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

Abstract

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.

Cobalt Catalysts for Alkene Hydrosilylation under Aerobic Conditions without Dry Solvents or Additives

“…The first approach was focused on the rate law of the reaction. In order to face it, we varied the concentration of each component of the reaction, following a similar methodology than the one described by Bleith and Gade . For the hydrosilylation of 4‐vinylcyclohexene with PhSiH 3 with catalyst 3b , we found that the initial rate of the reaction was not appreciably affected as a function of 4‐vinylcyclohexene concentration, indicating a zeroth order dependence on alkene concentration (Figure S15).…”

“…In order to face it, we varied the concentration of each component of the reaction, following a similar methodology than the one described by Bleith and Gade. [28] For the hydrosilylation of 4-vinylcyclohexene with PhSiH 3 with catalyst 3b, we found that the initial rate of the reaction was not appreciably affected as a function of 4-vinylcyclohexene concentration, indicating a zeroth order dependence on alkene concentration ( Figure S15). The same effect was observed when the concentration of PhSiH 3 is varied, indicating a zeroth order dependence on silane concentration, as well ( Figure S16).…”

Low‐Temperature Hydrogenation of Carbon Dioxide to Methanol with a Homogeneous Cobalt Catalyst

“…Zuschriften 1918 www.angewandte.de several hours.Asimilar induction period was recently observed in iron-catalyzed hydrosilylation owing to the strong coordination of ac arboxylate ligand. [14] Following this reasoning,t he induction period should be avoided if the removal of acac ligands is assisted before the reaction starts.I ndeed, we found that when the reaction solution was heated at 100 8 8Cfor 2hbefore H 2 and CO 2 were added, the formation of methanol after 5.5 hwas improved by one order of magnitude (Scheme 2). Preheating in the presence of H 2 (70 bar) slightly improved the activity further, thus suggesting the reductive activation of the acac-containing precatalysts.…”

Radical‐Initiated Dismutation of Hydrosiloxanes by Catalytic Potassium‐Graphite

“…Hydrosilanes are convenient reagents in reduction reactions thanks to their polarized Si−H bond, which make them good hydride donors . They can be used in multiple reduction reactions using organometallic complexes or organic catalysts of inorganic salts . Among the most reactive hydrosilanes, silane (or monosilane), SiH 4 , can be obtained from the disproportionation of triethoxysilane (TES).…”

“…[1] They can be used in multiple reduction reactions using organometallic complexes or organic catalysts of inorganic salts. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Among the most reactive hydrosilanes, silane (or monosilane), SiH 4 , can be obtained from the disproportionation of triethoxysilane (TES). Silane has been used as precursor for the production of high purity silicon applications, [18][19][20] and for the hydrosilylation of olefins.…”