Dramatic Increase in the Rate of Olefin Insertion by Coordination of Lewis Acids to the Oxo Ligand in Oxorhenium(V) Hydrides

In this work we show that classic coordination of the oxo group in an oxorhenium hydride complex to M(C6F5)(3) (M = Al, B) leads to dramatic increases in the rate of migratory olefin insertion. Combined experimental and computational studies have been utilized to understand the reasons for the rate enhancement upon coordination of the oxo group to the Lewis acid. The mechanism for migratory insertion involves coordination of the olefin to rhenium in the equatorial plane. This induces mixing of the rhenium-hydride sigma bond with a rhenium-oxygen pi* orbital. This results in an accumulation of electron density on the oxo ligand. The Lewis acid lowers the barrier for migratory insertion by diminishing the electron density on the oxo ligand in the transition state.