Hydrosilation of Carbonyl-Containing Substrates Catalyzed by an Electrophilic η-Silane Iridium(III) Complex

Hydrosilation of a variety of ketones and aldehydes using the cationic iridium catalyst, (POCOP)Ir(H)(acetone)(+), 1, (POCOP = 2,6-bis(di-tert-butyl phosphinito)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. Hydrosilation of esters and amides results in over-reduction and cleavage of C-O and C-N bonds, respectively. The diastereoselectivity of hydrosilation of 4-tert-butyl cyclohexanone has been examined using numerous silanes and is highly temperature dependent. Using EtMe(2)SiH, analysis of the ratio of cis:trans hydrosilation 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 e.u., 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)(HSiR(3))(+). The silane complex transfers R(3)Si(+) to ketone forming the oxocarbenium ion, R(3)SiOCR'(2) (+), which is reduced by the resulting neutral dihydride 3, (POCOP)Ir(H)(2), to yield product R(3)SiOCHR'(2) and (POCOP)IrH(+) which closes the catalytic cycle.