ANOMERIC-LIKE SUBSTITUENT EFFECTS ON THE CHAIR CHAIR CONFORMATIONAL EQUILIBRIUM OF THE 2-OXO-1,3,2-OXAZAPHOSPHORINANE RING-SYSTEM

The chair-chair equilibria for a series of 5,5-dimethyl-2-oxo-(2-p-X-anilino)-1,3,2-oxazaphosphorinanes were determined by H-1 NMR. The percentage of chair conformer with the p-X-anilino group axial is increased by the presence of electron withdrawing X, while the opposite is true for electron-donor para X. Reasonably good linear plots of log K vs sigma-p were obtained in the solvents acetone-d6, CD3CN, and CD3NO2 with rho = 0.28-0.36. These results are interpreted in terms of the dominance of the endo anomeric effect involving overlap of the endocyclic N(3) and O(1) p lone pairs with the axial P-N sigma* orbital (p-XC6H4NHP).