Remarkable kinetic stability of alpha-thiocarbamoyl substituted 4-methoxybenzyl cations

4-Methoxybenzyl cations bearing alpha-(N,N-dimethylcarbamoyl) and alpha-(N, N-dimethylthiocarbamoyl) substituents have been generated photochemically u pon irradiation of precursors with pentafluorobenzoate or 4-methoxybenzoate leaving groups. The ions have been observed with flash photolysis in 40:60 acetonitrile:water and in 50:50 methanol:water, and rate constants were me asured for their decay in solvent alone and for their capture by azide ion. The cations so studied and their lifetimes in 40% acetonitrile are 6, ArCH-CONMe2, 0.6 mu s; 2, ArC+H-CSNMe2, 7 ms; and 4, ArC+(CH3)-CSMe2. 6 ms, wh ere Ar = 4-MeOC6H4. The cation 4 reacts with solvent by elimination of a pr oton from the a-methyl group, and the rate constant for solvent addition mu st be less than l s(-1). The CSNMe2 substituted cations are 10(5)-10(7)-fol d longer lived than analogs where the thioamide group has been replaced wit h an alpha-methyl. The UV-visible absorption spectra of these two cations a lso show significant differences from those of typical 4-methoxybenzyl cati ons. Thus, both the lifetimes and spectra point to a strong interaction of the benzylic centre with the thioamide group.