Convenient criterion for the distinction between electrophilic and electron transfer reactions of electron-rich alkenes

Abstract

Both experimental and theoretical studies confirm that the formation of aryl vinyl ether and aryl vinyl sulfide cation radicals from the corresponding neutral substrates correlates with the Brown ' parameters as opposed to Hammett ' values. Peak oxidation potentials for both classes of substrates correlate preferentially with ' , as do gas-phase ionization energies calculated by both semi-empirical and ab initio methods. In contrast, the protonation energies of the same substrates, which relate to carbocation formation, correlate preferentially with ' values, as do rates of protonation and other electrophilic additions. These observations permit a sharp distinction between electrophilic and electron transfer reactions of these two common classes of electron-rich substrates. Using this criterion, the cycloadditions of tetracyanoethylene to these substrates are found to proceed via an electrophilic mechanism, rather than by a previously proposed electron transfer mechanism.

Accurate Oxidation Potentials of Benzene and Biphenyl Derivatives via Electron-Transfer Equilibria and Transient Kinetics

“…This provided an opportunity to determine if the oxidation potentials could be correlated with Hammett-type substituent constants. Prior correlations between oxidation potentials and Hammett/Brown σ þ values 23 have been observed for a variety of electron donors, including triarylamines, 24 aryl sulfides, selenides and tellurides, 25 vinyl ethers and thioethers, 26 styrenes, 27 and arylmethyl radicals. 28 As shown in Figure 10, there is an excellent correlation between E ox and the sum of the σ þ values for the biphenyl derivatives, which should be helpful in estimating oxidation potentials of other biphenyl derivatives.…”

A Non-Outer Sphere Mechanism for the Ionization of Aryl Vinyl Sulfides by Triarylaminium Salts

“…Extensive experimental and theoretical studies have previously established that substituent effects upon the ionization of meta- and para-substituted aryl vinyl sulfides and ethers in solution and in the gas phase preferentially correlate with the Brown σ + parameters as opposed to Hammett σ values (Scheme ) …”

The development of strong covalent interactions in inner sphere electron transfer reactions between cation radicals and neutral molecules

“…Experimental studies have previously demonstrated that the one electron oxidation of m-and p-substituted aryl propenyl ethers and also aryl vinyl sulfides to the corresponding cation radicals correlates extremely well with the Brown σ ϩ parameters. 10, 12 The correlation with Hammett σ values is far inferior and the preference for the σ ϩ correlation is found to be statistically significant at the 95% confidence level in both reaction series. Ab initio and semi-empirical calculations of the gas phase ionization energies of aryl vinyl ethers were also found to correlate preferentially with σ ϩ .…”