First isolation and characterization of an anti-apicophilic spirophosphorane bearing an oxaphosphetane ring: a model for the possible reactive intermediate in the Wittig reaction.

An anti-apicophilic phosphorane bearing an oxaphosphetane ring, in which the ring carbon is apical and the ring oxygen is equatorial (C-apical), has been prepared as a thermally less stable stereoisomer of a phosphorane with an ordinary equatorial carbon-apical oxygen array in the oxaphosphetane ring (O-apical). This novel C-apical phosphorane, which could be considered to be a model compound of the reactive intermediate in the Wittig reaction, was fully characterized by NMR and X-ray structural analysis. The compound was found to easily isomerize to its more stable O-apical isomer, especially in the presence of proton sources, and the latter O-apical compound was found to furnish olefin at elevated temperatures.