Dihydrosilane, hydrosilylation with

Asymmetric Hydrosilylation with Dihydrosilanes. Chiral alkoxysilanes have also been obtained in the rhodium catalyzed hydrosilylation of carbonyl compounds (68, 78, 79) (eq. [29]). 

The catalytic reduction of C=0 and C=N double bonds of a hydrogen acceptor (A) molecule may be carried out by direct addition of hydrogen (1), by transfer hydrogenation (2), or through hydrosilylation (3). In the case of transfer hydrogenation, the hydrogen necessary for reduction is abstracted from donor (D) organic compounds like secondary alcohols. Hydrosilylation with mono- or dihydrosilanes leads to alkoxi- or alkyl-aminosilanes, which subsequently have to be hydrolized. 

If prochiral ketones are hydrosilylated with dihydrosilanes carrying different alkyl groups R ) and the siloxane is treated with a Grignard reagent, optically active silanes are produced (7) ... 

Ojima reported on the reduction of R R C=NR type Schiff bases by hydrosilylation with several catalysts. The reaction was also affected by the quality of hydrosilane. Using dihydrosilanes, which were found to be more active than monohydrosilanes and trihydrosilanes, the activity of the catalysts used decreased in the order RhCl(PPh3)3 Rh(CO)Cl-(PPh3)2 > Py2RhCl(dmf)BH4 > [Rh(HD)Cl]2 > [Rh(COD)Cl]2 > PdCb > PdCl2(PPh3)2. 

Axially chiral spirosilane 61 was efficiently prepared by double intramolecular hydrosilylation of bis (alkenyl) dihydrosilane 60. By use of SILOP ligand, a C2 symmetric spirosilane which is almost enantiomerically pure was obtained with high di-astereoselectivity (Scheme 3-24) [65]. SILOP ligand is much more stereoselective for this asymmetric hydrosilylation than DlOP (5) though they have similar structure. 

The diruthenium complex having Ru-H-Si interactions Ru(CO)2SiTol2H) 2(ju-dppm)(p-q2 q2-H2SiTol2), where dppm is bis(diphenylphosphine)methane and Tol isp-tolyl, has been recently reported to be very effective in the hydrosilylation of various ketones (Eq. 95) and imines (Eq. 96) with dihydrosilanes [150]. 

With regard to the limitation of the applicability of this selective reduction method, the hydrosilylation of chalcone proceeds only by 1,4-addition even when dihydrosilanes and trihydrosilanes are employed128. However, the selective 1,2-addition is achieved by using CsF as catalyst129 (equation 70). 

The asymmetric synthesis at a prochiral silicon center in the sense of catalytic asymmetric reactions has been effected in the hydrosilylation of ketones with dihydrosilanes using rhodium(I) complexes as catalysts137,155. 

In contrast to the results discussed above, hydrosilylation of unsaturated ketones with rhodiumphosphine catalysts makes the selective saturation of either the C=C or the C=0 bond possible. Using monohydrosilanes, 1,4-addition takes place and saturated ketones are formed, while dihydrosilanes prefer the 1,2-addition and unsaturated alcohols are the products (9) ... 

In connection with the catalytic hydrosilylation, asymmetric alcoholysis of dihydrosilanes in the presence of chiral rhodium complexes as catalysts deserves to be described. 

Asymmetric hydrosilylation of acetophenone, methylbenzyl ketone, and isobutyro-phenone can be achieved using soluble chiral rhodium(i)-diop complexes (49) and polymer-supported catalyst. The optical yield is dependent on the silane used, dihydrosilanes giving higher yields than monohydrosilanes. Similar results are obtained with a-keto-esters. ... 

Asymmetric synthesis of chiral tCTtiary hydrosilanes can be realized by means of catalytic as3munetiic hydrosilylations of ketones with dihydrosilanes H2SiR R (R R ). 

Mechanistic investigation of the N-heterocyclic carbene (NHC)-initiated hydrosilylation of styrenes with dihydrosilane suggests a general base catalysis mechanism rather than activation of the dihydrosilane with the NHC. ... 

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