Enantioselective hydrosilation

An important example in this context is the hydrosilation reaction [121]. Hydrosilation is the formal addition of a silane to an alkene in presence of a hydrosilation catalyst. This reaction has numerous applications e.g., with a suitable catalyst, an enantioselective hydrosilation is possible [122] and also hydrosilation and double hydrosilation of alkines [123] are known. 

Enantioselective Hydrosilation of Ketones and Other Carbonyl Compounds... 

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(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

In the phase-transfer processes discussed in Section 11.2 it is assumed that the anionic hydride source, i.e. borohydride or a hypervalent hydrosilicate, forms an ion-pair with the chiral cationic phase-transfer catalyst. As a consequence, hydride transfer becomes enantioselective. An alternative is that the nucleophilic activator needed to effect hydride transfer from a hydrosilane can act as the chiral inducer itself (Scheme 11.6). 

Interestingly, the enantioselectivity for the hydrosilation of acetophenone was found to be independent of the amount of chiral ligand present in the reaction mixture, although the reactions slow down as excess ligand is added. Thus, catalyst solutions prepared by adding from 1-10 equivalents of MDPP per rhodium to the (5-Binap)Rh dimer all gave ca. 55% e.e. of the expected 5-sec alcohol. The addition of excess achiral phosphines such as triphenylphosphine or tricyclohexylphosphine did not effect the optical yield of the alcohol as compared to reactions involving stoichiometric amounts of the monophosphine. 

Intramolecular hydrosilation has been extensively used to control regio-and enantioselectivity.149 This strategy has been applied by Bergens et a/.150... 

Using this system, 1-alkenes can be hydrosilated with excellent enantioselectivities (e.e. 91-97%). Subsequent Tamao oxidation to the respective secondary alcohol occurs under retention of configuration (equation 21). 

Also norbomene and related systems, as well as dihydrofuranes, have been hydrosilated with good enantioselectivities with similar MOP ligands. Intramolecular rhodium catalyzed asymmetric hydrosUations have been achieved with good enantioselectivities, for example with meso- or other allylsilylethers. ... 

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. 

Asymmetric Intramolecular Hydrosilation. Intramolecular hydrosilation of allylic alcohols followed by oxidation is a convenient method for the stereoselective preparation of 1,3-diols. An enantioselective version is achieved by use of diene-free BINAP-Rh+ (eq 6). Both silyl ethers derived from cinnamyl alcohol and its cis isomer give (iJ)-l-phenylpropane-l,3-diol in high ee regardless of alkene geometry. 

Titanocene difluorides also catalyze the hydrosilation of imines, presumably through the formation of the trivalent titanocene hydride Cp 2Ti—H. This reaction can be made enantioselective, as shown in (equation 43) (255). 

In a series of studies published in the 1970s, Ojima and co-workers " demonstrated that hydrosilation of imines, isocyanates, and carbodiimides could be catalyzed by Pd complexes, as shown in Scheme 12. A related study of the enantioselective reduction of acetophenone with Ph2SiH2 was also reported. ... 

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