Hydrosilanes, hydrolysis

Hydrosilane HSiR.3 behaves similar to H2 toward transition metal complexes in some cases. When HSiR.3 is used instead of hydrogen in hydroformylation, two reactions are expected. One is a hydrocarbonylation-type reaction, by which formation of the silyl enol ethers 62 via the acylmetal intermediate 61, and the acylsilanes 64 via the acyl complex 63, are expected in practice both reactions are observed. The other possibility is silylformylation to form 65, which is unknown, even though silylformylation of alkynes is known. When Co2(CO)8 is used, the silyl enol ether of aldehyde 66 is obtained [36], However, the silyl enol ether 67 of acylsilane 68 is obtained when an Ir complex is used, and converted to the acylsilane 68 by hydrolysis [37],... 

Reduction of carbonyl groups can be achieved by catalytic hydrosilylation, followed by hydrolysis. Hydrosilanes add to ketones and aldehydes more easily than to alkenes... 

Reaction of an enantiomeric hydrosilane with KH yields a racemic silyl anion (77). Thus, treatment of (-l-)-(l-Np)PhMeSiH with KH at 50°C in DME for 24 hours and subsequent addition of n-BuBr give racemic ( )-(l-Np)PhMeSi-n-Bu, via the formation of the corresponding silyl anion with loss of optical activity. Hydrolysis or deuterolysis also gives a racemic product. These observations clearly rule out a mechanism in which the silyl anion is formed by proton abstraction from the hydrosilane, because retention of configuration would be expected. A possible mechanism involves a pentacoordinate dihydrosilyl anion formed via coordination of H in an initial fast, reversible process, and its decomposition to the racemic silyl anion with loss of molecular hydrogen [Eq. (7)]. A gas-... 

This mechanism consists of several steps (1) oxidative addition of hydrosilane to the rhodium(I) complex (2) and (3) coordination and insertion of the ketone into the rhodium-silicon bond to form a diastereomeric a-silyloxyalkylrhodium intermediate (4) reductive elimination of alkoxysilane as a primary product and (5) hydrolysis of the alkoxysilane yielding an optically active alcohol. Hydrosilylation of prochiral ketones by prochirally disubstituted silanes leads to asymmetry on the silicon atom as well as on the carbon atom and, in the presence of chiral rhodium complexes, results in optically active monohydrosilanes (eq. (5)) [2] ... 

Stereoselective hydrosiiyiation of terpene ketones such as camphor and menthone catalyzed by Rh(PPh3)3Cl followed by hydrolysis produces different stereochemistry from reductions using metal hydrides . Bulky hydrosilanes favor the production of the more stable alcohols ... 

The hydrosilylations of a,/ -unsaturated esters, such as acrylates or methacrylates catalyzed by H2PtCl6-6H20132 or RhCl(PPh3)3132,133, are rather complicated, i.e., the selectivity of the reaction is markedly dependent on both the substituents of the substrate and structure of the hydrosilane. Nevertheless, high selectivities are achieved with the proper choice of reactants133. The reaction provides a convenient route to ketene silyl acetals as well as a selective reduction method in combination with hydrolysis. 

Scheme 10. Possible mechanisms for conversion ofDMPS to the silanol by NDO. B) Oxidation by molecular oxygen B) Enzyme-assisted hydrolysis of the hydrosilane function.

As mentioned earlier, hydrosilylation can be used as an alternative route of ketone reduction. By performing the addition of hydrosilanes in the presence of suitable chiral transition metal catalysts, prochiral ketones are transformed in the subsequent hydrolysis step into optically active alcohols (5) ... 

The asymmetric reduction of keto-esters via hydrosilylation has also been achieved in the presence of chiral rhodium catalysts. a-Keto-esters give the corresponding lactates after hydrolysis, and by varying the hydrosilane the optical yield can be increased to 85%. Acetoacetates give the corresponding 3-hydroxy-butyrate, but in much lower optical yield (ca. 20%), whereby levulinates give chiral 4-methyl-y-butyrolactones with optical yields of up to 84% [equation (4)]. 

It is most striking that the hydrosilylation of aj3-unsaturated carbonyl compounds using wono-hydrosilanes was found to proceed in a manner of 1,4-addition, while c i-hydrosilanes very specifically underwent 1,2-addition to carbonyl functionalities [35]. Since the resulting silyl enol ethers and allylic silyl ethers can readily be converted by hydrolysis to saturated carbonyl compounds and a,j3-unsaturated alcohols, respectively, these reactions may furnish a unique method for selective reduction of carbonyl compounds, equation (13). The results are summarized in Table 4. 

Somewhat different but still distinct effects of hydrosilanes on the stereoselectivity were observed when a rhodium(I) complex with DIOP was employed. As is seen from Table 8, monohydrosilanes such as phenyldimethylsilane reacted with ketones under rather forced conditions to afford, after hydrolysis, the corresponding alcohols in low optical yield. However, both chemical and optical yields were remark-... 

Although silicon nanocrystals are now more commonly prepared by a variety of means which are easier to scale up, e.g., pyrolysis of silanes (Xuegeng et al. 2004), thermal treatment of silsesquioxanes (Hessel et al. 2006, 2010), and from reactions of molecular silicon compounds (Wilcoxon et al. 1999 Bley and Kauzlarich 1996), this review will concentrate on routes which proceed via the formation of porous silicon. More general reviews of silicon nanocrystals Irom physics and chemistry perspectives are available (Shirahata 2011 Kang et al. 2011 Heitmann et al. 2005). Derivatization of porous silicon and SiNCs usually relies on the chemistry of the hydrogen-terminated silicon surface, which shares some of the organic reactivity of hydrosilanes (Buriak 2002). Reaction with alcohols results in Si-O-C bonded monolayers (Sweryda-Krawiec et al. 1999), but these are suseeptible to hydrolysis under ambient conditions. Alternately, addition of surface Si-H aeross a C = C double bond produces Si-C bonded monolayers, which are very stable. 

In order to quantify the dependence of the preferred ring size obtained in cyclopolysilane synthesis on the bulk of the substituent groups on the silicon Cartledge postulated a set of parametas derived from the rates of acid catalyzed hydrolysis of appropriate hydrosilanes. The model, however, was applied only with limited success. ... 

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