Abstract

The conformations of organic compounds determined in the solid state are important because they can be compared with those in solution and/or from theoretical calculations. In this work, the crystal and mol-ecular structures of four closely related diesters, namely methyl isopropyl 2-(triphenyl- phosphoranylidene)malonate, C25H25O4P, ethyl isopropyl 2-(triphenyl- phosphoranylidene)malonate, C26H27O4P, methyl tert-butyl 2-(triphenyl- phosphoranylidene)malonate, C26H27O4P, and ethyl tert-butyl 2-(triphenyl- phosphoranylidene)-malonate, C27H29O4P, have been analysed as a preliminary step for such comparative studies. As a result of extensive electronic delocalization, as well as intra- and inter-molecular inter-actions, a remarkably similar pattern of preferred conformations in the crystal structures results, viz. a syn-anti conformation of the acyl groups with respect to the P atom, with the bulkier alk-oxy groups oriented towards the P atom. The crystal structures are controlled by nonconventional hydrogen-bonding and intra-molecular inter-actions between cationoid P and acyl and alk-oxy O atoms in syn positions. © 2008 International Union of Crystallography.