Hydrosilylation of Carbonyl Groups

The reduction of ketones with silicon hydrides has been occasionally performed by radical chemistry for a synthetic purpose. The radical adduct is stabilized by the a-silyloxyl substituent and for RsSi (R = alkyl and/or phenyl) the hydrogen abstraction from the parent silane is much slower than a primary alkyl radical (cf. Chapter 3). On the other hand, (TMS)3SiH undergoes synthetically useful addition to the carbonyl group and the reactions with dialkyl ketones afford yields 70% under standard experimental conditions, i.e., AIBN, 80-85 °C [45,51]. Reaction (5.25) shows as an example the reduction of 4-tcrt-butyl- 

It has been reported that (TMS)3SiCl can be used for the protection of primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable to the usual conditions employed in organic synthesis for the deprotection of other silyl groups and can be deprotected using photolysis at 254 nm, in yields ranging from 62 to 95%. Combining this fact with the hydrosilylation of ketones and aldehydes, a radical pathway can be drawn, which is formally equivalent to the ionic reduction of carbonyl moieties to the corresponding alcohols. 

Reactions (5.27) and (5.28) provide good evidence for the participation of radical intermediates in the initial hydrosilylation process [51,56]. Indeed, the 

R = cyclohexyl R = 1-naphthyl Ph Vp O-SiCb CbSiH Ph O-s iCb 93%, synianti = 64%, synianti = 

A variety of carbonyl compounds react with PhSeSiMes in the presence Bu3SnH/AIBN to afford the corresponding hydrosilylation derivatives [59,60]. Generally good yields are obtained only for aromatic substituted aldehydes or ketones. Reactions (5.28) and (5.29) show this case for a few aldehydes 

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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. 

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). 

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]. 

Figure 10.16 (a-c) Well-defined Ni(II) complexes catalyzing the hydrosilylation of carbonyl groups. 

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],... 

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... 

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. 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 ... 

Reduction of carbonyl groups can be achieved by catalytic hydrosilylation, followed by hydrolysis. Hydrosilanes add to ketones and aldehydes more easily than to alkenes... 

Formation of C-H Bonds by Reduction of Carbonyl Groups via Hydrosilylation and Subsequent Hydrolysis... 

Catalytic Hydrosilylation of Unsaturated Carbon-Heteroatom Bonds. Hydrosilylation of ketone produces silyl ether, which can be easily converted to alcohol via an additional hydrolysis (deprotection) step (Scheme 28). Analogously, hydrosilylation of imine leads to the formation of silylamine, which can be conveniently transformed to amine. Hydrosilylation of carbonyl (imine) group with subsequent hydrolysis is often referred to as the reduction by silanes. 

Regarding the use of well-defined nickel complexes as catalysts for reduction of carbonyl groups, only three examples are described in the literature. In 2009, Guan and coworkers [77] described the efficiency of a nickel PCP-pincer complex performing the hydrosilylation of aldehydes. In the same year, the catalytic hydrosilylation of ketones via a transient Ni-H complex supported by a monoanionic bidentate amidophosphine ligand was reported by Mindiola [78]. Later, Jones investigated well-defined PNP nickel pincer complexes, which catalyzed the hydrosilylation of aldehydes [79] (Fig. 10.16). 

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