The role of pentaco-ordinate species in the mechanism of the Arbusov reaction

Abstract

The Arbusov reaction between cis- and trans-2-ethoxy- and 2-isopropoxy-4-methyl-1,3,2-dioxaphosphorinan and alkyl iodides is non-stereospecific. In general, the amount of stereomutation appears to depend upon the nature of the alkoxy groups on the reactant P^III ester. Unchanged 2-alkoxy-4-methyl-1,3,2-dioxaphosphorinan recovered from interrupted Arbusov reactions shows that some of the less stable cis-phosphite has been converted to the trans-isomer, demonstrating that the Arbusov reaction pathway provides a route for this isomerisation. Pentacovalent intermediates are postulated to account for these observations. An approximate model of the conventional two-step mechanism for this reaction, which involves no pentaco-ordinate species, was set up and shown to proceed stereospecifically.