Study of the Conformational Equilibria of 2-Z-3-Methyl-1,3,2-oxazaphosphorinanes. Steric and Stereoelectronic Influences on the Orientation of the Me2N Substituent on Three-Coordinate Phosphorus

Article abstract of DOI:10.1021/jo00120a020

The conformations of a series of 1,3,2-oxazaphosphorinanes containing tree-coordinate phosphorus, 1-9, have been determined by the use of 1H, 31P, and 13C NMR spectroscopy.The rings were substituted at ring nitrogen, N(3), with a methyl group to compare its effect on conformational energies with those of 1,3,2-oxazaphosphorinanes reported earlier that featured a larger substituent at N(3), Ph or i-Pr.Quite expectedly, like those rings previously studied with Ph or i-Pr at N(3), a MeO or (CF3)2CHO substituent at phosphorus has a strong preference to be axial on a chair-form ring, 1-4, cis-7, and cis-8, or pseudoaxial on a ring in a twist/boat conformation, trans-7.However, when Me2N is attached to phosphorus, the newly studied N(3)Me rings display a chair-chair conformational equilibrium, 10 ->/<- 11, with the Me2N equatorial ring, 11, mildly dominant (58/42, 5; 65/35, 6).This contrasts with ratios of 17/83 and 20/80 for the corresponding N(3)Ph analogs, A, and 23/77 for the N(3)-i-Pr compound, B.The observed change in the free energy of the equilibrium 10 ->/<- 11, 1.2-1.3 kcal/mol, is ascribed to the dominant influence of a decrease in repulsion experienced in conformation 11 between the equatorial Me2NP and the smaller Me at N(3) (Me2N(eq)/N(3)Me destabilization) compared to that experienced with the N(3)Ph and N(3)-i-Pr analogs.This steric influence of N(3) substituents on the equilibrium 10 ->/<- 11 is opposite to that found for four-coordinate phosphorus containing 1,3,2-oxazaphosphorinanes in which Me2NP(ax)/N(3)Ph repulsions that destabilize 10 appear to be dominant.