Nucleophilic vinylic substitutions on unactivated substrates. : The behaviour of styryl alkyl sulphides and selenides towards sulphur and selenium nucleophiles

Sodium alkanethiolates or lithium methyl selenide react with styryl alkyl sulphides and selenides, in DMF at 100°C, to give the products of vinylic or aliphatic substitution. The two nucleophilic reagents are extremely selective. In the case of RSNa the attack at the vinylic carbon atom is much faster than that at the aliphatic carbon atom and the (Z)- or (E)- styryl alkyl sulphides are obtained as the result of a stereospecific vinylic substitution which occurs with retention of configuration. On the contrary, in the case of MeSeLi, under the same experimental conditions, the only reaction occurring is the aliphatic substitution which affords the vinyl thiolate anions, as an equilibrium mixture of the (E)- and (Z)- isomers, or the vinyl selenide anions which retain the configuration of the starting styryl alkyl selenides.