Manganese Carbonyl Complexes as Catalysts for the Hydrosilation of Ketones: Comparison with RhCl(PPh₃)₃

Abstract

Manganese carbonyl complexes catalyze the hydrosilation of ketones with PhMe2SiH and Ph2SiH2 in C6D6 solutions. Efficacy of the manganese carbonyl precatalysts (2.4 mol %) toward acetone hydrosilation with 1.1 equiv of PhMe2SiH to give (CH3)2CH(OSiMe2Ph) (3) varied: (PPh3)(CO)4MnC(O)CH3 (1) (<5 min) ≫ (CO)5MnC(O)Ph > (CO)5MnC(O)CH3 > (CO)5MnCH3 > (CO)5MnBr (6.0 h) ≫ Mn2(CO)10 ≈ (PPh3)(CO)4MnBr ≈ (CO)5MnSiMe2Ph (2). A turnover frequency of 27 min-1 was measured for catalysis using 1% 1; rapid catalysis was possible with 0.1% 1 in the absence of solvent. As a precatalyst, 1 is much more reactive than Rh(PPh3)3Cl for the PhMe2SiH hydrosilation of acetone, acetophenone, and cyclohexanone; both catalysts exhibit similar reactivity with Ph2SiH2. With 1 as the precatalyst, isolated yields of the alkoxydimethylphenylsilanes exceeded 90%, with no evidence of competing dehydrogenative silation to yield vinyl silyl ethers. Photochemical activation of (CO)5MnSiMe2Ph (2) affords moderate hydrosilation catalytic activity in transforming acetone to 3. In contrast, (CO)4CoSiMe2Ph (10) or Co2(CO)8 in the presence of the excess HSiMe2Ph, with or without photochemical activation, were ineffective acetone hydrosilation catalysts. A reaction pathway is presented for the manganese carbonyl-catalyzed hydrosilation of ketones that involves coordinatively unsaturated manganese silyl intermediates, (L)(CO)MnSiR‘‘3, as the active catalysts.