Nucleophilic substitution at the imidoyl carbon atom: Intermediate mechanistic and reactivity behavior between carbonyl and vinyl carbon substitution

Gas-phase nucleophilic substitution reactions at the imidoyl carbon have been investigated using chloride exchanges, Cl- + RY= CHCl = EY=CHCl + Cl- with Y = N and R = F, H or CH3, at the MP2, B3LYP and G2(+) levels using the MP2/6-311+G** geometries. The results are compared with those for the vinyl (Y = CH) and carbonyl (Y = O) carbon substitution. The mechanism and reactivity of substitution at the imidoyl carbon are intermediate between those of carbonyl (S(N)pi) and vinyl carbon (S(N)sigma) substitution, which is directly related to the electronegativity of Y, CH < N < O. The prediction of competitive S(N)sigma With S(N)pi path for the imidoyl chloride is consistent with the S(N)1-like mechanism proposed for reactions in solution. The important factors in favor of an in-plane concerted S(N)2 (S(N)sigma) over an out-of-plane pi -attack (S(N)pi) path are (i) lower proximate sigma-sigma* charge-transfer energies (DeltaE(CT)), (ii) stronger electrostatic stabilization (DeltaE(NCT)), and (iii) larger lobe size on C-alpha for the sigma*- than pi*-LUMO despite the higher sigma* than pi* level. The electron correlation energy effects at the MP2 level are overestimated for the relatively delocalized structure (S(N)pi TS) but are underestimated for the localized structure (S(N)sigma TS) so that the MP2 energies lead to a wrong prediction of preferred reaction path for the vinyl chloride. The DFT at the B3LYP level predicts comet reaction pathways but overestimates the electron correlation effects.