Dynamical Origin of Rebound versus Dissociation Selectivity during Fe-Oxo-Mediated C–H Functionalization Reactions

The mechanism of catalytic C–H
functionalization of alkanes
by Fe-oxo complexes is often suggested to involve a hydrogen atom
transfer (HAT) step with the formation of a radical-pair intermediate
followed by diverging pathways for radical rebound, dissociation,
or desaturation. Recently, we showed that in some Fe-oxo reactions,
the radical pair is a nonstatistical-type intermediate and dynamic
effects control rebound versus dissociation pathway selectivity. However,
the effect of the solvent cage on the stability and lifetime of the
radical-pair intermediate has never been analyzed. Moreover, because
of the extreme complexity of motion that occurs during dynamics trajectories,
the underlying physical origin of pathway selectivity has not yet
been determined. For the reaction between [(TQA_Cl)FeIVO]+ and cyclohexane, here, we report explicit solvent
trajectories and machine learning analysis on transition-state sampled
features (e.g., vibrational, velocity, and geometric) that identified
the transferring hydrogen atom kinetic energy as the most important
factor controlling rebound versus nonrebound dynamics trajectories,
which provides an explanation for our previously proposed dynamic
matching effect in fast rebound trajectories that bypass the radical-pair
intermediate. Manual control of the reaction trajectories confirmed
the importance of this feature and provides a mechanism to enhance
or diminish selectivity for the rebound pathway. This led to a general
catalyst design principle and proof-of-principle catalyst design that
showcases how to control rebound versus dissociation reaction pathway
selectivity.