Computational Insight into the Chemistry of β-(Phosphatoxy)alkyl Radicals: [3,2]- and [1,2]-Phosphatoxy Rearrangements and a New Pathway for syn-Elimination of Phosphate

Abstract

The [3,2]- and the [1,2]-phosphatoxy rearrangements have been studied in the 2-(phosphatoxy)ethyl radical, the 2-(dimethylphosphatoxy)ethyl radical, and the 2-(phosphatoxy)propyl radical with the Becke3LYP/6-31G(d) density functional method. Barriers have also been calculated through single-point energy calculations at the B3LYP/ 6-311+G(d,p), PMP2/6-31+G(d,p), and, in part, QCISD/6-31G(d) levels of theory. In contrast to acyloxy rearrangements in otherwise identical systems, the [1,2]-shift pathway is slightly preferred in phosphatoxy rearrangements. The degree of charge separation is much more significant in the [3,2]- than the [1,2]-shift pathway. Barriers for phosphatoxy and dimethylphosphatoxy shifts are rather similar for both pathways. Introduction of a methyl group adjacent to the radical center lowers the barrier quite significantly. The effect is larger for the [1,2]- than for the [3,2]-shift. Inspection of the charge and spin density distributions indicates that this effect is composed of steri...