Kinetics and Mechanism of the Addition of Benzylamines to Benzylidenemalononitriles in Acetonitrile

Nucleophilic addition reactions of benzylamines (BA) to benzylidenemalononitrile (BMN) have been studied in acetonitrile at 15.0 degrees C. The rate is first-order with respect to both BA and BMN and no base catalysis is observed. The rate decreases as the electron-withdrawing power of the substituent (Y) in the substrate increases (rho Y < 0). This is in contrast to the similar reactions of beta-nitrostyrenes (NS) with BAs in acetonitrile and the addition reactions of NS and BMN in aqueous solution (rho Y > 0). This sign change of rho Y is considered to result from the strong electron-withdrawing power of the (CN)2 group in BMN, which leads to polarization of C alpha delta+C delta- (CN)2 in the transition state. The mechanism of amine addition to BMN in acetonitrile is radically different from that in water. The reaction is predicted to proceed concertedly in a single step with a hydrogen-bonded, four-center cyclic transition state.