Conformations of diester triphenylphosphonium ylides with an ylidic ester or keto and ester ylidic groups.

The structures of three related keto diester and diester ylides, namely diethyl 3-oxo-2-(triphenylphosphoranylidene)glutarate, C(27)H(27)O(5)P, (I), diethyl 3-oxo-2-(triphenylphosphoranylidene)glutarate acetic acid monosolvate, C(27)H(27)O(5)P·C(2)H(4)O(2), (II), and diethyl 2-(triphenylphosphoranylidene)succinate, C(26)H(27)O(4)P, (III), are presented. The syn-keto anti-ester conformations in the crystalline keto diesters are governed by electronic delocalization between the P-C and ylidic bonds and an acyl group, and by intra- and intermolecular interactions. There are also intramolecular attractive and repulsive interactions of different types (C-H...O and C-H...π) controlling the molecular conformations. The mono-ylidic diester (III) has an anti-ester conformation, while those for (I) and (II) are related to pyrolytic formation of acetylene derivatives. The terminal nonylidic ester group in (I) was disordered over two sets of almost equally populated positions.