Formyl-, aroyl, and acyl-alkylidenetriphenylphosphoranes: conformational analysis by proton nuclear magnetic resonance spectroscopy

Abstract

The conformations of eleven oxoalkylidenetriphenylphosphoranes have been assigned from their ¹H n.m.r. spectra. Mixtures of (Z)cisoid and (E)transoid conformers were detected in four compounds. The Z-geometry of β-oxomethylenetriphenylphosphoranes is favoured by the presence of β-substituents and the E-geometry is favoured by the presence of α-substituents. The conformational preferences are rationalised in terms of steric and electrostatic interactions. The absence of peak coalescence in the p.m.r. spectra up to 150° shows that interconversion of the conformers is very slow relative to the n.m.r. time scale.