Large Transition State Imbalance in the Expulsion of Arylnitromethane Anions from Aniline Adducts of α-Nitrostilbenes

Article abstract of DOI:10.1021/jo00097a051

Aniline adducts of α-nitrostilbenes, PhCH(HNC6H4Z)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 1H NMR and reactivity data.The kinetics of the breakdown of the adducts into the respective benzanilines and arylnitromethane anions was studied in acidic 50percent Me2SO-50percent water solutions at 5 deg C.The rate constants, k_exp, were found to be independent of buffer concentration and pH, indicating a unimolecular mechanism.There is a strong dependence on the aniline substituent (ρ = -2.2) and the pK_a of the arylnitromethanes (β_lg = -1.28).By estimating the substituent dependence of the equilibrium constant (K_exp), approximate normalized structure-reactivity parameters α_expⁿ = (dlog k_exp/dlog K_exp)_Z ca. 1.28 and β_expⁿ = (dlog k_exp/dlog K_exp)_Y ca. 0.58 have been calculated.This suggests the presence of a substantial transition state imbalance (α_expⁿ - β_expⁿ ca. 0.70), similar to that in the deprotonation of arylnitromethanes.