Hydrosilylation of carbonyls

Scheme 2.9 Hydrosilylation of carbonyl compounds to silyl ethers...

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

Scheme 2.10 Intermediates in the Cu-catalysed hydrosilylation of carbonyl compounds...

While it is beyond the scope of this chapter to cover the asymmetric hydrosilylation of ketones and imines in any detail, a number of the more catalytically active ML combinations will be mentioned here. A full review of the area has recently appeared.138 Asymmetric hydrosilylation of carbonyl groups is usually performed with rhodium or titanium catalysts bearing chelating N- or P-based ligands. Representative results for some of the most active Rh/L combinations (Scheme 32) for addition of Si H to acetophenone are given in Table 11. 

The reaction of carbonyl compounds, R(R )C=0, with PhSeSiMe3 and Bu3SnH in the presence of a catalytic amount of AIBN as a radical initiator results in the hydrosilylation of carbonyl compounds giving the corresponding silyl ethers, R(R )CH-OSiMe3.132 133... 

PhSeSiRs reacts with BusSnH under free radical conditions and affords the corresponding silicon hydride (Reaction 1.8) [19,20]. This method of generating RsSi radicals has been successfully applied to hydrosilylation of carbonyl groups, which is generally a sluggish reaction (see Chapter 5). 

Scheme 5.9 Propagation steps for hydrosilylation of carbonyl compounds using...

Ojima and co-workers first reported the RhCl(PPh2)3-catalyzed hydrosilylation of carbonyl-containing compounds to silyl ethers in 1972.164 Since that time, a number of transition metal complexes have been investigated for activity in the system, and transition metal catalysis is now a well-established route for the reduction of ketones and aldehydes.9 Some of the advances in this area include the development of manganese,165 molybdenum,166 and ruthenium167 complex catalysts, and work by the Buchwald and Cutler groups toward extension of the system to hydrosilylations of ester substrates.168... 

Supplement to Chapter 6.3 Hydrosilylation of Carbonyl and Imino Groups... 

Brunner H (1998) Hydrosilylation of carbonyl compounds. In Beller M, Bolm C (eds) Transition metals for organic synthesis, vol 2. Wiley-VCH, Weinheim, p 131... 

Nishiyama H (1999) Hydrosilylation of carbonyl and imino groups. In Jacobsen EN, Pfaltz A, Yamamoto H (eds) Comprehensive asymmetric catalysis, vol 1, chap 6.3. Springer, Berlin Heidelberg New York... 

Hydrosilylation of carbonyl compounds catalyzed by fluoride salts such as CsF and KF, originally developed by Corriu and coworkers161-165, has been modified to homogeneous systems and found applications in organic syntheses as an efficient and selective reduction method. 

Hydrosilylation of carbonyl compounds. The definitive report on reduction of carbonyl compounds by hydrosilylation catalyzed by Wilkinson"scatalyst is available.1 Hydrosilylation can be used to effect regioselective 1,2- or 1,4-reduction of a,/ -enals or -enones by proper choice of the hydrosilane. In general, monohydrosilanes favor 1,4-adducts, whereas dihydrosilanes favor 1,2-adducts. The regioselectivity is also influenced by the substituents on silicon and on the substrates. The presence of a phenyl group on the enone system can effect dramatic changes in the selectivity. Examples ... 

The hydrosilylation of carbonyl compounds with polymethylhydrosiloxane (PMHS) or other alkoxysilanes can be catalyzed by TBAF with high efficiency [20]. The asymmetric version of this process has been developed by Lawrence and... 

High-valent rhenium dioxo complexes, ReC>2l(PR3)2, catalyse the hydrosilylation of carbonyl compounds.264 DFT calculations suggest rate-determining dissociative addition of the Si-H bond (of H-Si-alkyl3) across the Re=0 bond, with subsequent carbonyl coordination and reduction. 

The hydrosilylation of carbonyl compounds with polymethylhydrosiloxane (PMHS) or other alkoxysilanes can be catalyzed by TBAF, at high efficiency [9]. The asymmetric version of this process has been developed by Lawrence and coworkers using chiral ammonium fluoride 7c prepared via the method of Shioiri [10]. The reduction of acetophenone was performed with trimethoxysilane (1.5 equiv.) and 7c (10 mol%) in THF at room temperature, yielding phenethyl alcohol quantitatively with 51% ee (R) (Scheme 4.6). A slightly higher enantioselectivity was observed in the reduction of propiophenone. When tris(trimethylsiloxy)silane was used as a hydride source, the enantioselectivity was increased, though a pro-... 

Tris(pentafluorophenyl)borane [B(CgFj)3], is a powerful and selective Lewis acid catalyst used in many reactions in organic chemistry [1 ]. Parks and Piers [5] found that B(CgFj)3 catalyzes the hydrosilylation of carbonyl compounds. The silylation of alcohols with the formation of Ft as the only by-product [6] and the cleavage of silyl ether and ether bonds catalyzed by B(C F5)3 [7, 8] provide an... 

Rhodium-phosphine complexes are usually active and effective in the asymmetric hydrosilylation of olefins, ketones, and aldehydes, allowing for the virtual synthesis of optically active alkoxysilanes and organic compounds of high purity. Chiral rhodium-phosphine catalysts predominate in the hydrosilylation of pro-chiral ketones. This subject has been comprehensively reviewed by several authors who have made major contributions to this field [52-54]. A mechanism for the hydrosilylation of carbonyl groups involving the introduction of asymmetry is shown in Scheme 3 [55]. 

After oxidative addition of an unsymmetrical cr bond (E-H) to the metal, an unsaturated compound can insert into either the M-E or the M-H bond. In some cases, such as the hydrosilylation of carbonyl compounds, the 7r bond of the substrate inserts into the M-E bond, whereas, in others, insertion of the ir bond of the substrate into the M-H bond occurs faster. In any case, either pathway gives the same product after reductive elimination. 

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