Carbonyl groups hydrosilylation

The transition metal catalysed addition of a hydridosilane to a multiply-bonded system is known as hydrosilylation (1). Under such conditions, alkynes undergo clear cis-addition, so providing one of the most direct routes to vinylsilanes (Chapter 3). Hydridosilanes also add to the carbonyl group of saturated aldehydes and ketones, to produce alkyl silyl ethers. Fot example, under suitable conditions, 4-t-butylcyclohexanone (2) can be reduced with a high degree of stereoselectivity. 

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). 

In the late 1950s it was established that silanes add across the carbonyl moiety of ketones to give the corresponding silyl ethers either by heating at ca 250 °C or by UV irradiation [40]. The occurrence of Reaction (5.22) as the key step in the hydrosilylation of a carbonyl group was substantiated in 1969 when the EPR spectra of the first adduct radicals were observed for acetone and trifluoroacetic acid [41]. At present, a large number of EPR parameters for this class of radicals are available and structural information has been obtained [42,43]. 

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

Hydrosilylation is the term used for the addition of a silicon-hydrogen bond to unsaturated linkages such as carbonyl groups, alkenes, and alkynes (Equations Si5.2-5). 

The hydrosilylation/methanolysis reaction sequence. One of the most convenient methods is to use a sequence of hydrosilylation/methanolysis reactions as shown in equation 4 for the formation of 39. This direct synthetic pathway was reported in cases of the alkyl family of precursors 1290. This sequence is also used for the preparation of dendrimers and arborols121,123. Alternatively, hydrosilylation with HSi(OEt)3 can be performed advantageously in the case of dendrimers125,126 but it is of more interest in the case of carbonate precursors 26111. Indeed, hydrosilylation of double bonds can be achieved selectively in the presence of a carbonyl group with hexachloroplatinic acid or a rhenium catalyst130. 

Although hydrosilanes reduce ketones, in trifluoroacetic acid, to the corresponding methylene compounds or dimeric ethers via ionic hydrogenation, the reduction of a-amino and a-oxy ketones and p-keto acid derivatives with hydrosilanes, particularly PhMe2SiH, under these conditions proceeded with high anti selectivity to the alcohols. No racemization was observed at the carbon a to the carbonyl group. Intramolecular hydrosilylation catalyzed by Lewis acids provided a highly stereoselective route to anti-1,3-diols from p-hydroxy ketones (Section 1.1.3. ). ... 

Although -3-pinanyl-9-borabicyclo[3.3.1]nonane and related substances have also been developed as efficient asymmetric reducing agents for carbonyl compounds (Volume 8, Chapter 1.3), we discuss here only asymmetric reductions using chirally modified metal hydride reagents. The asymmetric hydrosilyl-ation of a carbonyl group catalyzed by a chirally modified transition metal is mentioned briefly. 

Addition of the elements of Si—H to a carbonyl group produces silyl ethers which are synthetically equivalent to chiral secondary alcohols since the silyl groups are easily hydrolyzed. Hydrosilylation can be catalyzed by acids or transition metal complexes. Enantioselective hydrosilylation of prochiral ketones has been extensively studied using platinum or rhodium complexes possessing chiral ligands such as BMPP (86), DIOP (87), NORPHOS (88), PYTHIA (89) and PYBOX (90)." ... 

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]. 

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

Hydrosilylotion of carbonyl compounds. This rhodium complex is an effective catalyst for hydrosilylation of aldehydes, ketones, a,/3-unsaturated aldehydes and ketones, and a-diketones. Hydrosilylation followed by hydrolysis is equivalent to reduction of either the carbonyl group or the a, 3-un-saturation. 

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