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
σ bond with a rhenium–oxygen π* 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.