Abstract
The substituent effects on the solvolysis of 2,2-dimethyl-1,1-diphenylpropyl system were analyzed by the Yukawa-Tsuno equation and discussed in terms of changing coplanarity of two aryls in the transition state.In the 1,1-diaryl system where X=Y, the carbocationic transition state adopts a propeller-like twisted E-conformation to minimize steric repulsion, while in unsymmetrical (X is not equal to Y) ones the propeller shape is distorted. It is considered that in the present a-t-Bu system carbocationic species adopt extremely twisted conformation owing to steric hindrance of t-Bu group.The result of ab initio calculation of the carbocation-intermediate shows as follows. The symmetrically disubstituted carbocation(X=Y) adopts E-conformation where both aryls are equivalently twisted for the whole range of substituents. The carbocation (X is not equal to Y), when the variable substituents X are far more electron-donating than the fixed Y, takes a PXTY-conformation where X-phenyl being coplanar and Y-phenyl being orthogonal. On the other hand, when variable X-substituents are far less activating than the fixed Y, the carbocation takes a PYTX-conformation. Consequently, this system is ideal for considering the conformational dependence of substituent effects.
