Silicon—Carbon Bond Formation

Carbon-shielding tensors, alkene and alkyne transition metal complexes, 1, 472 Carbon—silicon bond formation... 

B. CARBON-SILICON BOND FORMATION B.i. Reactions of Aryl, Alkyl, and Benzyl Halides... 

Abstract The recent progress in catalytic asymmetric carbon-boron and carbon-silicon bond formation catalyzed by chiral copper complexes is tremendous. Within less than a decade, the majority of fundamental bond-forming reactions in this arena, that is, conjugate addition, 1,2-addition and aUylic substitution, were accomplished. These enantioselective transformations had been either elusive or not even known before. This chapter summarizes these fascinating developments together with a brief mechanistic discussion as these copper catalyses share transmetalation of interelement bonds such as B-B and Si-B as a common feamre. 

Procter and co-workers employed chiral C2-symmetric NHC-precursor L12 [37] for asymmetric carbon-silicon bond formation, also applying it to kinetic resolution (Scheme 11) [38]. Conjugate silyl transfer onto 5-substituted butenolides 55a-c furnished the corresponding enantiomerically enriched a ri, 5-disubstituted lactones 56a-c in acceptable yields. Furthermore, the scope of this 1,4-addition was extended to lactones of different ring sizes (not shown). It is worthy of note that the parent lactone that had failed to react with Hoveyda s setup was obtained with good enantiomeric excess (55d 56d). 

Sawamura M, Ito H (2014) Carbon-boron and carbon-silicon bond formation, h Alexakis A, Krause N, Woodward S (eds) Copper-catalyzed asymmetric synthesis. Wiley, Weinheim,... 

An example of C—Si bond formation concludes this overview of carbon heteroatom bond formation. Reflux of bromide 62 in benzene and in the presence of small amounts of (TMS)3SiH and AIBN afforded the silabicycle 63 in 88 % yield (Reaction 7.64) [76]. The key step for this transformation is the intramolecular homolytic substitution at the central silicon atom, which occurred with a rate constant of 2.4 x 10 s at 80 °C (see also Section 6.4). The reaction has also been extended to the analogous vinyl bromide (Reaction 7.65) [49]. 

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... 

Organic silane derivatives with hydrolyzable groups on silicon (—OR, —Cl, —0C(0)R, etc.) are usually derived from a chlorosilane. Before or after formation of a carbon-silicon bond by hydrosilylation, the chlorosilanes are commonly converted to alkoxysilanes ... 

Various activated olefins can also be employed instead of organic halide for the formation of a carbon-silicon bond. Thus, cathodic reduction of a,j -unsaturated esters, nitriles... 

As described in Section II, Lewis acid catalyzed desilylative carbon-carbon bond formation with an electrophile has been shown to be very versatile in organic synthesis. Occasionally, depending on the nature of the substrates (e.g. the presence of appropriate functional groups), the carbon-silicon bond may remain intact. For example, treatment of 132 with a Lewis acid affords a mixture of cyclization products 133-135 (equation 113). The isolation of 133 indicates that the carbocation intermediate thus formed is trapped by the oxygen nucleophile before elimination of the silyl moiety occurs204. 

Aldimines can be trifluoromethylated at the imine carbon using Me3SiCF3 in dimethyl formamide at —20 °C, using a lithium carboxylate as catalyst.71 It is proposed that the carbon-silicon bond of the reagent is activated via formation of a lithium silicate bearing carboxylate and DMF ligands on silicon. A similar process has been used for diastereoselective addition to sulftnylimims.12... 

Dioxolane formation is not observed in allyl silanes as a result of silicons diminished ability in comparison to tin to stabilize a P positive charge. Nevertheless, the propensity of the carbon-silicon bond to donate electron density to electron deficient sites leads to population of a perpendicular conformation (Sch. 10) and ultimately to an unusually high yield of the sterically less stable Z-allylic hydroperoxide (e.g., 17). 

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>. 

The corresponding inter-and intra-raolecular additions of allyl-silanes to iminium salts give rise after desilylation to synthetically useful biradical intermediates. Evidence for the existence of two mechanisms for such reactions, differing only in the timing of carbon-silicon bond cleavage and carbon-carbon bond formation, has been described. Analogous electron transfer... 

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