Activations silyl ethers

The steps involved are (I) oxidative addition of hydrosilane to the rhodium(I) complex (5) to give 6 (ii) insertion of the carbonyl into the resulting silicon-rhodium bond of 6 to form diastereomeric a-siloxyalkylrhodium hydride intermediate 8, and (3) formation of an optically active silyl ether of sec-alcohol by reductive elimination ... 

It was shown that activated silyl ethers proceed in high yield (56 — 57) whereas non-activated silyl ethers (58 — 59) proceed in moderate yields and require much longer reaction times. 

Addition of methyldichlorosilane to a series of alkyl phenyl ketones catalyzed by these chiral phosphine-platinum(II) complexes gives partially optically active silyl ethers of 1-phenylalkanols, equation (14). 

Benzyloxy-2-fluoro-2-methylpropionaIdehyde was prepared in optically active form from (5)-monoethyl 2-fluoro-2-methylmalonate, which had itself been prepared by enzymatic hydrolysis A number of enol silyl ethers or enolates were added to the aldehyde in processes that occur with fair to good diastereoselectivity [6] (equation 6) (Table 2)... 

A proposed mechanism for silyl ether displacement is shown in Scheme 6.14. In the first step, the fluoride anion converts the trimethyl siloxy group into a phe-nolate salt. In the following step, the phenolate anion attacks the activated fluoro monomer to generate an ether bond. The amount of catalyst required is about 0.1-0.3 mol%. Catalyst type and concentration are crucial for this reaction. 

Scheme 9). Although cyanohydrin acetonide 64 could conceivably have been used, the silyl ether 75 was chosen. This compound is readily available from (l)-malic acid, and can undergo electrophilic activation under far more mild conditions than compound 64. Alkylation of the 1,3-diol synthon 75 with bromide 76 created the C11-C26 framework of roflamycoin, in 85% yield. A two-step conversion of the terminal siloxy group to the primary iodide (78) proceeded in 80% overall yield. 

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],... 

Intramolecular asymmetric hydrosilylation-oxidation of (alkenyloxy) hydrosilanes provides an efficient method for the preparation of optically active polyols from al-lylic alcohols. Cyclization of silyl ethers 54 of a meso-type allyUc alcohol in the pres-... 

This must reflect activation of the carbonyl group by magnesium ion, since ketones are less reactive to pure dialkylzinc reagents and tend to react by reduction rather than addition.141 The addition of alkylzinc reagents is also promoted by trimethylsilyl chloride, which leads to isolation of silyl ethers of the alcohol products.142... 

Instead of the absorption of chiral modifiers on metal surfaces, a new method using a slightly different approach attaches chiral moieties directly to metal surfaces through chemical bonds. Chiral silyl ethers have been attached to Pd surface atoms these new catalysts have the form (Pd)s=Si-0-R(,< orS) 42 Their synthesis arose from studies of the effects of siliconation on the catalytic activities and selectivities of dispersed, supported Pd and Pt.43-47 The results from... 

Kibayashi and coworkers have used enantiometrically pure allylic silyl ethers obtained from amino acids in cycloaddition with nitrones (Eq. 8.49).71 Cyclic nitrone reacts with a chiral allyl ether to give selectively the exo and erythro isomer (de 90%). Optically active alkaloids containing a piperidine ring such as (+)-monomorine,71c (+)-coniine,71a and (-)-oncinotine71b have been prepared from the addition product. 

The rhodium-catalyzed intramolecular hydrosilylation of allylic alcohol derived silyl ethers has been described. Oxidative cleavage of the resulting cyclized hydrosilylation products affords a route to optically active diols (Scheme 28).129,130... 

The notion of enol silyl ethers (ESE) as electron donors was first provided by Gassman and Bottorff,34 who showed that selective (carbonyl) deprotection can be readily achieved in the presence of an alkyl silyl ether group via an electron-transfer activation (e.g., equation 9). 

Having shown that the enol silyl ethers are effective electron donors for the [D, A] complex formation with various electron acceptors, let us now examine the electron-transfer activation (thermal and photochemical) of the donor/ acceptor complexes of tetranitromethane and quinones with enol silyl ethers for nitration and oxidative addition, respectively, via ion radicals as critical reactive intermediates. 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

Several classes of silyl ethers have been shown to be excellent substrates for the C-H insertion chemistry of donor/ acceptor-substituted carbenoids.81 Effective C-H insertions predominantly occur at methylene sites. Primary sites are not sufficiently activated electronically while tertiary sites are sterically too crowded. Rl -DOSP -catalyzed functionalization of the allyl silyl ether 3 resulted in a highly diastereoselective transformation, leading to the formation of the /3-hydroxyester 4 in 94% yield and 82% ee (Equation (17)).81 This example illustrates the regioselectivity of this chemistry, because 3 contains two allylic sites but only the methylene site adjacent to the siloxy group was functionalized. Even better substrates are the commercially available tetraalkoxysilanes such as 5, because with these substrates, the high diastereoselectivity was retained while the enantioselectivity was increased (Equation (18)).81... 

The intermediate enol silyl ether permits further regioselective substitutions such as bromination followed by dehydrobromination (Eq. 81)49> and alkylation (Eqs. 82 93) and 83 103)). Thus, in addition to activating the rearrangement, the oxygen substituent regioselectivity creates an enol silyl ether, a powerful enolate synthon. 

The galactosyl glycal 72 was bound to solid phase via a silyl ether linker. Epoxidation of the glycal using 2,2-dimethyldioxirane and subsequent electrophilic activation of the epoxide resulted in the glycosylation of 3,4-di-O-benzyl glucal 73. After acetylation of the product, the polymer-linked trisaccharide 74 was obtained. 

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. 

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