Hydrosilylation chiral catalyst

Asymmetric hydrosilylation can be extended to 1,3-diynes for the synthesis of optically active allenes, which are of great importance in organic synthesis, and few synthetic methods are known for their asymmetric synthesis with chiral catalysts. Catalytic asymmetric hydrosilylation of butadiynes provides a possible way to optically allenes, though the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (2S,4S)-PPM turned out to be the best catalyst for the asymmetric hydrosilylation of butadiyne to give an allene of 22% ee (Scheme 3-20) [59]. 

BINAP complexes (7 in Fig. 7.7) are among the most efficient chiral catalysts for enantioselective hydrogenations, hydrosilylations, etc. Heterogeniza-tion of this complex is highly desired because of the high price of the complex. 

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)]. 

Highly enantioselective hydrosilylation of N-aryl imines derived from aliphatic ketones was achieved by the use of (S)-ll as a chiral catalyst (Scheme 10)... 

The enantiomeric purity of the byproduct indicated that the chiral catalyst also promoted isomerization of the C=N bond, although the isomerization rate was much slower than that of hydrosilylation. 

Asymmetric hydrosilylation of prochiral ketones continues to be the most popular reaction to examine the efficacy of new chiral ligands or chiral catalyst systems. This asymmetric catalytic process gives enantiomerically enriched secondary alcohols after facile desilylation of the resulting silyl ethers (equation 80). 

In the reaction between an activated diene and a prochiral aldehyde, which is catalyzed by chiral Lewis acids, 5,6-dihydropyranoncs such as 6 are formed enantiomerically enriched. By attachment of a chiral auxiliary (3-heptafluorobutanoyl camphor derivatives such as 7 to a soluble poly-siloxane by hydrosilylation the catalyst should be recyled easily by precipitation or ultrafiltration while the cycloaddition reaction can be performed in homogeneous solution [lOj. 

Single-electron transfer processes have been implicated." When combined with asymmetric hydrosilylation using a chiral catalyst, optically active alcohols can be generated. ... 

Catalytic hydrosilylation of alkenes performed in the presence of a chiral catalyst results in the formation of chiral silanes. Initially platinium catalysts of the type L PtCl2, L = (/ )-benzyl-(methyl)phenylphosphine (BMPP) or (/ )-methyl(phenyl)propylphosphine and 1,1-disubstituted prostereogenic alkenes, such as a-methylstyrene, 2,3-dimethyl-l-butene and 2-methyl-l-butene, were used however, the stereoselectivity was low4,5. A slightly higher stereoselectivity is obtained when dichlorobis[(/ )-benzyl(methyl)phenylphosphine]nickel [Ni(BMPP)2Cl2] is used as the catalyst. Note that two chiral silanes are formed in this reaction, both of which are products of anti-Markovnikov addition. The major product is the expected dichlorosilane 3, while the byproduct is an anomalous chlorosilane 4 both products were separated by fractional distillation and the major product methylated to give the trimethylsilanes 56 7. 

The cluster itself was also recovered in a racemic form (140). This result means that the photo-induced racemization of the chiral catalyst is faster than the hydrosilylation, and it may be used as an argument for or against cluster integrity throughout the catalytic process. 

First attempts to achieve silyl enol ethers ) are known since the late 195O s when hydrosilylation-type procedures were applied to a,d-unsaturated ketones is—i is) based upon the observation by Duffaut and Galas ) that simple ketones are able to add trichlorosilane (2) under UV irradiation. These hydrosilylation reactions were widely expanded and intensively studied ). a,/3-unsaturated ketones react via 1,4-addition ) to silyl enol ethers ) affecting only the conjugated double bonds. It is worth mentioning that the employment of chiral catalysts induces an asymmetrical reaction " ) (Scheme 24). 

When prochiral silane and ketone are used, hydrosilylation, in the presence of a chiral catalyst, results in asymmetric induction at both the silicon and carbon centers. Treatment of the diastereomeric alkoxysilane by a Grignard reagent leads to recovery of an organosilane and an alcohol of different optical purity. Results obtained in the asymmetric hydrosilylation of ketones and aldehydes by prochiral silanes in the presence of an asymmetric catalyst are summarized in Tables 3 and 4. 

Hydrosilylation. Chiral a-arylethanol can be synthesized from styrenes via hydrotri-chlorosilylation and oxidative desilylation. The first step is accomplished with a Pd catalyst containing ligand 6. ... 

The addition of silanes across multiple bonds occurs in the presence of catalysts, mostly complexes of VIII B group elements (e.g. Co, Ni, Pd, Pt) (equation 15). Concerning the course of reaction it can be generally said that, predominantly, (a) the hydrosilylation of simple alkenes and alkynes places the silicon atom at the less substituted carbon atom and (b) via catalysts and reaction conditions a stereospecific course of reaction can be accomplished. Furthermore, as a very positive side-effect, asymmetrical hydrosilylation can be realized if chiral catalysts are employed33a 33bf 33c. For further details we recommend comprehensive review papers on this subject33a c. 

It has been shown by Ojima in the synthesis of depsipeptides (see the last five lines in Table 6) that optical selectivities are much higher in the hydrosilylation reactions, where asymmetric induction by the chiral catalyst predominates over that of the chiral center already present in the N- (a- ketoacyO-a- aminoester substrates. 

Intramolecular asymmetric hydrosilylation of allylic silyl ethers 150 bearing a silacyclohexyl group gives spirobicyclic oxasilacyclopentanes 151 with extremely high enantiopurity when [Rh(BINAP)]+C104 is used as the chiral catalyst (Scheme 70) (128,129). 

Asymmetric reduction of ketones by hydrosilylation in the presence of a chiral catalyst followed by hydrolysis has been studied by several research groups independently. In this Section, results so far obtained are properly compiled and plausible mechanisms of the asymmetric hydrosilylation of prochiral ketones are discussed. 

These facts may well imply that the stereoselectivity for addition of a hydrosilane to the enantiotopic faces of a ketone is different from that of an olefin which is undoubtedly tr-coordinated to the chiral catalyst the difference is again explained in terms of the intermediacy of an a-siloxyalkyl-platinum complex similar to the a-siloxyalkyl-rhodium complex described in the hydrosilylation of terpene ketones (see Scheme 4). 

The imidotitanium and titanocene complexes have been used as the chiral catalysts in the hydroamination/hydrosilylation sequences of alkynylamines in one pot to yield the cyclic amines with up to 66% ee (Scheme 14.140). Although the results have not yet been satisfactory, this sequential reaction opens up the field for using other ligands and substrates [303]. 

An enantioselective synthesis of chiral diols via intramolecular hydrosilylation using a chiral catalyst followed by oxidative cleavage of the Si-C bond (with retention of configuration) has... 

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