Axial and Equatorial Dimethylamine Substitution on Phosphorus in the 1,3,2-Oxazaphosphorinane Ring System: Structures of 2-Dimethylamino-4-Phenyl-2H-1,3,2-Oxazaphosphorinane-2-Oxides

Single crystal x-ray structures of the diastereomeric 2-dimethylamino-4-phenyl-2H-1,3,2-oxazaphosphorinane-2-oxides, 5c and 6c, have been determined. The pair has been prepared in a straightforward manner by reaction of 3-amino-3-phenylpropanol with Mo₂P(O)Cl₂, and separated into pure forms by column chromatography. Crystal data of 5c: space group P2₁/n, a = 11.087(3), b = 6.087(6), c = 18.465(6)Å, β = 98.40(3)°, R = 0.035 for 1845 reflections. Crystal data of 6c: space group P2₁/n, a = 11.176(2), b = 6.893(2), c = 16.673(3)Å, β = 95.29(1)°, R = 0.037 for 1253 reflections. The fast-migrating 5c is cis (Ph and P = O group cis) with the configuration of 2RS, 4SR. The slow migrating 6c is trans with the configuration of 2RS, 4RS. In both structures the 1,3,2-oxazaphosphorinane rings are chair like with the P-end essentially flattened, The skeleton made up with 4-phenyl-1,3,2-oxazaphosphorinane is essentially the same. With a conformationally demanding 4-phenyl substituent, 5c has an axial Mc₂N whereas 6c has an equatorial Me₂N. For 6c, the exocyclic P-N bond has a partial double bond character the geometry about exocyclic N is planar. On the other hand, for 5c, the geometry about exocyclic N deviates considerably from planarity, the sum of the angles around N being 348.1°. In the crystalline state, the screw-related 5c molecules are hydrogen bonded whereas in the crystalline state, the centro-symmetrically related 6c molecules are paired-up by hydrogen bonds, both through the N-H and P = O system.