Conformations of monoylidic diester triphenylphosphonium ylides

In ylidic triphenylphosphonium carboxylic esters the ester oxygen can be oriented towards (syn) or away (anti) from phosphorus, but except for small ylidic ester groups, e.g., Me, the anti conformer is dominant. With suitable crystals conformations are established by X-ray crystallography, but HF and DFT computations, with NMR and IR spectroscopy, are useful methods. Bulky ylidic or nonylidic groups strongly favor the anti conformer and even with small carboxylic groups, e.g. ethoxy, anti conformers are preferred in solution and are dominant in the crystal. The balance of attractive interactions between anionoid oxygen and cationoid phosphorus and nonbonding interactions, controls conformations, as indicated by evidence from NMR and IR spectroscopy, HF and DFT calculations, and X-ray observations.