Asymmetric hydrosilation

X = OMe, OCH2Ph,OPr , Et, CN, CH2NMC2, C02Me, COjH, OH, H(R), C6H2-3,5-dimethyl-4-OMe, C6H2-3,5-dimethyl-4-OMe with bis(6,6 - octyl) substituents on binaphthyl 

E = CO2R, etc. [Pd] = Pd(COD)MeCl, or preformed complex with 33 used Ar = 3,5-bis(trifluoromethyl)phenyl 

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. 

Rhodium complexes derived from chelating bisphosphines anchored to an asymmetric, ancillary Cp-Re complex exhibit moderate enantioselectivity in the hydrosiiation of phenyl/alkyl ketones with Ph2SiH2. A tra i-chelating, planar-chiral bisphosphine 40, based on a biferrocenyl motif, forms Rh complexes that are 

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Optically active 3-(trimethylsilyl)cyclopentene of moderate enantiomeric excess is available by asymmetric hydrosilation (see Section 1.3.3.3.5.1.5.) and reacts with aldehydes with reasonable stereoselectivity in the presence of titanium(IV) chloride36. 

E. Asymmetric Hydrosilations with Chiral Platinum Complexes. 427... 

The recent discovery that a chiral phosphine ligand in a platinum(II) complex can give rise to a catalytic asymmetric hydrosilation of prochiral olefins seems to prove that a phosphine ligand can be included in the coordination sphere of platinum in an active catalytic species, but that when a phosphine ligand is so included, the activity of the species is reduced by several orders of magnitude. 

Asymmetric hydrosilation of prochiral olefins other than 1,1-disubsti-tuted ones was unsuccessful because they were unreactive under the conditions employed. Styrene afforded a 20% yield of 1-phenylethyl- and 50% of 2-phenylethylmethyldichlorosilane, both inactive. The 1-phenylethyl isomer had a possibility of activity with its asymmetric center, PhMeC HSi=. 

Kumada and his co-workers (33) later showed that a chiral Ni(II) complex induced asymmetric hydrosilation of a-methylstyrene by meth-yldichlorosilane at 90°C for 60 hours. By use of the trans-(R) isomer of (PhCH2—PhMeP )2NiCl2, they isolated 8% PhMeC HCH2SiMeClH with [a]D + 6.43°and 31% PhMeC HCH2SiMeCl2 with [a] > + 6.50°. The latter compound was treated with methyllithium to prepare PhMeC HCH2SiMe3, [a], + 10°, which they estimated as 17.6% optically pure. 

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... 

An important potential target of asymmetric hydrosilation catalysis is to the reduction of the cyclopenten-l,4-diones, 1, producing the 4-silyloxyeyclopentenones, 2, which are key intermediates in the synthesis of singleisomer prostaglandins (18-22). 

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. 

Figure 4. Proposed mechanism for asymmetric hydrosilation using tetraphosphine Rh(I) catalysts.

The asymmetric hydrosilation of prochiral alkenes has been a challenging subject, which eventually experienced a... 

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. 

The use of prochiral secondary silanes in the asymmetric hydrosilation can give an optically pure sUylether, which can be alkylated with a Grignard reagent with retention of the... 

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). ... 

Another recent report described the asymmetric hydrosilation of styrene using chiral phosphine ligands such as methyldiphenylphosphine 160). Enantiomeric excesses of 5% of S isomer of 2-phenylethyl-trichlorosilane were obtained. The mechanism almost certainly involved insertion of Pd(II) into a silicon-hydride bond, followed by addition of Pd-hydride across the olefinic bond and elimination of Pd(II) to give the silicon-carbon bond. 

MoFeCo(/i3-S)(//-dppe)(Cp )(CO)6] in hydroformylation has been associated with the fact that the bidentate dppe ligand bridges the Fe-Co bondJ °°i Asymmetric hydrosilation of acetophenone was attempted with the optically active cluster (+)-5 but photoracemization proceeds faster than the hydrosilation reactionJ " ... 

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