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)(2)FeBN(CH(3))(2+) (8) with EX, where EX = H(3)PO (9ap), H(3)AsO (9bp), H(3)PS (9aq), H(3)AsS (9bq), CH(3)CHCH(2) (9cr), (NH(2))(2)CCH(2) (9dr), H(2)CO (9ep), and (NH(2))(2)CO (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 sigma* molecular orbital (weak sigma 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 beta-hydride-transfer reaction in the Fe-borylene complex.