Studies on Sulfenamides. XV. Semi-Empirical Calculation of Reactivity of 49-Substituted Benzenesulfenanilidyl Radicals

(OMe (1a), Me (1b), Cl (1c)) has attracted chemists’ attention because it has easily produced benzenesulfenanilidyl radicals (49-OMe (Aa), 49-Me (Ab), 49-Cl (Ac)) and yielded phenazines as one of final products. The coupling of benzenesulfenanilidyl radicals also seems interesting from a synthetic viewpoint for one-pot synthesis of 2,7-disubstituted phenazines (OMe (2a), Me (2b), Cl (2c)). However, the mechanism of the formation of the phenazine ring has been unclear. Sayo et al. and Benati et al. proposed the mechanism of the formation of 2 independently, but their two mechanisms are apparently incompatible with each other. Sayo carried out the oxidation of 1 using lead dioxide and an electrochemical method, and obtained 2 and diphenyl disulfide (3) as the products. The presence of A in the reaction solution was proved by using electron spin resonance (ESR) and visible spectra. The reaction products have been explained on the basis of the homolytic cleavage of the S–N bond of A, which afforded a phenylthio radical (B) and nitrenium; the former dimerized to 3 and the latter dimerized to 2, as shown in Chart 1. On the other hand, Benati reported that the oxidation of 1a with lead dioxide at 30 °C had afforded N,N-bis(phenylthio)p-anisidine, 2a and 3 where as at 10 °C N,N-bis(phenylthio)p-anisidine and di-imines which might be 1-[aza(2-azaphenylthiomethylene)-5-methoxycyclohexa-3,5-dienylidene]methyl]-4-methoxybenzene were observed. The later was thermally unstable and thus gave 2a and 3. Benati concluded that the dimerization of A could explain the formation of the products, as shown in Chart 2. Then, molecular orbital (MO) calculation was applied to elucidate the reactivity of A, as well as the mechanism of the formation of 2. Although the oxidation of 1 gives various products, this paper was confined to studying the mechanism of phenazine formation to avoid a confused discussion.