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 .deg. C. A greater degree of N-C_α bond formation (larger β_X) is obtained with a stronger electron-withdrawing substituent in either α- (δσ_Y > 0) or β-ring (δσ_Y' > 0). A stronger charge development is observed in the TS on C_β (ρ_Y' = 1.06 for X=Y=H) rather than on C_α (ρ_Y = 0.62 for X=Y'=H) indicating the lag in the resonance development into the activating group (CN) on C_β in the transition state. Similarly, the magnitude of ρ_XY' (.0.72) is greater than ρ_XY (.0.66) due to a stronger interaction of the nucleophile with β-ring than α-ring. The positive sign of ρ_YY' correctly reflects π bond cleavage between the two rings in the TS. Relatively large kinetic isotope effects (k_H/k_D ≥ 2.0) involving deuterated nucleophiles (XC₆H₄CH₂ND₂) suggest a four-membered cyclic TS in which concurrent N-C_α and H(D)-C_β bond formation occurs