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Kinetic mechanism for dimerization of an α‐thioamide substituted benzyl carbocation in aqueous solution
Author(s) -
Williams Kathleen B.,
Richard John P.
Publication year - 1998
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(1998100)11:10<701::aid-poc26>3.0.co;2-4
Subject(s) - chemistry , solvolysis , carbocation , yield (engineering) , solvent , reaction rate constant , medicinal chemistry , nucleophile , aqueous solution , substrate (aquarium) , hydrolysis , organic chemistry , kinetics , catalysis , physics , quantum mechanics , materials science , oceanography , metallurgy , geology
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‐O 2 CAr ) 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, NaClO 4 ) gives mainly the adducts to solvent. In 50:50 (v/v) trifluoroethanol–water ( I = 0.50, NaClO 4 ), 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, NaClO 4 ) is independent of the leaving group at 1‐O 2 CAr and remains constant as the concentration of the substrate is increased fourfold. These data show that the rate‐determining step for dimerization of 1‐O 2 CAr 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, NaClO 4 ) were k s / k c = 0.8 M −1 and k alk / k s = 540 000 M −1 . © 1998 John Wiley & Sons, Ltd.