Formation of cyclopropanes in γ-eliminations of sulphoxides

Abstract

A number of 2-alkyl-1-methylsulphinyl-3-phenylpropanes (alkyl = H, Me, Et, Prⁱ, or Bu^t) have been prepared; their decomposition has been studied in the systems potassium amide–triethylamine and sodium methylsulphinyl-methanide–dimethyl sulphoxide. 1,3-Elimination produced exclusively the trans-1-alkyl-2-phenylcyclopropane and 1,2-elimination gave mainly the trans-β-alkyl-β-methylstyrene by isomerisation of the initially formed 2-alkyl-3-phenylpropene. The relative amounts of substituted cyclopropane increased through the series H < Me < Et < Prⁱ < Bu^t, and for each substrate more cyclopropane was formed in dimethyl sulphoxide than in triethylamine. The increasing proportions of 1,3-elimination are discussed in terms of an entropy effect, and the decreasing amounts of 1,2-elimination is in terms of the steric interactions in the transition state for a concerted elimination. That the 1,3-elimination is favoured in dimethyl sulphoxide is explained in terms of the different natures of the two solvents, and their effect on an E1cB 1,3-elimination and a concerted 1,2-elimination. 2,2-Dimethyl-1-methylsulphinyl-3-phenylpropane, 1-methylsulphinyl-2,3-diphenylpropane, and 1-methylsulphinyl-3,3-diphenylpropane have been prepared, and their decomposition in triethylamine and dimethyl sulphoxide has been studied.