Silicon reaction with catechol

We have solved this problem taking into account previous German work [9] which describes the preparation of hexacoordinated silicon species directly by reaction of silica with catechol in presence of ammonia. 

The Lewis acidic character of reactive pentacoordinate silicon compounds has been unequivocally confirmed by Corriu, Sakurai and Hosomi [90]. Allylsilicates prepared from allylsilanes and catechol can undergo allylation reaction with aldehydes in the absence of Lewis acid promoter (Sch. 51). 

Silicon/boron exchange. For the sake of increasing reactivity in coupling an alkenylsilane is converted to a boronate by reaction with BCI3 and then with catechol. Such a product can be used in Suzuki coupling. 

Thus, the reactions with alkali metal glycolates or with catechol do not differentiate between SiO, elemental Si or Si02 (except that silicon is oxidized in the former two cases). 

The reaction of decamethylsilicocene with catechol gives the unusual silicon hydride 12, the structure of which was determined by NMR spectroscopy (equation 6). The Si NMR chemical shift for 12 is 386 ppm deshielded when compared with Cp Si and the Si—H coupling constant is 302 Hz both of which conld be regarded as consistent with a silicenium ion. However, althongh the silicon in 12 is formally only bound to three snbstitnents, its coordination nnmber can be regarded as higher because the Cp groups are tt- rather than a- bonded. 

Reaction of functionalized ketones such as a-hydroxy ketones or 1,3-ketones with allyltrifluorosilane in the presence of Et3N has also been investigated [95]. Although catecholate does not play a key role in the pentacoordination of silicon, chelating hexacoordinate intermediates are formed to give allylation products with high yields and selectivity (Sch. 54). 

Considering the thermodynamic parameters of the reaction, it has been determined that the silicic acid with a hexacoordinated atom of silicon H2[Si(OH)6] formed in die process of reaction displaces ammonium ion from phenolates, so that this acid is stronger than catechol. This is in agreement with our experimental data at the beginning of the reaction process the decrease in the pH of the reaction mixture from 7.16 to 5.85 was observed before the pH became constant. 

To conclude, it is necessary to note that the reaction ability of biogenic silica depends on its structure and solubility, determined by various factors. It has been found that soluble forms of silica could be stabilized by S - 7% of glycerin and catechol. It has been determined that the amorphous part of the biogenic silica of siliceous rocks formed the complex with triethylphosphate and actively reacts with polyphenols (the simple one, catehol and complex ones, humic acids), with formation of ethers. The silicon organic derivatives and complexes formed are inert to hydrolysis. 

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