PHOSPHONIUM SALTS AND PHOSPHORANES .6. P-31 NMR-SPECTRA OF ACYLMETHYLENEPHOSPHORANES

The 31P NMR spectra of 36 acylmethylenephosphoranes have been measured and the chemical shifts have been interpreted in terms of steric interactions and the resonance stabilization of the ylides. It is proposed that the observed solvent shift effects are best accounted for by a change in the resonance stabilization and not, as previously suggested, by a change in the conformational equilibrium position. The inductive electron-withdrawing interaction of α-aryl groups leads to a deshielding of the phosphorus nucleus, whilst the deshielding effect produced by α-acyl substituents can be rationalised in terms of a combination of the mesomeric and inductive interactions. Syntheses of 21 new phosphoranes are described. © 1979.