Favoured conformations of methyl isopropyl, ethyl isopropyl, methyl tert-butyl, and ethyl tert-butyl 2-(tri-phenylphosphoranylidene)malonate

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 molecular structures of four closely related diesters, namely methyl isopropyl 2-(triphenyl-phosphoranylidene) malonate, C25H25O4P, ethyl isopropyl 2-(triphenylphosphoranylidene) malonate, C26H27O4P, methyl tert-butyl 2-(triphenylphosphoranylidene) malonate, C26H27O4P, and ethyl tert-butyl 2-(triphenylphosphoranylidene)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 intermolecular interactions, 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 alkoxy groups oriented towards the P atom. The crystal structures are controlled by nonconventional hydrogen-bonding and intramolecular interactions between cationoid P and acyl and alkoxy O atoms in syn positions.