Exchange and insertion reactions involving borane adducts of phosphirane and phosphirene: a G2(MP2) ab initio investigation

Ab initio molecular orbital calculations at the G2(MP2) level have been employed to investigate the insertion and exchange reactions between unsaturated hydrocarbons and borane adducts of three-membered phosphorus heterocycles. Exchange is predicted to be energetically highly preferred over insertion, despite a large thermodynamic driving force for the latter. However, the present results suggest that the barriers for exchange are too high for the reaction to be observed experimentally. The barriers for exchange and insertion are considerably greater than those for the corresponding reactions of the related phosphiranium and phosphirenium ions. The higher barriers for exchange in the borane adducts can be rationalized in terms of the characteristic principal electronic interaction of exchange transition structures. (C) 1999 Elsevier Science S.A. All rights reserved.