The Effects of α-Substituents on the Kinetic and Thermodynamic Stability of 4-Methoxybenzyl Carbocations: Carbocation Lifetimes That Are Independent of Their Thermodynamic Stability

Article abstract of DOI:10.1021/jo00074a036

The following new rate constants for reaction of α-substituted 4-methoxybenzyl carbocations, 4-MeOC6H4CR1(R2)(1+), with a solvent of 50:50 (v/v) trifluoroethanol/water at 25 deg C and ionic strength 0.50 (NaClO4) are reported: 4-MeOC6H4CH(OMe)(1+), K_s = 2.2E7 s^-1; 4-MeOC6H4CH(N3)(1+), k_s = 3.3E5 s^-1; 4-MeOC6H4C(CH3)2(1+), k_s = 1.3E7 s^-1; 4-MeOC6H4CH(CO2Et)(1+), k_s = 1.4E7 s^-1; 4-MeOC6H4CCH3(CF3)(1+), k_s = 2.5E7 s^-1.The values of k_s for reaction of 4-MeOC6H4CR1(R2)(1+) with 50:50 (v/v) trifluoroethanol/water are nearly independent of very large changes in the thermodynamic stability of these carbocations caused by the addition of a wide range of electron-withdrawing or electron-donating groups at the α-position.In the most extreme case, the change from an α-methoxy to two α-(trifluoromethyl) substituents leads to a 23 kcal/mol thermodynamic destabilization of 4-MeOC6H4CR1(R2)(1+) relative to the neutral azide ion adducts but a 5-fold decrease in its reactivity toward solvent.The data show that the effects of α-substituents on the kinetic stability of 4-MeOC6H4CR1(R2)(1+) are complex and do not parallel the thermodynamic stability of these carbocations.The results are explained by consideration of the polar and resonance effects of the α-substituents on both the thermodynamic driving force and the intrinsic barrier for capture of the carbocations by solvent.These reactions are a new example of the consequences of the "principle of nonperfect synchronization".