Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part XIII. 1H Nuclear magnetic resonance studies of 3-aryl-2,3-dihydro-1,3,2-benzoxazaphosph(V)oles and their 2-oxoanalogues

Abstract

¹ H N.m.r. spectra of two new series of phosphorus compounds are described, namely the tetraco-ordinate 3-aryl-2,3-dihydro-2-oxo-1,3,2-benzoxazaphospholes (2) and the pentaco-ordinate trigonal bipyramidal 3-aryl-2,3-dihydro-1,3,2-benzoxazaphosph(V)oles (1). Both types of compound generally exhibit a marked shielding of one of the aromatic protons by the second aromatic ring, the extent of which points to significant sterically induced antiplanarity between the two aromatic systems. The spectra of compounds (1) are temperature-dependent, indicating ligand reorganisation in the trigonal bipyramid. Measurements of coalescence temperatures and hence of ΔG* for these processes indicate that the steric barriers already mentioned markedly influence the ease of ligand reorganisation.