Conformational dependence of substituent effects in the solvolyses of the 1,1-diphenyl-2,2,2-trifluoroethyl system

The substituent effects on the solvolysis of 1-X-phenyl-1-Y-phenyl-2,2,2-trifluoroethyl tosylates were analyzed on the basis of the Yukawa–Tsuno equation. For the solvolysis of the symmetrically disubstituted X = Y subseries, an excellent linear correlation was obtained. However, in the solvolyses of series of varying X with a fixed-Y subset, the substituent effects were found to give significantly concave Y–T correlations. The ρ value for the Y–T correlations changes significantly with the fixed Y substituent. There is a qualitative trend of a linear decrease in the ρ value as the fixed-Y substituent of the respective subset becomes more electron donating. The non-linearity of substituent effects was attributed to a substituent-induced conformational change of the transition state, which could be simulated by calculation of the preferred conformation of the intermediate carbocation. A molecular orbital optimization method was applied to determine the preferred conformations of the α,α-diarylcarbenium ions derived from the title systems. The symmetrical carbenium ions X = Y have a preferred propeller shape conformation with twist angles differing by 14° (Econformation), whereas in the unsymmetrical systems X ≠ Y the two aryl rings are much more (>30°) differently twisted in the preferred conformation (PTconformation). The linear correlation found for the symmetrical subseries is essentially an outcome of the Econformation of the transition state. In unsymmetrical cases, the substituent effect correlation in any Y subset should reflect the conformational arrangements, EX, PX and TX of the variable X-aryl group, depending on relative σ values of X and Y. The substituent effects in the Y-subsets were successfully treated by three different Y-independent correlations for the preferred conformational arrangements: E correlation for substituents when (σX − σY) ≅ 0, PX correlation when (σX − σY) ≪0 and TX correlation for the (σX − σY) ≫0 class, respectively. Copyright © 2002 John Wiley & Sons, Ltd.