Theoretical Studies of Substituent Effects on SN2 Transition States

摘要

Effects of substituents in the nucleophile(X), the substrate(Y) and the leaving group(Z) on the structure of SN2 transition states have been analyzed by considering effects of four components, electrostatic(Ees), exchange repulsion (Eex), polarization(Epl) and charge transfer(Ect) terms, of interaction between the reactants on the degree of bond making and bond breaking. Prediction of net effects of all substituents(X, Y and Z) on the degree of bond making were found to be clearcut whereas the effect of an electron withdrawing group on the substrate (Y = EWG) on the degree of bond breaking was complex; the substituent(Y = EWG) is normally carbon-leaving group(C*-L) bond tightening(Epl dominance) but becomes C*-L bond loosening when the bond is strongly antibonding (Ect dominance). Our model calculations on the reaction of CH2XNH2 with YCH2COOCH2Z using energy decomposition scheme have confirmed that predictions based on our analysis are correct.