Kinetic mechanism for dimerization of an α-thioamide substituted benzyl carbocation in aqueous solution

The products of reaction of the alpha-(N,N-dimethylthiocarbamoyl)-4-methoxybenzyl carbocation (If) intermediate of solvolysis of alpha-(N,N-dimethylthiocarbamoyl)-4-methoxybenzyl benzoate esters (1-O2CAr) show a strong dependence on solvent. The only product from reaction in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) is 2-dimethylamino-6-methoxybenzothiophene (2) from intramolecular cyclization of 1(+). The reaction of 1(+) in 50:50 (v/v) methanol-water (I = 0.50, NaClO4) gives mainly the adducts to solvent. In 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO4), 1(+) partitions between reaction with solvent (k(s), 27% yield), cyclization to form 2 (k(s), 3% yield) and nucleophilic addition of 2 to 1(+) (k(alk), 70% yield) to form dimeric product 3, The yield of 3 in 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO4) is independent of the leaving group at 1-O2CAr and remains constant as the concentration of the substrate is increased fourfold. These data show that the rate-determining step for dimerization of 1-O2CAr is ionization of substrate to form 1(+) and that the products of the reaction are determined by the rate constant ratio for partitioning of 1(+) between addition of solvent and cyclization. The rate constant ratios determined for the partitioning of 1(+) in 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO4) were k(s)/k(c) = 0.8 M-1 and k(alk)/k(s) = 540 000 M-1. (C) 1998 John Wiley & Sons, Ltd.