Cycloaddition reactions of unsaturated sulfones

Abstract

The reaction of a series of allyl-substituted bis(phenylsulfonyl)-methanes or dimethyl malonates with 2,3-bis(phenylsulfonyl)-1,3-butadiene in the presence of base afforded alkenyl-substituted allenes in good yield. The reaction proceeds by initial attack of the soft carbanion onto the terminal position of the diene and subsequent PhSO2- elimination to give the phenylsulfonyl substituted allene. The thermal reactions of these phenylsulfonyl allenes gave [2+2]-cycloadducts. Only the C1-C2 double bond of the allene participates in the [2+2]-cycloaddition. Stepwise bonding prefers to occur in a 1,6-exo manner rather than in a 1,7-endo fashion. The formation of all products can be rationalized by a mechanism which includes an initial carbon-carbon bond formation involving the central allene carbon to give a diradical intermediate. The product distribution is then determined by the substitution pattern of the alkene and the fate of the diradical intermediate.