MOLECULAR-STRUCTURE OF PHOSPHONIUM YLIDES

The structures of 12 phosphonium ylides were completely optimized at the HF/6-31G* level. Use of larger basis sets and electron correlation in optimizing the structures of methylenephosphorane and phosphorane cyclopropylide resulted in little geometric change. The phosphonium ylides can be grouped into two classes: (1) nonstabilized ylides and (2) stabilized ylides. All examples of the nonstabilized ylides have nonplanar ylidic carbon geometries, while the stabilized ylides have planar ylidic carbons. These structures are used to support the concept that the dominant resonance structure is the ylide form.