Computational Design of an Iron Catalyst for Olefin Metathesis

Olefin metathesis is a versatile
reaction in synthetic chemistry.
The use of iron compounds as olefin metathesis catalysts has attracted
interest but has not succeeded because most Fe complexes catalyze
cyclopropanation reactions preferentially to cycloreversion, which
is a key step in the metathesis reaction cycle. In the present study,
we performed density functional theory calculations to study the catalytic
effect of ancillary pincer ligands in an Fe organometallic catalyst
for olefin metathesis. We show here that the partial atomic charge
on Fe in the metallocyclobutane intermediate is a descriptor for the
tendency of the Fe complex to favor olefin metathesis over cyclopropanation.
The results suggest that the pincer ligands can decrease the partial
atomic charge on the Fe ion and that decreasing this charge can make
the desired cycloreversion reaction more energetically favorable than
the cyclopropanation side reaction. In this way, we found that the
Fe carbene stabilized by N-heterocyclic dicarbene ligands (Fe­(C–N–C)­(CHCH3), where C–N–C denotes 2,6-bis­(methylimidazol-2-ylidene)­pyridine),
is a potential candidate for catalyzing the olefin metathesis reaction.
We then calculated the whole catalytic cycle for the propylene metathesis
reaction catalyzed by the Fe­(C–N–C) carbene, and the
results indicate that the cyclopropanation reaction is disfavored
by ∼4 kcal/mol. We verified the stability of the proposed Fe­(C–N–C)
catalyst by considering two possible deformation pathways: the transformation
of the carbene ligand to a coordinated alkene molecule and the formation
of a hydride carbyne intermediate followed by the insertion of a propylene
molecule. We found that neither deformation pathway is energetically
favorable. Finally we propose a viable route to synthesize the Fe­(C–N–C)
carbene complex by utilizing the already available Fe­(N2)2 complex stabilized by an N-heterocyclic dicarbene ligand.