Substitution reactions at the azomethine carbon and nitrogen atoms

Morath, R. J.; Stacy, Gardner W.

doi:10.1002/9780470771204.ch8

Cited by 3 publications

(1 citation statement)

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“…Thus, the earlier TS for the β-elimination of imidoyl than vinyl chloride is in accord with the Bell−Evans−Polanyi (BEP) principle, which asserts that the earlier the TS along the reaction coordinate, the lower is the activation barrier. In almost all of the actual reactions in solution, however, the unsubstituted azomethines, for example, HNCHCl, are rarely used, but the RNCR‘Cl type, disubstituted imidoyl compounds are involved. , Therefore, the extremely facile β-elimination reactions are not observed and are not important in practice. The substitution reactions provide the major reaction pathway.…”

Section: Resultsmentioning

confidence: 99%

Nucleophilic Substitution at the Imidoyl Carbon Atom: Intermediate Mechanistic and Reactivity Behavior between Carbonyl and Vinyl Carbon Substitution

Kim

Lee

et al. 2001

J. Am. Chem. Soc.

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Gas-phase nucleophilic substitution reactions at the imidoyl carbon have been investigated using chloride exchanges, Cl- + RY=CHCl right harpoon over left harpoon RY=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 (SNpi) and vinyl carbon (SNsigma) substitution, which is directly related to the electronegativity of Y, CH < N < O. The prediction of competitive SNsigma with SNpi 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 SN2 (SNsigma) over an out-of-plane pi-attack (SNpi) path are (i) lower proximate sigma-sigma* charge-transfer energies (DeltaECT), (ii) stronger electrostatic stabilization (DeltaENCT), 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 (SNsigma 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 correct reaction pathways but overestimates the electron correlation effects.

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Section: Resultsmentioning

confidence: 99%