STUDY OF THE CONFORMATIONAL EQUILIBRIA OF 2-Z-3-METHYL-1,3,2-OXAZAPHOSPHORINANES - STERIC AND STEREOELECTRONIC INFLUENCES ON THE ORIENTATION OF THE ME(2)N SUBSTITUENT ON 3-COORDINATE PHOSPHORUS

Abstract

The conformations of a series of 1,3,2-oxazaphosphorinanes containing three-coordinate phosphorus, 1-9, have been determined by the use of H-1, P-31, and C-13 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)(2)CHO 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 Me(2)N is attached to phosphorus, the newly studied N(3)Me rings display a chair-chair conformational equilibrium, 10 reversible arrow 11, with the Me(2)N 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(S)-i-Pr compound, B. The observed change in the free energy of the equilibrium 10 reversible arrow 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 Me(2)NP and the smaller Me at N(3)(Me(2)N(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 reversible arrow 11 is opposite to that found for four-coordinate phosphorus containing 1,3,2-oxazaphosphorinanes in which Me(2)NP(ax/N(3)Ph repulsions that destabilize 10 appear to be dominant.