Insights into the nucleophilic substitution of pyridine at an unsaturated carbon center

Bimolecular nucleophilic substitution (S(N)2) is a fundamental reaction that has been widely studied. So far, the nucleophiles are mainly anionic species in S(N)2 reactions. In this study, we use density functional theory calculations to assess the mechanisms of substitution of carbonyl, imidoyl, and vinyl compounds with a neutral nucleophile, pyridine. Charge decomposition analysis is performed to explore the main components of the transition state's LUMO. For reactions of imidoyl or carbonyl compounds with pyridine or Cl(−), the LUMOs of the transition states are composed of mixed orbitals originating from the nucleophile and the substrate. Considering the unique mixed nature of the orbitals, the reaction mode is termed S(N)m (m means mix). Moreover, the main components of the transition state's LUMO are pure σ*(C–Cl) MO in the reactions of H(2)C[double bond, length as m-dash]CHCl with pyridine or Cl(−). Computations were also performed for RY[double bond, length as m-dash]CHX substrates with different X and Y groups (X= Cl(−), Br(−), or F(−); Y = O, N, or C).