The mechanism of phosphoryl to carbonyl migration of amino groups in mixed anhydrides. A MNDO SCF-MO study

Abstract

A MNDO SCF–MO study of the mechanism of acyl migration in mixed carboxylic–phosphoric anhydride systems supports the hypothesis that these reactions are concerted, involving no dipolar intermediates. The calculations reveal a marked specificity for migration to an amino group rather than to an alkoxy group. The stereoelectronic contributions to this effect are revealed by inspection of the SCF molecular orbitals. These show greater localisation along the axes of the forming/cleaving C–N and N–P bonds at the transition state for migration to N than for migration to O. The cyclic transition state for the latter also shows greater destabilising 1,3-interactions between the endocyclic heteroatoms. A more quantitative analysis in terms of localised orbitals also supports the bond localisation argument.