Catalyst Behavior in Metal-Catalyzed Carbonyl-Olefin Metathesis

Iron­(III)-catalyzed
carbonyl-olefin ring-closing metathesis employs
reactivity not typically observed in Lewis acid-catalyzed reactions.
In converting a ketone with a pendant olefin into a cycloalkene and
a simple carbonyl byproduct, the reaction requires the Lewis acid
catalyst to differentiate between the carbonyl of the substrate and
that of the byproduct. It is necessary to determine how this solution
interaction imparts the desired reactivity to best employ this method.
Herein, we report detailed kinetic, spectroscopic, and colligative
measurements applied toward the identification of the solution structures
of the active Fe­(III) and Ga­(III) carbonyl-olefin metathesis catalysts.
These data are consistent with formation of Lewis acid-carbonyl pairs
for both metal systems under stoichiometric conditions. However, they
diverge in the presence of higher equivalents of carbonyl, with Fe­(III)
forming highly ligated complexes, and no observed change for Ga­(III).
These findings are consistent with the resting state identity of the
Fe­(III) metathesis catalyst changing over the course of the reaction.