Ionic and radical intermediates of 3-substituted 5,6-dihydro-1,4,2-dioxazines: a theoretical (AM1) study

Stable carbocations (A), carbanions (B) and free radicals (C) are expected to result from 3-substituted-5, 6-dihydro-1,4,2-dioxazine 1 or their 3- (1-bromoalkyl) derivative 2, due to the presence of a CN double bond, adjacent to the active methylene group. However, experimental data indicate that while A and C could be obtained, B was apparently too unstable to act as an intermediate in alkylations. A theoretical study of the structures and stabilities of A-C resulting from 1 and 2 (R = CH₃), in the framework of a semiempirical all-valence electron molecular orbital approximation (AMI) is described. The results obtained indicate that A and C can be stabilized by delocalization and accommodation of the positive charge and unpaired electron respectively, by conjugation, hyperconjugation and field effects. Although B has a non-uniform distribution of the charge due to a less planar shape, it should be stabilized by delocalization. Calculated energies of reaction (E_r) for the formation of B, and deprotonation enthalpy (DPE) of 1 (R = CH₃) in comparison with the deprotonation of 3-methyl-4H-5,6-dihydrooxazine (6), a structurally related compound proven to lead to carbanions (B′ and B″), could not explain the failure to perform carbanion-mediated alkylations.