HYDROSILATION OF THE MANGANESE ACETYL (CO)(5)MNC(O)CH3 WITH MONOHYDROSILANES

Treatment of the manganese acetyl (CO)(5)MnC(O)CH3 (1) with 1-2 equiv of a monohydrosilane furnished mixtures of alpha-siloxyethyl (CO)(5)Mn CH(OSiR(3))CH3 (2) and alpha-siloxyvinyl (CO)(5)MnC(OSiR(3))=CH2 (3) c omplexes. Relative yields of 2 and 3 varied from 80% and 6% for HSiMe( 2)Ph to 28% and 59% for HSiEt(3). One of the latter compounds, (CO)(5) MnC(OSiEt(3))=CH2 (3j), was fully characterized, and seven examples of 2 were isolated in moderate yields. Four alpha-siloxyethyl complexes 2 were further characterized as stable derivatives (CO)(5)MnC(O)CH(OSi R(3))CH3 after carbonylation. Mechanistic studies on the HSiMe(2)Ph an d HSiEt(3) hydrosilation of 1 are noteworthy for (1) the absence of (R (3)SiO)CH=CH2 and CH(3)CH(2)OSiR(3) byproducts, (2) the presence of 3 but not (CO)(5)MnSiR(3), (3) inhibition by CO, phosphine, or acetonitr ile, but neither air nor light, (4) competitive hydrosilation of other substrates (e.g., acetone or Cp(CO)(2)FeC(O)R) for which 1 is a preca talyst, (5) degradation of 2 by excess HSiR(3), giving Mn-2(CO)(10) an d (R(3)Si)(2)O as the final products, (6) the fact that this degradati on results in autocatalysis by generating the transient active catalys t (CO)(4)MnSiR(3) (15), and (7) the fact that the hydrosilation induct ion period can be removed by independently generating the putative 15. These observations are consistent with an autocatalytic hydrosilation mechanism in which silane degradation of product 2 (or of other manga nese complexes) generates the active catalyst 15, which binds 1 and re arranges to the unsaturated bimetallic mu-siloxyethylidene (CO)(5)MnC( CH3)(OSiMe(2)Ph)Mn(CO)(4) as the key catalysis intermediate: silane ad dition affords 2 whereas beta-deinsertion produces 3.