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

Abstract

Abstract 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 1,1-diarylethyl p-nitrobenzoates. The non-linearity and non-additivity were significant in both cases. While an extended Brønsted relationship against ΔpKBH+ 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 ΔσR+, where the ΔσR+ is the Yukawa–Tsuno resonance parameter. This relationship can be rewritten in the form of an extended Brønsted correlation: log (kXY/kHH)Br = α [log KXY/KHH]Br, where [log KXY/KHH]Br = −ΔpKBH+ + 0.81 ΔσR+, for the thermodynamic stabilities of the intermediate in the bromination. The constant α 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 ΔσR+ 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.