Conformations of Saturated Six-Membered Ring Phosphorus Heterocycles. Axial, Nonplanar Dimethylamino Group of 5,5-Dimethyl-2-(dimethylamino)-2-oxo-1,3,2-oxazaphosphorinane As Determined by X-Ray Crystallography. (1)H NMR and IR Spectroscopic Analysis of Conformation in Solution

Article abstract of DOI:10.1021/ja00320a021

The title compound (7) crystallized in space group Pbca with a=7.409(2) Angstroem, b=12.180(2) Angstroem, c=12.912(4) Angstroem, Z=8, R=0.061, R_w=0.075.The molecule adopts a chair conformation with the 2-dimethylamino substituent axial.The sum of the bond angles about the exocyclic Me2N(2) (349.3(13) deg) shows a considerable deviation from planarity.The exocyclic P-N(2) bond length is unusually long (1.644(5) Angstroem).The molecules are intermolecularly hydrogen bonded in chains by way of the N(3)H and phosphoryl oxygen.From (1)H NMR measurements at 300 MHzalong with IR results, it is concluded that the same chair conformation is the predominant one populated in solution, by contrast to the known corresponding 1,3,2-dioxaphosphorinane for which Me2N is equatorial.It is concluded that the dimethylamino group is an effectively smaller substituent (lower axial conformational energy) in the 1,3,2-oxazaphosphorinane ring and in that system is also smaller than the mustard moeity N(CH2CH2Cl)2.Intermolecular hydrogen bonding can play a secondary role in determining conformation.The P-N(2) bond lengthening and pyramidal N(2) are attributed to the nonoptimal conformation about the P-N bond required for the axial Me2N to avoid potential 1,3-syn-axial repulsive steric interactions with the axial hydrogens at C4 and C6.