Stereoelectronic Effects on the Basicity and Nucleophilicity of Phosphites and Phosphates. Ab Initio Molecular Orbital Calculations and the Alpha-Effect

摘要

Ab initio molecular orbital calculations on phosphites, protonated phosphites, and protonated phosphates reveal important stereoelectronic effects. In the phosphites, an antiperiplanar lone pair on oxygen to the phosphite lone pair raises the energy of the molecule by 3.3 kcal/mol relative to a phosphite conformation with no antiperiplanar lone pairs to the phosphite lone pair. Upon phosphorus protonation of the phosphite the relative energy difference between the conformations reverses. The conformation with lone pairs on oxygen antiperiplanar to the P-H bond is now more stable than the conformation without this antiperiplanar lone-pair interaction. Finally, the origin of the alpha-effect, the enhanced nucleophilicity of a base possessing a heteroatom with an adjacent unshared electron pair, is suggested to arise from the stereoelectronic effect. This is attributed to a transition-state stereoelectronic effect. Whereas oxygen lone pairs antiperiplanar to the P-H(+) in the ground-state protonated phosphite stabilize the structure by only 1 kcal/mol at a P-H distance of 1.4 A, this stereoelecronic stabilization rises to >12 kcal/mol, at a P-H distance of approx. 3 A, falls off again at even longer P-H bond distances, and finally reverses energies for the two confirmations of the neutral phosphites.