Kinetics and mechanism of the addition of benzylamines to β-nitrostilbenes and β-cyano-4′-nitrostilbenes

Abstract

Nucleophilic addition reactions of benzylamines (BA) to β-nitrostilbenes (NSB) and β-cyano-4′-nitrostilbenes (CNS) have been studied in acetonitrile at 25.0 and 30.0 °C, respectively. The rate is first order with respect to BA and substrate. The rate of reaction with CNS is much lower than that expected from the rate sequence observed in aqueous solution indicating that the mechanisms of BA addition in acetonitrile and in water are different. The major factor determining reactivity of the amine addition in acetonitrile is the direct resonance effect (σ⁻ or R⁻) while that in aqueous solution is the polar electron-withdrawing effect (σ) of the activating groups. Due to steric inhibition the β-phenyl rings in NSB and CNS are prevented from π-overlap with the anionic center in the TS so that the reduced resonance effect leads to unduly low addition rates. The kinetic isotope effects and activation parameters are in line with the one step addition mechanism in which N–C_α and H–C_β bonds are formed concurrently with a hydrogen bonded four-center cyclic transition state. The cross-interaction constant ρ_XY is negative and the magnitude is somewhat larger than those for other similar addition reactions.