The behaviour of some functionally substituted α-allenic alcohol derivatives toward organocopper(I) species

Trimethylsilyl ethers of 2,3-hexadien-5-yn-1-ols (1) undergo carbocupration at the terminal triple bond, and this reaction has been used to prepare, among other species, the trimethylsilyl ethers of 8-trimethylsilyl-2,3,5-octatrien-7-yn-1-ols(6a and 7a). When the terminal triple bond of 1 is protected with a trimethylsilyl group S_N2′-like displacements take place upon addition of RCu, with predominant formation of E-dienynes in which H is present instead of R, especially when s-BuCu is used. Only when R is methyl is the R group transferred. On the other hand, smooth formation of dienynes bearing the R group are observed upon reaction of methanesulfinates with RCu; in this case, however, the configuration of the dienynes is mainly Z. A mechanistic rationalization of the results is proposed.