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

摘要

Nucleophilic addition reactions of benzylamines (BA) to β-nitrostilbenes (NSB) and β-cyano-4u0001-nitrostilbenesn(CNS) have been studied in acetonitrile at 25.0 and 30.0 u0003C, respectively. The rate is first order with respect to BAnand substrate. The rate of reaction with CNS is much lower than that expected from the rate sequence observed innaqueous solution indicating that the mechanisms of BA addition in acetonitrile and in water are different. The majornfactor determining reactivity of the amine addition in acetonitrile is the direct resonance effect (σu0004 or Ru0004) while thatnin aqueous solution is the polar electron-withdrawing effect (σ) of the activating groups. Due to steric inhibition thenβ-phenyl rings in NSB and CNS are prevented from π-overlap with the anionic center in the TS so that the reducednresonance effect leads to unduly low addition rates. The kinetic isotope effects and activation parameters are in linenwith the one step addition mechanism in which N–Cα and H–Cβ bonds are formed concurrently with a hydrogennbonded four-center cyclic transition state. The cross-interaction constant ρXY is negative and the magnitude isnsomewhat larger than those for other similar addition reactions.