Competition between vinylic substitution and conjugate addition in the reactions of vinyl selenoxides and vinyl selenones with nucleophiles in DMF

Vinyl selenoxides and vinyl selenones present a different reactivity towards thiolate or alkoxide anions in DMF. In the case of selenoxides the addition of the nucleophiles regioselectively occurs at the α-carbon leading to the formation of the vinylic substitution products with complete retention of configuration. These reactions occur under very mild conditions indicating that the seleninyl group markedly enhances nucleophilic vinylic substitution rates. The results obtained with vinyl selenones are consistent with competitive nucleophilic attack at the α- and at the β-carbon. The former yields irreversibly the vinylic substitution products, whereas attack at the β-carbon leads to the reversible formation of selenonyl stabilized carbanions. The fate of these intermediates depends upon the nucleophilic reagent employed. With thiolate anions the vinyl selenones are rapidly subtracted from the equilibrium and the carbanion does not give any other product. With methoxide anions, on the contrary, the vinylic substitution is a slow process and the carbanion can give rise to conjugate addition products also. Malonate anions react only at the β-carbon of vinyl selenones and the resulting carbanions suffer proton transfer and intramolecular displacement of the selenonyl group to afford cyclopropane derivatives.