of type with RMgCJ. Propargylic acetates, which also afford allenes in this reaction, but through a Cu(II1) intermediate, are not sensitive to this halogen effect. By close analogy to ethers, propargylic epoxides react with Grignard reagents and catalytic amount of Cu(1) salt, leading to a-allenic alcohols. The reaction is highly diastereoselective and its stereochemical outcome can be fully controlled.The diastereomer, probably arising through an addition-elimination mechanism, is better obtained with RM&l and copper (I) bromide, whereas the anti diastereomer is best obtained with RM& and a complexed copper (I) salt.
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