CONVENIENT CRITERION FOR THE DISTINCTION BETWEEN ELECTROPHILIC AND ELECTRON-TRANSFER REACTIONS OF ELECTRON-RICH ALKENES

Both experimental and theoretical studies confirm that the formation o f aryl vinyl ether and aryl vinyl sulfide cation radicals from the cor responding neutral substrates correlates with the Brown sigma(+) param eters as opposed to Hammett sigma values. Peak oxidation potentials fo r both classes of substrates correlate preferentially with sigma(+), a s do gas-phase ionization energies calculated by both semi-empirical a nd ab initio methods. In contrast, the protonation energies of the sam e substrates, which relate to carbocation formation, correlate prefere ntially with sigma values, as do rates of protonation and other electr ophilic additions. These observations permit a sharp distinction betwe en electrophilic and electron transfer reactions of these two common c lasses of electron-rich substrates. Using this criterion, the cycloadd itions of tetracyanoethylene to these substrates are found to proceed via an electrophilic mechanism, rather than by a previously proposed e lectron transfer mechanism. (C) 1998 John Wiley & Sons, Ltd.