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.