Migratory aptitudes of simple alkyl groups in the anionotropic rearrangement of quaternary chloromethyl borate species: a combined experimental and theoretical investigation.

A combined experimental and theoretical investigation at the DFT and MP2 levels on the boron-to-carbon 1,2-shift in "ate species", coming from the quaternization of boranes (A) and boronate (B), is reported. To discuss the different migratory aptitudes of various alkyl groups, we have examined the migration of primary (R = Me, Et), secondary (R = i-Pr), and tertiary (R = t-Bu) alkyl groups. The effect of the counterion Li(+) and of the solvent (polarized continuous model (PCM) method) has been considered. The following results are relevant: (a) in all cases, the reaction proceeds via a concerted-type mechanism which explains the retention of configuration at the migrating group and the inversion at the migration terminus experimentally observed. (b) The trend of the migration barriers along the direction primary --> secondary --> tertiary alkyl group observed in "ate" species A is reversed in boronate species B, in agreement with the experimental evidences. (c) A simple theoretical model is proposed where the barrier trend is the result of a delicate interplay between two opposite factors: (1) a "steric effect", which favors the most sterically demanding migrating groups, and (2) a "charge effect" associated with the partial carbanionic nature of the migrating carbon atom and which favors the less substituted migrating carbons.