Hydrosilylation alcohol synthesis

Alcohol synthesis Hydrosilylation of alkenes with (fluoroaryl)silanes (e.g., CgFjSiHj) is accomplished with CpjYMe at room temperature. The products are readily oxidized to alcohols with KF-H Oj. 

Qi S-B, Li M, Li S, Zhou J-N, Wu J-W, Yu F, Zhang X-C, Chan ASC, Wu J (2013) Copper-dipyridylphosphine-catalyzed hydrosilylation enantioselective synthesis of aryl- and hetero-arylcycloalkyl alcohols. Org Biomol Chem 11 929-937. doi 10.1039/C20B27040D... 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

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],... 

An alternative disconnection of homopropargylic alcohols substrates for intramolecular hydrosilylation is the opening of an epoxide with an alkynyl anion. This strategy was employed in a total synthesis of the macrolide RK-397 (Scheme 20). Epoxide ring opening serves to establish homopropargylic alcohol C with the appropriate stereochemistry. A hydrosilylation/oxidation protocol affords the diol E after liberation of the terminal alkyne. The... 

Hydrosilylation can be applied to alkenes, alkynes, and aldehydes or ketones. A wide range of metal compounds can be used as a catalyst. The most common and active ones for alkenes and alkynes are undoubtedly based on platinum. Hydrosilylation of C-0 double bonds gives silyl ethers, which are subsequently hydrolysed to their alcohols. The reaction is of interest in its enantioselective version in organic synthesis for making chiral alcohols, as the achiral hydrogenation of aldehydes or ketones does not justify the use of expensive silanes as a reagent. 

It has been reported that (TMS)3SiCl can be used for the protection of primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable to the usual conditions employed in organic synthesis for the deprotection of other silyl groups and can be deprotected using photolysis at 254 nm, in yields ranging from 62 to 95%. Combining this fact with the hydrosilylation of ketones and aldehydes, a radical pathway can be drawn, which is formally equivalent to the ionic reduction of carbonyl moieties to the corresponding alcohols. 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

Doyle and co-workers have employed Rh2(pfb)4 as a highly selective catalyst for the room temperature synthesis of silyl ethers from alcohols and triethylsilane.159 The selectivity of the catalyst is demonstrated by reactions of olefinic alcohols, in which hydrosilylation is not competitive with silane alcoholysis when equimolar amounts of silane and alcohol are employed. High yields (>85%) of triethylsilyl ethers are obtained from reactions of alcohols such as benzyl alcohol, 1-octanol, 3-buten-l-ol, cholesterol, and phenol. Tertiary alcohols are not active in this system. 

In 1990, the enantioselective intramolecular hydrosilylation of allylic alcohols was successfully applied to the synthesis of chiral 1,3-diol [63] (Scheme 2.6). The reaction of 3-diarysiloxy-1,4-pentadiene (104) catalyzed by (-)-DIOP-[Rh(C2H4)Cl]2 (2 mol%), followed by Tamao oxidation, gave (2S,3R)- 1,3-diol 105 (symanti = 98 2) with 93% ee [63]. 

Asymmetric Synthesis of Alcohols by Hydrosilylation of Ketones A Comparison of an Insolubilized (+) -DIOP Catalyst vs a Solution (+)-DIOP Catalysta... 

Hydrosilylation of terminal alkenes using the air-stable silane (EtO)2MeSiH in the presence of either HzPtCb or (PhsPbRhCl results in the introduction of silicon exclusively at the terminal carbon atom. When coupled widi oxidative cleavage, this protocol provides a simple one-pot synthesis of anti-Mar-kovnikov alcohols firom terminal alkenes (Scheme 7). 

Intramolecular hydrosilylation of allyl and homoallyl alcohols, with subsequent oxidative cleavage of the resultant C—Si bond, has provided a new approach to the regiocontrolled synthesis of 1,2-and/or... 

Synthesis. Synthesis of the copolymers was performed by a hydrosilylation reaction of poly(dimethylsiloxane-co-methylhydrosiloxane) (Petrarch System, Inc.) and a-olefins of various lengths (Aldrich). A round-bottomed flask equipped with a magnetic stirring bar, condenser, and calcium chloride tube was charged with a 50% solution of the reactants (up to 10% molar excess of a-olefin) in dry toluene. A solution of hydrogen hexachloroplatinate(IV) in diglyme-isopropyl alcohol (150 ppm Pt) was then added to the reaction mixture. The reaction mixture was stirred at 60 °C for 3 h. At the end of this period, the mixture was refluxed with activated charcoal for 1 h and filtered while hot. Finally the solvent and excess a-olefins were removed under reduced pressure (67 Pa at 100 °C). The reaction proceeded to completion as evidenced by the absence of the Si-H absorption at 2130 cm" in the IR spectra. Residual a-olefin in the purified polymers was determined by gas-liquid chromatography. In all polymers, residual a-olefin was less than 1.5 wt %. 

Hydrosilylation provided a novel alternative reduction of a-oxy ketones (29) with tunable dia-stereoselectivity (Scheme 3). Fluoride-catalyzed reduction with phenyldimethylsilane in HMPA provided the jyn-alcohols (28) with high selectivity (87 13-96 4). The absence of a coordinating cation and the bulkiness of the reducing species combined to favor the Felkin model for these reductions. Conversely, reduction in trifluoroacetic acid proceeded via a proton-bridged cyclic transition state to give the anti products (30 84 16-99 1), These complementary methods constitute a powerful tool in stereoselective synthesis. 

The catalytic hydrosilylation of acetophenone or t-butyl methyl ketone with diethyl-, methylphenyl- or diphenylsilane in the presence of rhodium(I) catalysts containing (R,i )-(-l-)- or (S,S)-(—)-170, followed by acid cleavage of the intermediate silyl ethers, affords the respective alcohols with optical yields of 10—42% . The synthesis of (ff)-(-l-)-l-phenylethanol from acetophenone and diethylsilane in conjunction with the catalyst derived from (S,S)-( —)-170 was the most effective reaction (equation 28). In... 

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