Carbonylate anions, reaction with silicon

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

Although there is some experimental evidence for the formation of hypervalent silicon species (Fukada et al. 2006) an alternative reaction pathway via preliminary carbene-mediated activation of the carbonyl or imine species respectively (1,2-addition), and subsequent reaction of the anionic species with TMSCN etc. (electrophilic trapping of the alkoxide) has been proposed (Suzuki et al. 2006b Marion et al. 2007). 

The intermediate metal hydride has been isolated on occasion for Co and Mn , and Eq. (b) has actually been used to prepare silicon-metal bonds (see 5.2.3.2.2.). Inspection of Table 1 reveals the ease of reaction of Co2(CO)g compared with the other carbonyls. Normally this reaction is performed simply by condensing volatile silane onto the carbonyl in the absence of solvent and then allowing rapid reaction in the liquid phase at room temperature, but for the remaining carbonyls it is necessary to use elevated temperatures and sealed, evacuated tubes. The products are volatile and readily purified by vacuum fractionation or sublimation, but are often oxygen and moisture sensitive. The route is most efficient for RjSi derivatives of Co, Mn and Re, which are not generally obtainable by the reactions of silicon halides with metal carbonyl anions (see S.8.3.3.I.). In this way lCo(SiR,)(CO -] = Et, Phj, Clj -, (OEt)j, F/, ... 

Reaction with ei,fi-Unsaturated Sulfoxides. The reaction of TMSI with a, -unsaturated sulfoxides in chloroform at ambient temperature is a mild, efficient, and general method for the preparation of carbonyl compounds (eq 63). The proposed reaction mechanism is shown in eq 63. Formation of a strong oxygen-silicon bond is followed by reduction of the sulfur function and oxidation of iodide to iodine, the latter precipitating in chloroform. The trimethylsiloxy anion attacks the unsaturated carbon linked to the sulfur function, which leaves the substrate, allowing the formation of the sUyl enol ether species. Finally, hydrolysis converts the silyl enol ether into the carbonyl compound. ... 

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—O" is then eliminated and a new C—C bond is formed. Such reactions take place because the formation of a Y—0 bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

In 1982, Sakurai [7] described a catalytic version of this reaction (Scheme 13.4). The addition of small quantities of fluoride anions to the allylsilane 1 generates the pentacoordinated silicon species 10, probably in equilibrium with the starting materials 1 and 11. This activated species can react with the carbonyl derivative 6 to yield the alkoxide 12 which is trapped by fluorotrimethylsilane. This last step not only furnishes the silylated compounds 13 but also regenerates the fluoride catalyst 11. Acidic work-up then leads to the desired homoallylic alcohol 7. 

In summary, this procedure will be most effective with an anion of high nucleophilicity, an iodosilane of low steric requirements, and a nonpolar solvent. Reactions usually begin at low temperatures, although long shaking at room temperature may be needed for completion. The method can be capricious, and occasionally a familiar system may yield no product at all, with no obvious explanation. Nevertheless, it has considerable extra potential, particularly for the synthesis of silicon derivatives of polynuclear metal carbonyls. 

Addition of R Li to the double bond gives an anion stabilized by two silicon atoms. Peteri-it reaction gives a vinyl silane and epoxidation and hydrolysis (p. 1302) gives the carbonyl compocril . There is no need to concern ourselves with stereochemistry as it all disappears in the final prod- -c The silicon atom is essential in every step except the final tautomerism. 

As a general mle, unless an anion-stabilizing group, such as phenyl, or a heteroatom such as sulfur is present, the alkylsilane is not readily deprotonated. The a-halosilane can be deprotonated but, unlike the readily available chloromethyltrimethylsilane, there are few general methods to this approach. Al-kyllithium reagents add to vinylsilanes ( ) to produce the carbanion (287). Silyl derivatives with heteroatoms, such as sulfur, selenium, silicon or tin, in the a-position (288) may be transmetallated (Scheme 41). Besides the difficulty in synthesizing the anion, alkene formation lacks specificity for simple di- and tri-alkyl-substituted alkenes. As a result, the Peterson reaction of an a-silyl carbanion with a carbonyl has found the greatest utility in the synthesis of methylene derivatives, (as discussed in Section 3.1.3), heterosubstituted alkenes and a,p-unsaturated esters, aldehydes and nitriles. 

The most intriguing results are those from the reaction promoted by fluoride. The reaction of fluoride with an allylmetal reagent is thought to proceed through either an allyl anion or an allyl fluorosiliconate intermediate [24]. Allyl anions [25] and pentacoordinate silicon species [26] have been proposed as intermediates in the fluoride-induced allylation of a carbonyl compound. The results obtained with the deuterium model rule out the intermediacy of a free allyl anion since the ratio 14/15 is different for the syn compared to the anti Sp/ pathways. 

Although the mechanism of the Mukaiyama reaction is not yet fully understood, several points have now been firmly established (a) a Lewis acid enolate is not involved (b) the Lewis acid activates the carbonyl group for the nucleophilic addition and (c) the Si—O bond is cleaved by nucleophilic attack of the anionic species, generally halide, on silicon. Point (a) has been established by the use of INEPT- Si NMR spectroscopy. Moreover, trichlorotitanium enolates have been synthesized, characterized and shown to give a completely different stereochemical outcome than the TiCU-mediated reactions of silyl enol ethers. Complexes between Lewis acids and carbonyl compounds have been isolated and characterized by X-ray crystallography and recently by NMR spectrometry. On the basis of these observations closed transition structures will not be considered here open transition structures with no intimate involvement between the silyl enol ether and the Lewis acid offer the best rationale for the after the fact interpretation of the stereochemical results and the best model for stereochemical predictions. 

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