Carbonylative hydrosilylation

Carbonylative hydrosilylation.1 lrCl(CO)j (or lr4(CO)i2) catalyzes this reaction with terminal alkenes to form the cnol silyl ethers of acylsilancs. Acetal, epoxide, and cyano groups arc not affected. 

Fig. 5a) with the analogous carbonyl hydrosilylation shown in Fig. 4. Formation of a pentacoordinate silyl-H oxonium complex 7 from 2 by addition of the alkoxysi-lane is followed by substitution at the SiH partner with inversion. Rearrangement to an alkane complex 8 is foUowed by decomposition to product. 

However, it is unclear whether the double bonds are hydrogenated simultaneously or involved in the catalyic process successively. This also refers to the residual double bonds in mixed-unit metaUopolymers. Metallopolymers with this type of unit variability can also undergo other analogous polymer transformations (i.e., reactions that do not affect the backbone) such as all l hydroperoxide-induced epoxidation, carbonylation, hydrosilylation, and hydroformylation. Realization of tiiese reactions could yield valuable products. However, solutions to these problems will require additional research. 

Nolan and Diez-Gonzalez introduced a new family of NHC-Cu complexes to the field of carbonyl hydrosilylation. These bis-NHC cationic compounds of formulae [(NHC)2Cu]X were shown to be more efficient than their neutral mono-ligated counterparts under identical conditions and even allowed for the direct reduction of an ester into the corresponding protected alcohol (Equation (11.3)). 

The hydrosilylation of aldehydes and ketones is of interest since it provides the corresponding alcohols protected as silyl ethers and the hydroxy function can subsequently be regenerated under mild conditions. Although the mechanism of carbonyl hydrosilylation is not as well-established as for olefin hydrosilylation, a few variations have been discussed by Ojima, Zhang and Chan and, more recently, Gade. ... 

Other procedures for carbonyl hydrosilylation of aldehydes and ketones are using [bis(imino)pyridine]iron dinitrogen and dialkyl complexes as precatalysts. Only 0.1-1.0 mol% catalyst are required to achieve this transformation. The reductants are either phenylsilane or diphenylsilane in this case. A number of enantioselective versions of the hydrosilylation reaction is described. This includes the application of 1,2-bis[(25, 55)-2,5-dimethylphospholano]benzene [(S,5)-Me-DuPhos] (Scheme 4-328) as chiral ligand, iron(II) acetate as a precatalyst and polymethylhydrosiloxane as hydride source. A large variety of ketones can be transformed into the corresponding alcohols in excellent yield and up to 99% enantiomeric excess. Catalytic ketone hydrosilylation is also achieved with the dialkyliron complexes (S,S)-... 

The transition metal catalysed addition of a hydridosilane to a multiply-bonded system is known as hydrosilylation (1). Under such conditions, alkynes undergo clear cis-addition, so providing one of the most direct routes to vinylsilanes (Chapter 3). Hydridosilanes also add to the carbonyl group of saturated aldehydes and ketones, to produce alkyl silyl ethers. Fot example, under suitable conditions, 4-t-butylcyclohexanone (2) can be reduced with a high degree of stereoselectivity. 

Highly enantioselective hydrosilylation of aliphatic and aromatic carbonyl compounds such as acetophenone, methyl phenethyl ketone 1813, or deuterobenz-aldehyde 1815 can be readily achieved with stericaUy hindered silanes such as o-tolyl2SiH2 or phenyl mesityl silane 1810 in the presence of the rhodium-ferrocene catalyst 1811 to give alcohols such as 1812, 1814, and 1816 in high chemical and optical yield [47] (Scheme 12.14). More recently, hydrosilylations of aldehydes... 

Beller and coworkers reported hydrosilylation reactions of organic carbonyl compounds such as ketones and aldehydes catalyzed by Fe(OAc)2 with phosphorus ligands (Scheme 21). In case of aldehydes as starting materials, the Fe(OAc)2/PCy3 with polymethylhydrosiloxane (PMHS) as an H-Si compound produced the corresponding primary alcohols in good to excellent yields under mild conditions [67]. Use of other phosphorus ligands, for instance, PPhs, bis(diphenylphosphino) methane (dppm), and bis(diphenylphosphino)ethane (dppe) decreased the catalytic activity. It should be noted that frans-cinnamaldehyde was converted into the desired alcohol exclusively and 1,4-reduction products were not observed. 

Scheme 2.9 Hydrosilylation of carbonyl compounds to silyl ethers...

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

Fig. 2.10 Hydrosilylation catalysts of carbonyl compounds based on Cu-NHC complexes...

Scheme 2.10 Intermediates in the Cu-catalysed hydrosilylation of carbonyl compounds...

Table 1.4 Examples of functionalization and defunctionalization with metal NPs in ILs carbonylation, hydroformylation, borylation, hydrosilylation, bond cleavage, hydrogenolysis, aminolysis, and dehalogenation. 

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. 

These complexes anchored to a solid via a ligand have been tested for a number of reactions including the hydrogenation, hydroformylation, hydrosilylation, isomerization, dimerization, oligomerization, and polymerization of olefins carbonylation of methanol the water gas shift reaction and various oxidation and hydrolysis reactions (see later for some examples). In most cases, the characterization of the supported entities is very limited the surface reactions are often described on the basis of well-known chemistry, confirmed in some cases by spectroscopic data and elemental analysis. 

The reaction of carbonyl compounds, R(R )C=0, with PhSeSiMe3 and Bu3SnH in the presence of a catalytic amount of AIBN as a radical initiator results in the hydrosilylation of carbonyl compounds giving the corresponding silyl ethers, R(R )CH-OSiMe3.132 133... 

Although there are now several catalysts useful for hydrogenation of saturated carbonyl compounds to alcohols (see Section XII), an alternative approach has involved initial hydrosilylation (Chapter 9 in this volume) followed by acid hydrolysis [Eq. (41)]. The area first developed using principally the RhCl(PPh3)3 catalyst (207-210), and has since proved particularly useful in asymmetric syntheses (see Section III,A,4). Besides simple aliphatic and aromatic aldehydes and ketones, the ter-pene-ketones camphor and menthone were stereoselectively reduced to mainly the less stable alcohols e.g., camphor gave 9 (209). 

One or both carbonyls in /3-diketones can be reduced, as well as the carbonyl function in acyl cyanides (210). Similar treatment of a,/3-unsat-urated ketones and aldehydes can lead to the saturated carbonyl products via selective reduction of the olefinic bond (207, 208, 210) see Eq. (51) in Section III,A,4. Some terpenes (a- and /3-ionone, pulegone) were reduced in this way (208). Platinum(II) phosphine complexes have been used for the hydrosilylation of saturated ketones and could be used for the reduction (211). 

The low induction for the acetoacetates was attributed to a transfer hydrogenation process within an enol form of the substrate, coordinated through the carbon-carbon double bond, CH3C(OH)=CH—C02R, rather than hydrosilylation of the carbonyl moiety (285). 

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