Hypervalent silicon compounds reactions

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

In this context, the existence of stable penta- and hexacoordinate silicon derivatives and their structure have elicited considerable interest. Isomerization processes of these compounds are also of importance, since penta- or hexacoor-dinate intermediates are implicated in the substitution and racemization of tetracoordinate silicon derivatives. For all these reasons, we include here (i) some available structural data for hypervalent silicon compounds (the review is not intended to be exhaustive), (ii) recent reports concerning the nucleophilically induced substitution and racemization reactions at silicon, and (ii) a criti-... 

Hypervalent silicon compounds 4. Reactions with organometallic reagents... 

Hypervalent silicon compounds have found wide utility in organic synthesis. In general, pentacoordinated anionic silicates are more reactive toward nucleophiles than are tetracoordinated silanes. For example, Mes2SiF2 is unreactive toward water, while (the 18-crown-6 potassium salt of) Mes2SiF3 is completely hydrolyzed within minutes. Similarly, the pentacoordinate anion HSi(OEt)4 is an effective reducing agent for aldehydes, ketones, and esters at or below room temperature (Scheme 2) no such reaction occurs with HSi(OEt)3. The difference in relative reactivities of hypervalent and nonhypervalent species is relevant to the intermediates proposed in Section 7.6. 

Summary Our investigations on silicon compounds of etfaylene-MiV"-bi (2 -hydroxyacetophenoneiminate) led to the synthesis and X-ray structure analysis of a new kind of salen complex — hypervalent silicon compounds with a threefold deprotonated salen ligand and an enamine structure. This stmctural unit provides access to new routes for synthesizing hypercoordinate silicon complexes. Addition reactions between various Brpnsted acids and these new pentacoordinate silicon compounds were carried out to precipitate complexes bearing hexacoordinate silicon atoms. 

The second major class of non-umpolung nucleophilic carbene catalysis comprises reactions by initial NHC-activation of various silicon compounds. Their proposed common pathway is thought to lead to a hypervalent silicon complex4 and thus provide carbene-catalyzed activation of the corresponding nucleophiles such as TMSCN, TMSCF3 etc. (Kano et al. 2006 Song et al. 2005 2006). It is not only certain carbon-silicon bonds that can be effectively activated, but a comparable activation of Si-O bonds, e.g. of trimethylsily enol ethers etc., allows for mild, NHC-promoted Mukaiyama aldol reactions (Scheme 6 Song et al. 2007). 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

V. Hypervalent Silicon Species as Intermediates in the Reactions of Organosilicon Compounds Dynamic Stereochemistry of... 

V. HYPERVALENT SILICON SPECIES AS INTERMEDIATES IN THE REACTIONS OF ORGANOSIUCON COMPOUNDS DYNAMIC STEREOCHEMISTRY OF PENTACOORDINATE ORGANOSIUCON... 

In addition to these stoichiometric reagents, many of the reactions of organosilicon compounds which are catalysed by nucleophiles, including those now commonly used in organic synthesis, involve hypervalent silicon intermediates. These systems are discussed in the last section of this chapter. 

Silicon-based Polymeric Materials Mechanistic Organosilicon Chemistry (a) Gas Phase and Photochemical Reactions (b) Hypervalent Silicon, Nucleophilic Substitution, and Biotransformations Structural Organosilicon Chemistry and New Organosilicon Compounds Organic Synthesis using Siiicon. 

Carbenium, silyl, or phosphonium cations are active Lewis acids. In addition, hypervalent phosphorus and silicon compounds can act as Lewis acids. Furthermore, ionic liquids, which are organic salts with a melting point below 100 °C, can act as Lewis acids. They can catalyze reactions either in substoichiometric amounts or, if used as the reaction medium, in stoichiometric or even larger quantities. The solvents can be efficiently recovered after the reaction. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

The sol gel chemistry of sihcon alkoxides is much simpler (see Silicon Inorganic Chemistry)P Si is fourfold coordinated (N = z = 4,) in the precursor as well as in the oxide so that coordination expansion does not occur. The electronegativity of Si is rather high compared to transition metals. Silicon alkoxides are therefore not very sensitive toward hydrolysis. Their reactivity decreases when the size of the alkoxy groups increases. This is mainly due to steric hindrance, which prevents the formation of hypervalent sihcon intermediates (see Hypervalent Compounds). Silicon alkoxides, Si(OR)4, are always monomeric. Heterometallic alkoxides have never been obtained via the reaction of a sihcon alkoxide with another alkoxide. Silicon alkoxides have to be prehydrolyzed before Si T M bonds can be formed. 

---