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

The products of reaction of the α-(N,N-dimethylthiocarbamoyl)-4-methoxybenzyl carbocation (1⁺) intermediate of solvolysis of α-(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-propanoI (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_c, 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.