Ketone hydrosilation, asymmetric

Rh(P2) 22+ dimers. The new [Rh(P2)(P)2]+ complexes can be isolated as their PFg- complexes. These Rh(I) complexes are air-stable in the solid state and serve as extremely active and selective (1,2- vs. 1,4-hydrosilation) asymmetric hydrosilation catalysts for the reduction of ketones with disubstituted silanes. 

Rhodium catalyzed asymmetric hydrosilation of ketones is an excellent route to chiral alcohols in reasonable chemical yields. The reaction occurs by treatment of alkyl... 

Discrete Chiral Rhodium Phosphine Complexes as Catalysts for Asymmetric Hydrosilation of Ketones... 

The catalytic asymmetric hydrosilation of a prochiral ketone to the corresponding chiral silyl ether followed by a mild hydrolysis is in principle an attractive route for the preparation of chiral alcohols that has die advantage that it would not require high hydrogen pressure to effect the reduction (1-6). Despite the synthetic potential only limited application of this technique to the synthesis of complex organic molecules has been made (7-9). This is in part due to the relatively low optical... 

Palladium-catalyzed asymmetric hydrosilation of cychc and linear 1,3-dienes proceeds as 1,4-addition with moderate to good enantioselectivities. a-Ketoesters snch as pyruvates can be hydrosilated with Rh(I)-(-t-)-DIOP catalysts to yield lactates with much better optical purity than simpler ketones (equation 22). This effect is likely to be cansed by a ligand fimction of the ester moiety in the transition state. 

Optically active, saturated carbonyl compounds and allylic alcohols were prepared via 1,4- and 1,2-asymmetric hydrosilation of enones using Rh catalysts bearing chiral ligands. For example, 1,4-hydro-silation of a,p-unsaturated ketones afforded the corresponding optically active ketones in 1.4-15.6% ee (Scheme 63). ... 

Asymmetric 1,2-hydrosilation in benzene of a,p-unsaturated ketones with dihydridosilanes and a chiral Rh catalyst produced allylic alcohols with up to 69% Highly stereoselective hydrosilation of an a,P-unsaturated aldehyde was achieved with triethylsilane and nonchiral Wilkinson catalyst. Dehy-drofaranal was thus stereoselectively reduced to the insect pheromone (3S,4/ )-faranal with 85% de (Scheme 64). 

Homogeneous catalytic hydrosilation of polar multiple bonds such as C=0 and C=N bonds is a useful synthetic tool as a better substitute for catalytic hydrogenation. Chiral rhodium complexes are employed for the asymmetric reduction of ketones (eq(32)) [29]. 

Directed and Asymmetric Reduction The principles of directed and asymmetric reactions were first developed for hydrogenation, as discussed in Section 9.2. Asymmetric hydrosilation of ketones can now be carried out cata-lytically with rhodium complexes of diop (9.22). The new chiral ligand Et-duPHOS, made by Burk at du Pont, allows chiral amination of ketones via Eq. 14.50. Note how the use of the hydrazone generates an amide carbonyl to act as a ligand, as is known to favor high e.e. (see Section 9.2). Noyori s powerful BINAP ligand has been applied to a large number of asymmetric reactions. 

Intramolecular Hydrosilation of Carbonyl Groups. Catalytic asymmetric intramolecular hydrosilation of the carbonyl group of an a-hydroxy ketone has been attained by using chlorodimethylsilane as the silylating agent and a rhodium catalyst containing an optically active bidentate ligand (eq 2). ... 

Unlike the asymmetric hydrosilation of alkenes and acetylenes where Pd catalysts seem to dominate activity, for enantioselective hydrosilation of ketones a preponderance of Rh-based catalysts have been used, although a few other metals including Pd have been examined and some ligand templates such as binaphthyl are in common. 

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