Conformation and stereodynamics of 2-dialkylamino-1,3-dioxa- and 3-methyl-1,3-oxaza- 2-phospholanes. A carbon-13 nuclear magnetic resonance and theoretical MNDO investigation

Abstract

Rotation around the exocyclic P–NR₂ bond in the title compounds (R = Me, Et, and Prⁱ) has been frozen on the ¹³C n.m.r. time-scale at –100 to –150 °C. The exocyclic PNC coupling constants confirm that the preferred conformation has one NR group eclipsing the phosphorus lone pair and the other lying over the face of the ring; this geometry is also supported by MNDO SCF MO calculations. The P-NR₂ rotational barriers, which lie in the range 6.0–9.2 kcal mol^–1, increase in the sequence 1,3-dioxa- < 1,3-oxaza- < 1,3-diazaphospholane and also with increasing size of the NR group. MNDO calculations of the barrier height reproduce the former sequence. The cyclic POC and PNC coupling constants are discussed in relation to the ring conformation, and some ¹⁵N n.m.r. data are reported.