Reactivity of vinyl sulphonic esters. Part V. Stereochemistry and rearrangements in SN1 reactions of β-thiovinyl trinitrobenzenesulphonates

Abstract

The S_N1 methanolysis of 2-phenylthio-1,2-di-p-tolylvinyl 2,4,6-trinitrobenzenesulphonates in 4:1 acetone–methanol at 25 °C is at least 97% stereospecific. ‘Retention’ of configuration is probably involved. The pair of structural isomers 2-phenyl-2-phenylthio-1-p-tolylvinyl and 1-phenyl-2-phenylthio-2-p-tolylvinyl trinitrobenzene-sulphonates was prepared in mixtures of different composition (from 3:1 to 1:2). The methanolysis in 4:1 acetone–methanol at 25 °C of such mixtures gives the two isomers, 1-methoxy-2-phenyl-2-phenylthio-1-p-tolylethylene and 1-methoxy-1-phenyl-2-phenylthio-2-p-tolylethylene in a ratio of ca. 4·5:1. This ratio is independent on the proportion of the two isomers in the starting material. Similarly, in nitromethane at 25 °C, mixtures of the p-tolylthio-analogues give the two isomers, 6-methyl-2-phenyl-3-p-tolylbenzo[b]thiophen and 6-methyl-2-p-tolyl-3-phenylbenzo[b]thiophen in a ratio of 5:1 independently on the isomeric distribution in the reagents.

Both retention of configuration and migration of the β-arylthio-group are evidence of β-sulphur participation and indicate that a bridged structure, probably a thiirenium ion, is involved as an intermediate in an S_N1 reaction of β-arylthiovinyl sulphonic esters.