Kb. Williams et Jp. Richard, KINETIC MECHANISM FOR DIMERIZATION OF AN ALPHA-THIOAMIDE SUBSTITUTED BENZYL CARBOCATION IN AQUEOUS-SOLUTION, Journal of physical organic chemistry, 11(10), 1998, pp. 701-706
The products of reaction of the alpha-(N,N-dimethylthiocarbamoyl)-4-me
thoxybenzyl 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-methoxybenzo
thiophene (2) from intramolecular cyclization of 1(+). The reaction of
1(+) in 50:50 (v/v) methanol-water (I = 0.50, NaClO4) gives mainly th
e adducts to solvent. In 50:50 (v/v) trifluoroethanol-water (I = 0.50,
NaClO4), 1(+) partitions between reaction with solvent (k(s), 27% yie
ld), 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 indep
endent of the leaving group at 1-O2CAr and remains constant as the con
centration of the substrate is increased fourfold. These data show tha
t the rate-determining step for dimerization of 1-O2CAr is ionization
of substrate to form 1(+) and that the products of the reaction are de
termined by the rate constant ratio for partitioning of 1(+) between a
ddition of solvent and cyclization. The rate constant ratios determine
d 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.