An ab initio study of the P-C bond rotation in phosphoryl- and thiophosphoryl-stabilized carbanions: Five- and six-membered heterocycles

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 P=X 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 P=X 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 chalcogen atom yields a distinct preference for the axial/equatorial orientation of the nitrogen substituents in the oxo species and for the diequatorial arrangement in the thioxo analog, in perfect agreement with X-ray crystallographic data. The X-ray crystal structure of lithio 2-(1-methylethyl)-1,3-dimethyl-1,3,2-diazaphosphorinane 2-sulfide·3THF is reported and it is consistent with existing theoretical and experimental geometries. The five-membered ring analogs (1,3,2-diazaphospholidines) exhibit the same conformational preference for the carbanion in the GS and the TS. The activation barrier for P-C bond rotation is higher in the thioxo derivatives as well. Whereas only one nitrogen substituent changes its orientation in the diazaphosphorinanes during the P-C rotational coordinate, the ring backbone responds strongly in the diazaphospholidines.