Hydrosilylation ketones, -NORPHOS

Enantioselective Hydrosilylation of C=0 Double Bonds in Ketones. The use of Rh-phosphorane catalyst systems to promote asymmetric hydrosilylation of prochiral ketones with silanes of the type RsSiH has met with only limited success. Thus, hydrosilylation of acetophenone with Ph2SiH2 promoted by [Rh(COD)Cl]2-(S,S)-DIOP catalyst afforded the (S)-(-)-phenylmethylcarbonyl with an optical yield of 32% ee. Similarly, the use of a Rh-NORPHOS catalyst in this reaction proceeded with an optical induction of only 16% ee. S... 

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)." ... 

Hydrosilylation of acetophenone to give silyl ether (M R = Me) can also be achieved using copper(I) complexes with the chiral phosphine ligands (-)-DIOP (87) or (+)-NORPHOS (88). The enantioselectiv-ity is rather low, but a nonphosphine auxiliary, PYTHIA (89) with a Rh(COD)Ch catalyst using neat diphenylsilane reduces aryl ketones to (R)-l-phenylethyl alcohol silyl ethers in high yield and with high enantiomeric excess (Scheme 18). " ... 

In the absence of xmsaturation, for example, in simple ketones or after 1,4-reduction has already occurred in a,p-unsaturated ketones, the 1,2-manifold is likely to be the main reaction pathway (27 28, Scheme 7). Since the pioneering report by Brunner [16] on a modestly enantioselective Cu-catalyzed hydrosilylation of acetophenone with diphenylsilane using (-l-)-NORPHOS (L6) (up to 39% ee) followed by the discovery by Lipshutz [17] of a highly enantioselective hgand-accelerated reduction of aryl ketones using BIPHEP (L4), various aspects of the Cu-catalyzed asymmetric 1,2-addition of a hydride to simple ketones have been under close scrutiny [2 ]. 

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