Enol ethers hydrosilylation

The 1,4-reduction of a,/3-unsaturated aldehydes is best carried out with diphenylsilane in the presence of zinc chloride and tetrakis(triphenylphosphine) palladium436 or a combination of triethylsilane and tris(triphenylphosphine) chlororhodium 437 Other practical approaches use phenylsilane with nickel (0) and triphenylphosphine438 and diphenylsilane with cesium fluoride.83 It is possible to isolate the initial silyl enol ether intermediate from the 1,4-hydrosilylation of o, /3-unsaturated aldehydes (Eq. 264).73,411 The silyl enol ethers are produced as a mixture of E and Z isomers. 

The 1,4-hydrosilylation of enones can be used as a method for the introduction of the silyl enol ether functionality, and may be accomplished with the combination of PhMe2SiH/RhH(PPh3)4 (Eq. 278),374 Et3SiH/RhCl(PPh3)3 411 (HMe2 Si)20/[CuH(PPh3) ], 455 Et3 SiH/Pt-complex,456 chloromethy ldimethy 1-... 

This intramolecular hydrosilylation can be extended to a-hydroxy enol ethers (2-alkoxy-l-alkene-2-ols) to provide access to 2,3-sy -l,2,3-triols.2 In this case a neutral catalyst, Pt(0)-vinylsiloxane,3 is preferred over H2PtCl6. 

Tamao, K. Nakagawa, Y. Ito, Y. Regio- and stereoselective intramolecular hydrosilylation of a-hydroxy enol ethers 2,3-syn-2-methoxy-methoxy-l,3-nonanediol. Org. Synth. 1998,... 

CH3P(C6H5)2. The reaction involves hydrosilylation of the enone to furnish an intermediate enol ether (a), which then undergoes coupling with an aldehyde. 

REG 10- AND STEREOSELECTIVE INTRAMOLECULAR HYDROSILYLATION OF a-HYDROXY ENOL ETHERS 2,3-syn-2-METhOXYM ETHOXY-1,3-NON ANEDIOL (1,3-Nonanedlol, 2-(methoxymethoxy)-, (R, R )-( )-)... 

In an extension of this process, the intramolecular hydrosilylation of a-hydroxy enol ethers has been presented as a new, syn selective route to 1,2,3-triols (Scheme 11). With such sensitive substrates, a neutral hydrosilylation catalyst, Pt [(CH2==CH)Me2Si]20 2, must be used. The utility of this method has been demonstrated in a synthesis of the pentitols, o-arabinol and xylitol (as their pentaacetates), in optically pure form. 

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). 

Nickel metal catalysts give mixtures of the corresponding silyl ether and silyl enol ether. The former is produced via hydrosilylation, while the latter is produced via de-hydrogenative silylation. The reaction catalyzed by zinc chloride proceeds under drastic conditions, and the product of aldehydes disproportionates. The reaction of a,jS-unsatu-rated carbonyl compounds with H2PtCl6 proceeds by 1,4-addition, while coupling is also observed on using metallic Ni as catalyst. ... 

In a side reaction enolizable ketones may give rise to varying amounts of silyl enol ethers. On hydrolysis, these silyl enol ethers are reconverted into the starting material. Thus, even on complete hydrosilylation, product formation is not quantitative and the secondary alcohol is accompanied by the ketone to the extent in which the silyl enol ether was formed. Thus, the hydrosilylation of 1-phenylethanone and its enol form with diphenylsilane affords the corresponding silyl ether and silyl enol ether and subsequent hydrolysis gives 1-phenylethanol and 1-phenylethanone13. 0 0H... 

Alternatively, triorganyl-//-silanes can be added across the carbonyl function under hydrosilylation-type conditions, e.g., triethylsilane (442) reacts with aldehydes in the presence of a Ni catalyst to yield the corresponding TES-enol ether 632 (equation... 

However, such a marked ligand effect of the ester moiety was not observed in the case of methyl acetoacetate (23.5% e.e.)153 or ethyl benzoylacetate (62.3-68.4% e.e.)207. This may be due to the fact that the silyl ether could be produced not only by the hydrosilylation, but also by in situ transfer hydrogenation of the silyl enol ether (a mixture of E and Z isomers), which is formed in the first step of the reaction by dehydrogenative coupling of the enol with the hydrosilane. 

Aldol reactions. a-Isocyanoalkanoic esters condense with aldehydes in the presence of BU4NF to afford oxazolines. The reaction of hydrosilyl enol ethers with aldehydes afford 1,3-diols as a result of aldol reaction and subsequent reduction. ... 

As an alternative to enolization and addition of a silyl halide or trillate, silyl enol ethers may be prepared by the 1,4-hydrosilylation of an a,3-unsaturated ketone. This can be done by using a silyl hydride reagent in the presence of a metal catalyst. Metal catalysts based on rhodium or platinum are most effective and provide a regiospecific approach to silyl enol ethers (1.25). 

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