LARGE TRANSITION-STATE IMBALANCE IN THE EXPULSION OF ARYLNITROMETHANE ANIONS FROM ANILINE ADDUCTS OF ALPHA-NITROSTILBENES

Aniline adducts of alpha-nitrostilbenes, PhCH(HNC(6)H(4)Z)CH(C6H4Y)NO2 have been synthesized for a variety of Z/Y combinations. In several cases a mixture of two diastereomers was obtained. Structural assignment of the diastereomers was based both on H-1 NMR and reactivity data. The kinetics of the breakdown of the adducts into the respective benzalanilines and arylnitromethane anions was studied in acidic 50% Me(2)SO-50% water solutions at 5 degrees C. The rate constants, h(exp), were found to be independent of buffer concentration and pH, indicating a unimolecular mechanism. There is a strong dependence on the aniline substituent (rho = -2.2) and the pK(a) of the arylnitromethanes (beta(1g) = -1.28). By estimating the substituent dependence of the equilibrium constant (K-exp), approximate normalized structure-reactivity parameters alpha(exp)(n)=(dlog K-exp)(Z) approximate to 1.28 and beta(exp)(n)=(dlog k(exp)/dlog K-exp)Y approximate to 0.58 have been calculated. This suggests the presence of a substantial transition state imbalance (alpha(exp)(n)-beta(exp)(n) approximate to 0.70), similar to that in the deprotonation of arylnitromethanes.