Stereochemical aspects of the Pummerer reaction. Regioselectivity as a criterion for the differentiation of ylide and E2 pathways in the product-determining step of the reactions of benzyl methyl halo- and oxysulfonium cations

Abstract

The conversion of [Formula: see text] (X = Cl, OCH3, OCOCH3, OCOCHCl2, OCOCF3) to a regioisomeric mixture of p-Y-C6H4CHXSCH3 and p-Y-C6H4CH2SCH2X has been studied as a function of X, Y, and deuteration in the benzyl and methyl positions. The conclusion is reached that, when X is acyloxy, the transformation of the sulfonium cation to an α-thiocarbonium ion and thence to the products proceeds via an E2 pathway. Trifluoroacetic anhydride is a much more reactive reagent than acetic anhydride for Pummerer reactions of benzyl methyl sulfoxides. However, with other sulfoxides, this reagent is not as effective as acetic anhydride. Acetyl trifluoroacetate is not effective as a Pummerer reagent.