NOVEL PI-LIGAND EXCHANGE AND INSERTION REACTIONS INVOLVING 3-MEMBEREDPHOSPHORUS-HETEROCYCLES - AN AB-INITIO INVESTIGATION

Ab initio molecular orbital calculations at the G2 level of theory hav e been used to characterize the potential energy surface for the react ions of phosphirenium or phosphiranium ions with acetylene or ethylene . Comparisons between two competing pathways for pi-ligand exchange ar e discussed along with comparisons between the lower energy exchange p athway and a competing insertion reaction. The lower energy exchange r eactions have barriers (4-39 kJ mol(-1)) much smaller than those of th e insertion reactions (215-234 kJ mol(-1)), despite the fact that the insertion products are strongly preferred thermodynamically. This is c onsistent with the exclusive observation of exchange experimentally. T he transition structures for the preferred exchange reactions contain PH2+ moieties that are rotated so that they are almost parallel to the CC bond of the ring and the CC bond of the incoming hydrocarbon, lead ing to the prediction that the reactions are stereospecific with respe ct to the configuration at phosphorus.