Teshima Minami, Tsuji Yutaka, Richard John P
Department of Biochemistry and Applied Chemistry, Kurume National College of Technology, Komorinomachi, Kurume 830-8555, Japan.
J Phys Org Chem. 2010 Aug 1;23(8):730-734. doi: 10.1002/poc.1642.
The sum of the rate constants for solvolysis and scrambling of carbon bridging and nonbridging oxygen-18 at 4-MeC(6)H(4)CH(CF(3))OS((18)O(2))Tos in 50/50 (v/v) trifluoroethanol/water, (k(solv) + k(iso)) = 5.4 × 10(-6) s(-1), is 50% larger than k(solv) = 3.6 × 10(-6) for the simple solvolysis reaction of the sulfonate ester. This shows that the ion pair intermediate of solvolysis undergoes significant internal return to form reactant. These data give a value of k(-1) = 1.7 × 10(10) s(-1) for internal return of the carbocation-anion pair to the substrate. This rate constant is larger than the value of k(-1) = 7 × 10(9) s(-1) reported for internal return of an ion pair between the 1-(4-methylphenyl)ethyl carbocation and pentafluorobenzoate anion to the neutral ester (4-MeC(6)H(4)CH(CH(3))O(2)CC(6)F(5)) in the same solvent. The partitioning of ion pairs to the 1-(4-methylphenyl)ethyl carbocation and to the highly destabilized 1-(4-methylphenyl)2,2,2-trifluoroethyl carbocation is compared and contrasted.
在50/50（v/v）的三氟乙醇/水中，4-甲基苯甲基三氟甲基磺酸酯（4-MeC(6)H(4)CH(CF(3))OS((18)O(2))Tos）的溶剂解速率常数与碳桥连和非桥连氧-18重排速率常数之和，即(k(solv) + k(iso)) = 5.4 × 10(-6) s(-1)，比磺酸酯简单溶剂解反应的k(solv) = 3.6 × 10(-6)大50%。这表明溶剂解的离子对中间体经历了显著的内部返回以形成反应物。这些数据给出了碳正离子-阴离子对返回底物的内部返回速率常数k(-1) = 1.7 × 10(10) s(-1)。该速率常数大于在相同溶剂中报道的1-（4-甲基苯基）乙基碳正离子与五氟苯甲酸根阴离子之间的离子对返回中性酯（4-MeC(6)H(4)CH(CH(3))O(2)CC(6)F(5)）的k(-1) = 7 × 10(9) s(-1)。对离子对分别生成1-（4-甲基苯基）乙基碳正离子和极不稳定的1-（4-甲基苯基）2,2,2-三氟乙基碳正离子的分配情况进行了比较和对比。
