Carbon-silicon bonds oxidation

Benzylic silanes bearing an electron-donating group experience an oxidative carbon-silicon bond cleavage selectively with oxovanadium(V) compounds to permit an intermolecular carbon-carbon bond formation the addition of TMSOTf resulted in a more facile coupling (eq 123)4 ... 

The selective carbon-silicon bond cleavage is of synthetic potential. Oxidative carbon-silicon bond cleavage is similarly achieved through oxovanadiumfV)-induced one-electron oxidation. The carbon-silicon bond of the dihydrofuran, which is obtained by the VO(OR)Cl2-induced cyclization of diketene with a-trimethylsilylstyrene is oxidatively cleaved by using VO(OEt)Cl2-Me3SiOTf, to give the furan [125]. 

Primary alcohols are available in good yields from terminal alkenes via the organopentafluorosilicate adducts (1) and a novel oxidative carbon-silicon bond cleavage using peracid (Scheme 2) This latter reaction is very medium-dependent DMF was found to be the solvent of choice. 

The view has also existed in the past that the carbon-silicon bond should be similar in behaviour to the carbon-carbon bond and would have a similar average bond energy. There is some measure of truth in the assumption about average bond energy but because silicon is more electropositive than carbon the C—Si bond will be polar and its properties will be very dependent on the nature of groups attached to the carbon and silicon groups. For example, the CH3—Si group is particularly resistant to oxidation but H13—Si is not. 

This excellent method of oxidative cleavage (/) of carbon-silicon bonds requires that the silane carry an electronegative substituent (2), such as alkoxy or fluoro. Either hydrogen peroxide or mcpba may be used as oxidant, and the alcohol is produced with retention of configuration (3). Fluoride ion is normally a mandatory additive in what is believed to be a fluoride ion-assisted rearrangement of a silyl peroxide, as shown below ... 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

The use of a silyl ether temporary linkage introduced by Nishiyama [82J and Stork [77,83] allows the facile cleavage of the five-membered ring formed in the cyclization by oxidation of the carbon-silicon bond. This procedure has been successfully used for the hydroxymethylation of sugars at position 3, 4, and 6 [84-86] (Scheme 24). 

Oxidative cleavage of the carbon-silicon bond has been the subject of a recent and comprehensive review119, in which emphasis has been placed on the compatibility of the oxidation conditions with various functional groups, with the inclusion of very useful compatibility tables. 

The high electropositivity of silicon means that the carbon-silicon bond is readily oxidized. Yoshida has carried out an extensive study of the anodic oxidation of benzyl and allyl silanes95. Oxidation converts the silane to a carbocation, which then reacts with a nucleophilic component of the medium (Scheme 18). Yoshida has shown that the reaction... 

The fact that oxidation of carbon-silicon bond is possible in the presence of a silyl ether increases the attractiveness of siletanes as masked hydroxyl groups. 

Ring expansion in conjunction with Tamao-type oxidation of carbon-silicon bonds provides access to 1,4-diols. The l-(l-iodoalkyl)-l-silacyclobutanes are available from 1-chlorosilacyclobutanes (addition of vinyl, Scheme 34) <1991TL6383>. The utility of silacyclopentanes formed by the ring expansion of SCB for the synthesis of diols has been reported <1992TL7031, 1995BCJ625>. 

To develop new methods for organic synthesis, Woerpel and coworkers exploited the inherent reactivity of di -fc/ f-butylsilacyclopropanes to create new carbon-carbon bonds in a stereoselective fashion (Scheme 7.7).62 They discovered that transition metal salts catalyze the insertion of carbonyl compounds into the strained carbon-silicon bond to form oxasilacyclopentanes. The regioselectivity of insertion could be controlled by the identity of the catalyst. Copper promoted the insertion of croto-naldehyde into the more substituted C-Si bond of 52 to afford oxasilacyclopentane 53,63 whereas zinc catalyzed the insertion of butyraldehyde into the less substituted bond of 52 to provide the complementary product, 54.64 Oxasilacyclopentanes (e.g., 55) could be transformed into useful synthetic intermediates through oxidation of the C-Si bond,65 66 which provided diol 56 with three contiguous stereocenters. 

It has seemed quite natural to think of silicon only in terms of the oxide, for practically all of the earth s silicon is bound up with oxygen. Together these two elements constitute some 76 per cent of the solid crust of the earth, and there is more than enough oxygen to combine with all the silicon. Free silicon therefore does not occur in nature, nor do its organic compounds. The only natural substance which has been demonstrated to have carbon-silicon bonds is the rare mineral moissanite, which is silicon carbide, and this ordinarily is not thought of as an organosilicon compound. 

Exhaustive cleavage of the carbon-silicon bond followed by treatment with an acid converted the complex benzo[f]furan 261 to phenol 262, as illustrated in Equation (154) <2003JA12994>. Villeneuve and Tam were able to interrupt this phenol formation by choosing Cp"Ru(COD)Cl as the catalyst. Thus, the reaction of 1,4-epoxy-1,4-dihydronaphthalene 263 with a ruthenium catalyst in 1,2-dichloroethane at 60 °C afforded the 1,2-naphthalene oxide 264 (Equation 155) <2006JA3514>. 

Oxidation of Si—C bonds. The carbon-silicon bond of organoalkoxysilanes is cleaved by 30% H202 or m-chloroperbenzoic acid in the presence of a base or under neutral conditions in the presence of KHF2 (equation I). Only one alkoxy group on silicon... 

An impressive synthesis of estrone (51) was reported Vrdlhardt and cowoikers. G>bait-catalyzed cooligomerization of the diyne (49) with bis(trimethylsilyl)acelylene gave the estratrienone (50) in 71% yield. Introduction of the hydroxy substituent at C-3 was then cleverly achieved by selective proto-desilylation at C-2, followed by oxidation of the carbon-silicon bond using LTFA (Scheme 19). 

This chapter concentrates on those processes in which oxidative cleavage of a carbon-silicon bond results in pr uction of the alkyl/aiyl fragment as an alcdiol henol. Other cleavage processes are dealt with, but more briefly. 

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