Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

A detailed mechanistic study of the
catalytic hydrosilylation of
ketones with the highly active and enantioselective iron­(II) boxmi
complexes as catalysts (up to >99% ee) was carried out to elucidate
the pathways for precatalyst activation and the mechanism for the
iron-catalyzed hydrosilylation. Carboxylate precatalysts were found
to be activated by reduction of the carboxylate ligand to the corresponding
alkoxide followed by entering the catalytic cycle for the iron-catalyzed
hydrosilylation. An Eyring-type analysis of the temperature dependence
of the enantiomeric ratio established a linear relationship of ln­(S/R) and T–1, indicating a single selectivity-determining step over the whole
temperature range from −40 to +65 °C (ΔΔG‡sel, 233 K = 9
± 1 kJ/mol). The rate law as well as activation parameters for
the rate-determining step were derived and complemented by a Hammett
analysis, radical clock experiments, kinetic isotope effect (KIE)
measurements (kH/kD = 3.0 ± 0.2), the isolation of the catalytically active
alkoxide intermediate, and DFT-modeling of the whole reaction sequence.
The proposed reaction mechanism is characterized by a rate-determining
σ-bond metathesis of an alkoxide complex with the silane, subsequent
coordination of the ketone to the iron hydride complex, and insertion
of the ketone into the Fe–H bond to regenerate the alkoxide
complex.