Asymmetric hydrosilylations of ketones

An iron complex-catalyzed asymmetric hydrosilylation of ketones was achieved by using chiral phosphoms ligands [68]. Among various ligands, the best enantios-electivities (up to 99% ee) were obtained using a combination of Fe(OAc)2/(5,5)-Me-Duphos in THF. This hydrosilylation works smoothly in other solvents (diethylether, n-hexane, dichloromethane, and toluene), but other iron sources are not effective. Surprisingly, this Fe catalyst (45% ee) was more efficient in the asymmetric hydrosilylation of cyclohexylmethylketone, a substrate that proved to be problematic in hydrosilylations using Ru [69] or Ti [70] catalysts (43 and 23% ee, respectively). 

In 2008, Gade and coworkers reported that the asymmetric hydrosilylation of ketones was catalyzed by the Fe complex with a highly modular class of pincer-type ligand (Scheme 22) [71]. This Fe catalyst system showed both moderate to good... 

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. 

Supported cationic rhodium(I) phosphine complexes, chiral at a men-thyl moiety, effected hydrogenation of ketones, but the 2-butanol produced from methylethylketone was optically inactive (348). Polystyrene-or silica gel-supported DIOP systems, however, are particularly effective for production of optically active alcohols (up to 60% ee) via asymmetric hydrosilylation of ketones (10, 284, 296, 366, 368 see also Section III, A,4). 

Asymmetric hydrosilylation of ketones has developed enormously since these early reports and probably there are few catalytic reactions for which the variety in ligand structure is so immense as for this reaction. Numerous reports have been published and in general oxazoline-based ligand systems seem to give the highest enantioselectivities. In the following we will mention a few... 

For many years nitrogen-based ligands were the ligands of choice for the asymmetric hydrosilylation of ketones and especially pyridincoxazolincs were highly effective [25], In Figure 18.14 a few typical examples have been collected. 

A number of P-N ligands has been reported as efficient ligands for the asymmetric hydrosilylation of ketones. We mention the phosphinooxazolines developed by Helmchen, Pfaltz, and Davis, we have seen before and mixed ligands containing planar-chiral heterocycles such as ferrocene [31] (Figure 18.17). For several ketone and silane combinations e.e. s in the high nineties were obtained. 

Other silicon derivatives containing Si—X—C bonds (where X is O and/or N) can be successfully prepared by using iridium-catalyzed reachons such as the asymmetric hydrosilylation of ketones and amines, the silylcarbonylation of alkenes, and the alcoholysis of Si—H bonds. Indeed, oxygenation of the latter bond to silanol also proceeds smoothly in the presence of iridium compounds. 

ASYMMETRIC HYDROSILYLATION OF KETONES AND IMINES WITH RH AND RU CATALYSTS... 

ASYMMETRIC HYDROSILYLATION OF KETONES AND IM1NES WITH TITANIUM CATALYSTS 125... 

TABLE 2.3. Asymmetric Hydrosilylation of Ketones with Titanium Catalysts ... 

Scheme 4.27 Asymmetric hydrosilylation of ketone 66 with iron catalysts according to Nishiyama and Furuta [59],...

Metal complexes of a C2-symmetric bisproline catalyse the asymmetric hydrosilylation of ketones 29Si NMR spectra provide evidence of the mechanism. 265 ... 

Recent advances in the asymmetric hydrosilylation of ketones and imines have been reviewed.276... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

More impressive results were obtained for the asymmetric hydrosilylation of ketones over the insolubilized catalyst (34). Acetophenone, for example, was hydrosilylated with phenylnaphthylsilane and the intermediate was hydrolyzed with HC1 to give (-)-S-phenylmethylcarbinol (58% ee). Similar results were obtained with the homogeneous DIOP catalyst. Using diaryl or arylalkyl silanes, yields from 52 to 100% and optical purities from 7 to 59% were realized (Table V). 

Complex 32 has been employed in the asymmetric hydrosilylation of ketones, displaying good activity and excellent enantioselectivities (92% < ee < 98%) for aryl-alkyl ketones, while the selectivity observed in the transformation of the more demanding dialkyl ketones is somewhat lower (67% < ee < 96%). 

Table 2 Asymmetric hydrosilylation of ketones with catalyst 68...

The optically active (A)-(+)- and (i )-(—)-2,2 -bis[diarylstibano]-l,l -binaphthyls (BINASb) have been prepared and used as chiral auxiliaries in the Rh-catalyzed asymmetric hydrosilylation of ketones with diphenylsilane.49,49a When acetophenone is reduced using 0.25 mol% of [Rh(COD)Cl]2 as the catalyst and 0.5 mol% of (i )-BINASb (aryl = />-tolyl) as the ligand, (i )-l-phenylethanol is formed in 78% yield and in 32% ee (Equation (15)). When (i )-BINAP is used as the chiral ligand instead of (i )-BINASb, the yield and enantiomeric excess of (7 )-l-phenylethanol are 42% and 0.6%, respectively. 

Kumada et al. have examined a number of chiral ferrocenylphosphines as ligands for asymmetric reactions catalyzed by transition metals. They are of interest because they contain a planar element of chirality as well as an asymmetric carbon atom. They were first used in combination with rhodium catalysts for asymmetric hydrosilylation of ketones with di- and trialkylsilanes in moderate optical yields (5-50%). High stereoselectivity was observed in the hydrogenation of a-acetamidoacrylic acids (equation 1) with rhodium catalysts and ferrocenylphosphines. ... 

Diselenides as Chiral Ligands for Asymmetric Hydrosilylation of Ketones and Related Reactions... 

Rhodium-Catalyzed Asymmetric Hydrosilylation of Ketones. Complex 2 is a good catalyst for catalytic asymmetric hydrosilylation of ketones (eq 7). The reactions are carried out by using 1-naphthylphenylsilane at -40 °C in THF in the presence of 2 (1 mol%) for 3-4 days. Several types of ketones are hydrosilylated to afford optically active alcohols after acidic work-up. 

For asymmetric hydrosilylation of ketones a rhodium complex coordinated with the ferrocenyl(dimethyl)phosphine 3b has been reported to be more effective than other ferrocenylphosphines to give optically active alcohols (up to 49% ee) after hydrolysis (Scheme 2-49) [7]. 

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