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

4-Methoxybenzyl cations bearing α-(N,N-dimethylcarbamoyl) and α-(N,N- dimethylthiocarbamoyl) substituents have been generated photochemically upon 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 measured 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, ArC⁺H- CONMe₂, 0.6 μs; 2, ArC⁺H-CSNMe'2, 7 ms; and 4, ArC⁺(CH₃)-CSMe₂, 6 ms, where Ar = 4-MeOC₆H₄. The cation 4 reacts with solvent by elimination of a proton from the α-methyl group, and the rate constant for solvent addition must be less than 1 s^-1. The CSNMe₂ substituted cations are 10⁵-10⁷- fold longer lived than analogs where the thioamide group has been replaced with an α-methyl. The UV-visible absorption spectra of these two cations also show significant differences from those of typical 4-methoxybenzyl cations. Thus, both the lifetimes and spectra point to a strong interaction of the benzylic centre with the thioamide group.