A theoretical study on the mechanism of boron metathesis

Abstract

The mechanism of the boron metathesis reaction of the transition-metal-aminoborylene complex Cp(CO)₂FeBN(CH₃)₂⁺ (8) with EX, where EX = H₃PO (9ap), H₃AsO (9bp), H₃PS (9aq), H₃AsS (9bq), CH₃CHCH₂ (9cr), (NH₂)₂CCH₂ (9dr), H₂CO (9ep), and (NH₂)₂CO (9dp) is investigated at the B3LYP/LANL2DZ level. The analysis of bonding and charge distribution shows that the Fe-borylene complex (8) is a Fischer-type carbene analogue. The attack of the olefin takes place at the metal end of the M=C bond of the metal-carbene complex in olefin metathesis and proceeds via [2 + 2] cycloaddition, while in boron metathesis, the initial attack of the substrates takes place at the positively charged B atom of the Fe-borylene complex and forms the preferred acyclic intermediate. The energetics of boron metathesis is comparable to that of the olefin metathesis. Substrates that are polar and a have low-lying σ^∗ molecular orbital (weak s bond) prefer the boron metathesis reaction. The relative stability of the metathesis products is controlled by the strength of the Fe-E and B-X bonds of the products 13 and 14, respectively. We have also investigated the possibility of a β-hydride-transfer reaction in the Fe-borylene complex.