Mechanism of the Bis(imino)pyridine-Iron-Catalyzed Hydromagnesiation of Styrene Derivatives

Iron-catalyzed hydromagnesiation
of styrene derivatives offers
a rapid and efficient method to generate benzylic Grignard reagents,
which can be applied in a range of transformations to provide products
of formal hydrofunctionalization. While iron-catalyzed methodologies
exist for the hydromagnesiation of terminal alkenes, internal alkynes,
and styrene derivatives, the underlying mechanisms of catalysis remain
largely undefined. To address this issue and determine the divergent
reactivity from established cross-coupling and hydrofunctionalization
reactions, a detailed study of the bis­(imino)­pyridine iron-catalyzed
hydromagnesiation of styrene derivatives is reported. Using a combination
of kinetic analysis, deuterium labeling, and reactivity studies as
well as in situ 57Fe Mössbauer spectroscopy, key
mechanistic features and species were established. A formally iron(0)
ate complex [iPrBIPFe­(Et)­(CH2CH2)]− was identified as the
principle resting state of the catalyst. Dissociation of ethene forms
the catalytically active species which can reversibly coordinate the
styrene derivative and mediate a direct and reversible β-hydride
transfer, negating the necessity of a discrete iron hydride intermediate.
Finally, displacement of the tridentate bis­(imino)­pyridine ligand
over the course of the reaction results in the formation of a tris-styrene-coordinated
iron(0) complex, which is also a competent catalyst for hydromagnesiation.