Substituent Effects on the Solvolysis of 2,2,2-Trifluoro-1,1-diphenylethyl Tosylates. II. 3-Chlorophenyl- and 3,5-Dichlorophenyl-Fixed Systems

Article abstract of DOI:10.1246/bcsj.70.3081

The solvolysis rates of 2,2,2-trifluoro-1-(3-chlorophenyl)-l-(substituted phenyl)ethyl and 2,2,2-trifluoro-1-(3,5-dichlorophenyl)-1-(substituted phenyl)ethyl tosylates or bromides were conductimetrically measured at 25.0°C in 80% aqueous ethanol. The 3-chlorophenyl-fixed series showed a bilinear Yukawa-Tsuno correlation with ρ = -4.81 and r = 1.41 for substituents more deactivating than 3,5-dimethyl, and with ρ = -6.19 and r = 1.57 for the substituent range more activating than 4-methyl. The bilinear correlation was interpreted in terms of the changing coplanarity of the two aryl rings. The 3,5-dichlorophenyl-fixed substrates gave an excellent linear Yukawa-Tsuno correlation for the substituents ranging from 4-MeO to 4-Cl with ρ = -5.95 and r = 1.69. The variable aryl rings in this series show the largest extent of resonance interaction with the extraordinary deactivated carbenium ion center by α-CF₃ and α-(3,5-dichlorophenyl) groups in the transition state. The optimization of the geometries of 2,2,2-trifluoro-1,1-diphenylethyl cation and some ring-substituted derivatives was performed at the RHF/6-31G* level. The symmetric diaryl cations have equivalently twisted conformations while in unsymmetric ones the more stabilizing aryl group is less twisted from the plane of the positive sp² carbon. The observed variation of r and ρ parameters is accommodated by geometrical changes of the incipient cation.