An Ab initio Study of the P−C Bond Rotation in Phosphoryl- and Thiophosphoryl-Stabilized Carbanions: Five- and Six-Membered Heterocycles

Abstract

The potential energy surfaces for the P−C bond rotation in the 2-oxo- and 2-thioxo-2-methyl-1,3,2-diazaphosphorinane and -1,3,2-diazaphospholidine anions have been investigated at MP4(SDQ)/6-31+G*//HF/6-31+G* + ZPE. Four stationary points have been found for the six-membered ring species. The lowest energy structures exhibit a completely or nearly planar carbanion with its substituents parallel to the PX axis (X = O, S). The transition state (TS) structures have a strongly pyramidalized carbanion in which the lone pair (LP) is approximately perpendicular to the PX bond. Isodesmic equations, bond length comparisons, and orbital interactions indicate a superior ground state (GS) stabilization of the thioxo derivative and a favorable TS stabilization of the oxo species. Both effects cooperate to furnish the computationally and experimentally observed higher (ca. 2.5 kcal/mol in both cases) P−C rotational barrier for the 2-thioxo-1,3,2-diazaphosphorinane based anions. Coordination of a lithium cation to the c...