The Phosphorus Version of the Oxaspiropentene−Cyclobutenone Rearrangement

The phosphatriafulvene complex 1 is sulfurized at phosphorus by reaction with propylene sulfide at room temperature. The X-ray crystal structure of 2 shows a long P−S bond (2.0039(5) Å) compatible with a zwitterionic formulation. The reaction of 1 with propylene oxide takes place at 110 °C and leads to the ring-expanded 1-phosphacyclobutenone 3. DFT calculations support the intermediacy of a phosphatriafulvene epoxide and its rearrangement to a four-membered ring, paralleling the oxaspiropentene−cyclobutenone conversion.