Asymmetric hydrosilylation, ketone

Chiral diamino carbene complexes of rhodium have been merely used in asymmetric hydrosilylations of prochiral ketones but also in asymmetric addition of aryl boron reagents to enones. 

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... 

The Rh-catalysed asymmetric hydrosilylation of prochiral ketones has been studied with complexes bearing monodentate or heteroatom functionalised NHC ligands. For example, complexes of the type [RhCl(l,5-cod)(NHC)] and [RhL(l,5-cod)(NHC)][SbFg ], 70, where L = isoquinoline, 3,5-lutidine and NHC are the chiral monodentate ligands 71 (Fig. 2.11). 

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. 

The in situ generation of CuH from organosilanes in the presence of either a BIPHEP (99) or a SEGPHOS (100) type ligand represents a general method for the asymmetric hydrosilylation of aryl alkyl ketones at low temperatures. 

As outlined in Section II,E, ketone and imine groups are readily hydrogenated via a hydrosilylation-hydrolysis procedure. Use of chiral catalysts with prochiral substrates, for example, R,R2C=0 or R,R2C=N— leads to asymmetric hydrosilylation (284, 285 Chapter 9 in this volume) and hence optically active alcohols [cf. Eq. (41)]. 

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. 

The enantiomerically pure dithiourea derivative L6 has been successfully applied to the iridium-catalyzed [ Ir( x-Cl)(cod) 2] asymmetric hydrosilylation of acetophenone, as well as of various alkylaryl ketones [51]. In the case of acetophenone, when the iridium catalytic system ([Ir]/L6) was used with a 10-fold excess of L6 (R = Ph), an encouraging enantioselectivity was achieved (74% ee), together with a yield of 30% at 50 °C after 24h. 

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. 

Keywords Asymmetric hydrosilylation, optically active alcohols, amines, Chiral Titanocene Catalysts, Acyclic Imines, Cyclic Imines, Chiral Rhodium Catalysts, aromatic ketones... 

Racemic N-methylimines derived from 4-substituted 1-tetralones were ki-netically resolved by asymmetric hydrosilylation with phenylsilane (1 equivalent) as a reducing agent using the titanocene catalyst (R)-ll (substrate Ti= 100 1) at 13 °C, followed by a workup procedure to afford the corresponding chiral ketones and chiral cis amines with very high enantio- and diastere-oselectivity (Scheme 12) [28], The extent of the enantiomeric differentiation, kfast/kslow was calculated to be up to 114. The ris-selectivity of this reaction was... 

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

TABLE 2.2. Asymmetric Hydrosilylation of Other Ketones with Rhodium Catalysts ... 

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

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