Aryl vinyl sulfides as probes for electrophilic versus electron transfer mechanisms

Abstract

The distinction between electrophilic and electron transfer mechanisms of addition reactions to the vinyl double bond of aryl vinyl sulfides, ethers and selenides has been achieved by studying substituent effects. Specifically, the effects of meta and para substituents on the rates of electrophilic addition reactions correlate with Hammett σ values, while ionization of the substrates to the corresponding radical cations correlates with σb+. The significance of the respective correlations are confirmed by statistical tests. The σ (electrophilic)/σb+ (ET) dichotomy is in accord with the conventional paradigm for σ/σb+ correlations and is further supported by ab initio SCF MO calculations. The application of this criterion to the reactions of aryl vinyl sulfides and ethers with tetracyanoethylene indicates that formation of cyclobutanes occur via direct electrophilic addition to the electron-rich alkene and not via an electron transfer mechanism. This conclusion is further supported by the results of previous radical cation probe studies.