Kinetic silicon atoms

Since the LUMO is n-antibonding, the kinetically controlled interaction of a donor molecule (HMPA) with the silicon leads to a decrease of multiple bonding between Cr and Si concomitant with a pyramidalization at the silicon atom. The resulting MSi bond distance and pyramidalization effect are strongly influenced by the respective substituents. 

The reduction of tributyltin methoxide with optically active methyl-phenyl-1-naphthylsilane involves retention of configuration at the silicon atom and follows second-order kinetics (2 72). The reaction between tributyltin methoxide and ring-substituted dimethylphenylsilanes shows a Hammett p-value of -t-0.903, and that between dimethyl-phenylsilane and ring-substituted tributyltin phenoxides shows a p-value of -1.319 this is compatible with the reactions proceeding through a 4-centered (SNi-Si) transition state (272, 173). 

The unsaturated silicon atoms form five bonds because of the donor. They have a tetrahedral environment and are four-coordinate. The adducts can be hydrolyzed very easily and are kinetically stable under normal conditions. One of the adducts, the THF adduct of Me2Si=N—Si(CMe3)3, has been isolated by Wiberg et al., as shown in Eq. (10).15... 

Corriu et al. have reported that the coupling reaction of 2-(iV,iV-dimethylaminomethyl)phenyllithium with (McvSi)vSiCI 53 affords 2-(iV,iV-dimethylaminomethyl)-l-[tris(trimethylsilyl)silyl]benzene 894. No evidence has been found that the intramolecular iV-ligand coordinates to the silicon atom of 894. Upon UV irradiation, the trisilane forms a transient silyene 895, which has been trapped with 2,3-dimethyl-2,3-butadiene and triethylsilane to give the oligosilanes 896 and 897 as well as 898-900, (Scheme 126).859 Apparently, the bulk on the two ligands is insufficient to provide kinetic stabilization of the silylene intermediate 895. 

Lekishvili N.G., Katsitadze M.G., Nakaidze L.I., Khananashvili L.M. Some Kinetically Regularities of Polymerization Condensation Reaction of Organocyclosilazanes with Spacial Groups at Silicon Atoms with Aromatic Dihydroxy compounds. Bulletin of the Academy of Sciences of Georgia. Series of Chemistry. 152, 3 529-531 (1995) (Rus.). 

The kinetics of the acid catalyzed hydrolysis of ethoxysilanes has been studied. Each of the silanes that were used had a phenyl or para-substituted phenyl group attached to the silicon atom. This permitted a study of the linear free energy relationships of this reaction. The reaction is of interest because of its role in silane coupling agent chemistry, in the preparation of zinc-rich silicate coatings, in the sol-gel process and in the preparation of silicones in general. 

Enantioselective enzymatic ester hydrolyses have also been used for the preparation of optically active silicon compounds with the silicon atom as the center of chirality. An example of this is the kinetic resolution of the racemic 2-acetoxy-l-silacyclohexane rac-(SiR,CR/SiS,CS)-79 with porcine liver esterase (PLE E.C. 3.1.1.1) (Scheme 16)65. Under preparative conditions, the optically active l-silacyclohexan-2-ol (SiS,CS)-80 was obtained as an almost enantiomerically pure product (enantiomeric purity >96% ee) in ca 60% yield [relative to (SiS,CS )-79 in the racemic substrate]. The biotransformation product could be easily separated from the nonhydrolyzed substrate by column chromatography on silica gel. 

Enantioselective enzymatic amide hydrolyses can also be applied for the preparation of optically active organosilicon compounds. The first example of this is the kinetic resolution of the racemic [l-(phenylacetamido)ethyl] silane rac-84 using immobilized penicillin G acylase (PGA E.C. 3.5.1.11) from Escherichia coli as the biocatalyst (Scheme 18)69. (R)-selective hydrolysis of rac-84 yielded the corresponding (l-aminoethyl)silane (R)-85 which was obtained on a preparative scale in 40% yield (relative to rac-84). The enantiomeric purity of the biotransformation product was 92% ee. This method has not yet been used for the synthesis of optically active silicon compounds with the silicon atom as the center of chirality. 

Enzymatic kinetic resolutions via enantioselective esterifications have been successfully used for the preparation of optically active (hydroxymethyl)silanes, with the silicon atom... 

It can be seen in Figure 5 that when the amount of silicon in SAPO-37 is increased, the conversion increases up to 44 Si/unit cell and then decreases. In order to avoid the possible contribution of the free alumina (Table I), present in minor amounts in these samples, the first order kinetic rate constant per unit of BET surface area (calculated at cat/oil = 0.4) has been plotted as a function of the silicon content, Figure 6. The same behavior is observed. This behavior can be explained in an analogous way as in the case of zeolites, but considering that the acid sites in SAPO are related to the introduction of silicon. Therefore, in SAPO, the number and density of acid sites will be related to the number of silicon atoms. Consequently,... 

The kinetics of nucleophilic substitution at the silicon atom assisted by uncharged nucleophiles have been studied by Corriu et at. (248-251). Hydrolysis of triorganochlorosilanes induced with HMPA, DMSO, and DMF was used as the model. The reaction proceeded according to the third-order kinetic law, first order with respect to the nucleophile, the silane, and the silylation substrate. Very low values of activation enthalpy and high negative entropy of activation were observed (Table VI). These results were taken as evidence for the intermediacy of silicon hypervalent species (249,251) however, they are also perfectly consistent with... 

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