Hydrosilylation silyl functional groups

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],... 

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

The ruthenium carbene complex (Grubbs catalyst) which has shown high efficiency in alkene methathesis and related processes, since it displays tolerance toward a wide variety of common functional groups, has also appeared of synthetic utility in the hydrosilylation of ketones to yield silyl ethers-one of the most widely used classes of protecting groups in synthetic chemistry (Eq. 97) [ 151 ]. The reaction requires temperatures above 50 °C, which generate a slightly increased amount of silylated by-products. 

In recent years, we have been investigating easy and economical functionalization of widely nsed carbon based polymers snch as polybntadienes. The preliminary results of these studies have led our group to discover a highly selective and mild synthetic route to silyl-functionalization of 1,2-polybutadienes (PBD) via Pt-nanocluster catalyzed hydrosilylation of olefin bonds. Unlike other catalytic systems, our system was found to be equally effective with all varieties of functional silanes such as halo-, alkyl-, aryl- and alkoxy- silanes affording high yields and selectivities. In addition, all the hydrosilylation reactions were found to be very clean with the ease of product separation and purifications (Scheme 2). 

Fully substituted alkenes are hydrosilylated with aluminum chloride without double-bond migration. The products have tertiary alkyl groups and thus may be utilized as silyl protecting groups for alcohols and related functional groups. " For example, chlorodimethylthexylsilane (49) is readily prepared from 2,3-dimethyl-2-butene (equation 42). 

Asymmetric hydrosilylation of 2-phenyl-1-butene yields enantiomeric excess ee) values as high as 68% [149]. Products obtained by sequential cyclization/ silylation reactions of 1,5-dienes and 1,6-dienes feature in the suggested mechanistic scenario (Scheme 8) [149, 155]. Furthermore, hydrosilylation of terminal olefins achieved both excellent chemoselectivity in the presence of any internal olefin, and functional-group compatibility with halides, ethers, and acetals [155]. 

The sequential cyclization/silylation reactions of 1,5-dienes and 1,6-dienes are catalyzed by Cp 2YMe(THF). The reaction tolerates a number of functional groups and proceeds with good yields and diastereoselectivities to give phenylsilane products which can be converted easily to synthetically more versatile alcohols (Scheme 276). The hydrosilylation of dienes is also effectively catalyzed by the neodymium alkyl complex Cp 2NdCH(SiMe3)2. 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

Allyl derivatives of hydrocarbons with functional group were often used as starting materials. The hydrosilylation of allylperfluoroethers with methyl(chloro)silanes is a frequently used method for synthesis (after hydrolysis and polycondensation) of fluorine-containing polymers (162). Unsaturated esters of unsaturated carboxylic acids in the reaction of hydrosilylation give silyl derivatives of carboxylic acid (e.g., 3-methacryloxypropyltriethoxysilane). 

This hydrosilylation method can also be applied to the reduction of esters. The silyl acetal products can be hydrolyzed, resulting in net reduction of esters to aldehydes. For example, ethylbenzoate can be fully reduced in the presence of 0.1 mol % [Ir(coe)2Cl]2 and 1.5 equiv of diethylsUane at room temperature for 1 h to give benzaldehyde after hydrolysis (eq 3). The functional group compatibility is analogous to that of the amide reduction. 

This chapter aimed to provide a summary relating to major catalytic routes for obtaining mono-, di-, and octafunctionalized silsesquioxanes. From the synthetic point of view the choice of the optimum method, ie, silylative coupling, cross-metathesis, hydrosilylation, or nucleophilic substitution, depends on the kind of functional groups to be introduced to the silsesquioxyl core (eg, saturated or unsaturated) and specificity of reaction system. A related family of silsesquioxanes, ie, spherosilicates and... 

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