Kinetics and Mechanism of the Addition of Benzylamines to β-Cyanostilbenes in Acetonitrile

Nucleophilic addition reactions of benzylamines (XC6H4CH2NH2) to β-cyanostilbenes (YC6H4CH=C(CN)C6H4Y’) have been studied in acetonitrile at 30.0 o C. A greater degree of N-Cα bond formation (larger βX) is obtained with a stronger electron-withdrawing substituent in either α- (δσY > 0) or β-ring (δσY’ > 0). A stronger charge development is observed in the TS on Cβ (ρY’ = 1.06 for X=Y=H) rather than on Cα (ρY = 0.62 for X=Y’=H) indicating the lag in the resonance development into the activating group (CN) on Cβ in the transition state. Similarly, the magnitude of ρXY’ (−0.72) is greater than ρXY (−0.66) due to a stronger interaction of the nucleophile with β-ring than α-ring. The positive sign of ρYY’ correctly reflects π bond cleavage between the two rings in the TS. Relatively large kinetic isotope effects (kH/kD ≥ 2.0) involving deuterated nucleophiles (XC6H4CH2ND2) suggest a four-membered cyclic TS in which concurrent N-Cα and H(D)-Cβ bond formation occurs.