Kinetic mechanism for dimerization of an α-thioamide substituted benzyl carbocation in aqueous solution
✍ Scribed by Kathleen B. Williams; John P. Richard
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 155 KB
- Volume
- 11
- Category
- Article
- ISSN
- 0894-3230
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✦ Synopsis
The products of reaction of the a-(N,N-dimethylthiocarbamoyl)-4-methoxybenzyl carbocation (1 ) intermediate of solvolysis of a-(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 2dimethylamino-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 À1 and k alk /k s = 540 000 M À1 .