Cycloaddition reactions between 2 H-phosphole and phosphaketene: Ab initio examination of [2 + 2] and [4 + 2] pathways

Abstract Staudinger ([2 + 2]) and Diels—Alder ([4 + 2]) addition reactions between 2 H -phosphole and phosphaketene were examined at the MP4SDQ/6-31G * //MP2/6-31G * + ZPE level. Electronic structures were analyzed with topological electron density and NBO analyses. The Diels—Alder reaction is favored kinetically by 6 kcal mol −1 and thermodynamically by 4 kcal mol −1 over the Staudinger reaction. This is in contrast to the reaction between ketene and cyclopentadiene, for which the Diels—Alder reaction requires a 12 kcal mol −1 higher activation energy than the Staudinger reaction. In both Staudinger and Diels—Alder reactions phosphaketene reacts at P C rather than at C O, as does ketene in the parent Staudinger reaction. The transition structure of the phospha Staudinger reaction is a closed ring, but the second bond is formed employing the phosphaketene phosphorus lone pair rather than the former P C bond. The reaction mechanism is therefore not [π 2 s + π 2 a].