Manganese carbonyl hydrosilation catalysis: Reducing organic carbonyl functionalities

Nearly all manganese carbonyl silyl complexes (CO)5MnSiR 3 (3) reported to date were synthesized via metalation of the requisite silyl chloride with a weakly nucleophilic manganese carbonylate lithium, sodium, and more recently potassium salts, (CO)5MnK. A photochemical procedure for conveniently preparing (CO)5MnSiMe 2Ph (3b) from the aryl manganese (CO)5Mn- p-CH2C6H4OCH3 ( 7) was developed. Irradiation of 7 in the presence of excess PhMe2SiH at 10--15°C for 1--1.5 h generates 3b in solution. Workup by low temperature column chromatography, -50°C to -35°C gives spectroscopically pure 3b in yields of 70--85%.;The thermal reaction of (CO)5MnCH3 (2) with 2--3 equivalents of PhMe2SiH produces (CO)4MnH(SiMe 2Ph)2 (4a) and 3b, 75% and 7% respectively. Reacting 2 with silane under 80 psi of CO at 40°C or treating 4a generated in situ results in complete transformation to 3b . It is believed that the presence of CO, under thermal conditions, inhibits the formation of 4a by favoring a pathway in which loss of silane by 4a gives an unsaturated silyl intermediate (CO) 4MnSiMe2Ph (1a) which then preferentially associates CO over silane forming 3b. Thermal and photochemical attempts to convert (CO)5MnSiMe2Ph (3b) to (CO) 4MnH(SiMe2Ph)2 (4a) in the presence of excess silane were unsuccessful.;As the most frequently used manganese silyl, (CO)5MnSiMe 3 (3a) has figured prominently in the development of the chemistry of the manganese-silicon bond. Gladysz and co-workers established that coordinatively saturated complex 3areacts with most oxygenated functional groups by ionic silation processes. The manganese silyl species, (CO)4MnH(SiMe2Ph)2 (4a), (CO) 5MnSiMe2Ph (3b), (CO)5MnSiMePh 2 (3d), and (CO)5MnSiPh3 ( 3e) were studied for their efficacy in hydrosilation and hydrosilation-deoxygenation of acetone and ethylacetate. The aforementioned manganese silyls efficiently catalyze the hydrosilation and the hydrosilation-deoxygenation of acetone and ethylacetate respectively.;Catalysis is believed to occur not by ionic silation processes but via a reversible oxidative addition and reductive elimination pathway. We postulate that the active catalyst in such a pathway to be (CO)4MnSiMe 2Ph (1a). Upon generation of (CO)4MnSiMe 2Ph (1a), the substrate binds to 1a and rearranges to give the requisite unsaturated siloxy adduct. Addition of silane and subsequent reductive elimination gives the hydrosilated substrate. In the case of ethylacetate, further reduction occurs by (CO)4MnH(SiMe2Ph)2 (4a) to afford ether and disiloxane (10), or siloxyether (9).