Hydrosilylation of Carbonyl-Containing Substrates Catalyzed by an Electrophilic η1-Silane Iridium(III) Complex

Abstract

Hydrosilylation of a variety of ketones and aldehydes using the cationic iridium catalyst (POCOP)Ir(H)(acetone)⁺, 1 (POCOP = 2,6-bis(di-tert-butylphosphinito)phenyl), is reported. With triethyl silane, all but exceptionally bulky ketones undergo quantitative reactions employing 0.5 mol % catalyst in 20−30 min at 25 °C. Hydrosilylation of esters and amides results in over-reduction and cleavage of C−O and C−N bonds, respectively. The diastereoselectivity of hydrosilylation of 4-tert-butyl cyclohexanone has been examined using numerous silanes and is highly temperature dependent. Using EtMe₂SiH, analysis of the ratio of cis:trans hydrosilylation products as a function of temperature yields values for ΔΔH^‡ (ΔH^‡(trans) − ΔH^‡(cis)) and ΔΔS^‡ (ΔS^‡(trans) − ΔS^‡(cis)) of −2.5 kcal/mol and −6.9 eu, respectively. Mechanistic studies show that the ketone complex (POCOP)Ir(H)(ketone)⁺ is the catalyst resting state and is in equilibrium with low concentration of the silane complex (POCOP)Ir(H)(HSiR₃)⁺. The silane complex transfers R₃Si⁺ to ketone, forming the oxocarbenium ion R₃SiOCR′₂⁺, which is reduced by the resulting neutral dihydride 3, (POCOP)Ir(H)₂, to yield product R₃SiOCHR′₂ and (POCOP)IrH⁺, which closes the catalytic cycle.