Hypervalent silicon compounds reactivity

The most recent comprehensive review focused primarily on the reactivity aspects of hypervalent silicon complexes6, while the latest review covers only silicon-oxygen coordination7. The present chapter focuses on synthesis and structure, silicon-29 NMR spectroscopy, and on the nonrigidity of hypervalent silicon compounds and the resulting kinetic and stereochemical studies. 

The single NMR resonance moved to low frequency with each addition of HMPA, and finally remained as a singlet at 8 = -78 ppm at all ratios of HMPA to silane of 3 1 or greater. The only reasonable structure for this new species is 5. These complex changes for a relatively simple system illustrate the subtle relationship between coordination and reactivity for silicon. It was observations such as these and others that stimulated us to try to make quantitative measures on hypervalent silicon compounds. 

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. 

Hypervalent silicon compounds attract interest from both the structural and reactivity point of view [1]. The azomethine AJV -ethylene-bis(2-hydroxyacetophenoneimine) (salen H2 1), was formed by condensation of ethylenediamine with 2-hydroxyacetophenone. We set out to synthesize hexacoordinate silicon complexes containing the salen ligand. The anion salen is able to chelate the silicon atom through four donor atoms. There are some rare examples of salen-silicon compounds known from the literature [2], but characterization of these compounds seems to be doubtful [3]. Structural aspects are uncertain due to the lack of crystal structure data. 

The Si-Cl distances at the octahedrally coordinated Si atom are lengthened compared with those at the tetrahedral silicon compounds. Hypervalent silicon compoimds are more reactive than tetracoordinate ones and exhibit their own pattern of reactivity. 

The first observation of penta- and hexaeoordinate silicon compounds was reported at the beginning of the 19th century by Gay-Lussac [87] and Davy [88], Subsequent investigation of hypercoordination in silicon compounds stimulated widespread use of nucleophilic activation and catalysis in the application of organosilicon compounds as reactive species in organic synthesis. Synthetic application for silicon-fluorine bond formation can be found in several reviews over the last two decades, and this section focuses on recent advances in the use of hypervalent organosilicon compounds in selective organic synthesis, in particular, selective carbon-carbon bond formation [89]. 

A number of important review articles have appeared in the area of pentaco-ordinate and hexaco-ordinate phosphorus chemistry. Robert Holmes has provided an extremely informative comparison of the hypervalency, stereochemistry and reactivity of silicon and phosphorus including the application of the latter to enzyme systems. The coordination chemistry of hydrophosphoranes including the formation of complexes from bicyclic-, tricyclic- and tetracyclic hydrophosphoranes has also been the subject of a comprehensive review with literature coverage to 1995. Numerous metal complexes are mentioned including Rh, Ru, Pd, Co, Fe, Mo, and W and the relevance to asymmetric catalysis is discussed. Neutral six-coordinate compounds of phosphorus, including mono-, di-, tri-, and tetracyclic examples, have also been reviewed. 

The work reported here is a continuation of our attempts to understand the basis of the reactivity of hypervalent compounds of silicon [7-8]. A particular emphasis of the current work is investigation of reactivity by quantitative methods such as thermodynamic and kinetic investigations. 

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. 

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