Substituent Effects in the 1,1-Diphenylcarbenium Systems: Hydration and Bromination of 1,1-Diphenylethylenes

Substituent effects log (kXY/kHH)Br on the bromination of 1,1-diarylethylenes were analyzed in comparison with those log (kXY/kHH)OPNB in the solvolysis of 1,1diarylethyl p-nitrobenzoates. The non-linearity and non-additivity were significant in both cases. While an extended Bronsted relationship against (delta)pKBH+ for diarylethylenes holds in the latter process, of particular importance is the finding of a precise linear correlation between the bromination and solvolysis for the whole substituents; log (kXY/kHH)OPNB = 1.07 log (kXY/kHH)Br - 0.83 (delta)(sigma)R+, where the (delta)(sigma)R+ is the Yukawa-Tsuno resonance parameter. This relationship can be rewritten in the form of an extended Bronsted correlation: log (kXY/kHH)Br = (alpha) [log KXY/KHH]Br, where [log KXY/KHH]Br = -(delta)pKBH+ + 0.81 (delta)(sigma)R+, for the thermodynamic stabilities of the intermediate in the bromination. The constant (alpha) coefficient should suffice as convincing evidence for the absence of a coordinate shift of the transition state that is induced by varying substituents in the bromination process. The additional contribution of 0.81 (delta)(sigma)R+ should be ascribed to the differential cationic charge at the benzylic position in the bromination transition state; a constant coefficient indicates a fixed position of the bromination transition state, irrespective of the fixed-Y subset.