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Synthesis and X-ray diffraction study of cationic bischelates MeSi(SCH(2)CONMe(2))(2)(+)Cl(-) and MeGe(SCH(2)CONMe(2))(2)(+)Br(-) are reported. According to X-ray data, the Si and Ge atoms in these compounds have distorted TBP environments with two coordinating oxygen atoms in axial positions.

Biotransformations make use of biological systems to catalyze or promote specific chemical reactions. Transformations that utilize enzymes as "greener" and milder catalysts compared to traditional reaction conditions are of particular interest. Recently, organosilicon compounds have begun to be explored as non-natural enzymatic substrates for biotransformations. The aims of this study were to screen readily available (approximately eighty) enzymes for their ability to catalyze in vitro siloxane bond formation under mild reaction conditions using a model monoalkoxysilane as the substrate and to make a preliminary evaluation of potential factors that might lead to activity or inactivity of a particular enzyme. Several new hydrolase enzymes were observed to catalyze the formation of the condensation product when compared to peptide controls, or buffer solutions at the same pH, as judged from quantitative analyses by gas chromatography. Aspergillus ficuum phytase, Aspergillus niger phytase, chicken egg white lysozyme, porcine gastric mucosa pepsin, and Rhizopus oryzae lipase all catalyzed the condensation of silanols in aqueous media. Factors involved in determining the activity of an enzyme towards silanol condensation appear to include: the presence of imidazole and hydroxyl functions in the active site; solvent; the presence of water; the surface properties of the enzyme; possible covalent inhibition; and steric factors in the substrate., (Copyright © 2010 Elsevier Inc. All rights reserved.)

The potential for expanding the variety of enzymic methods for siloxane bond formation is explored. Three enzymes, Rhizopus oryzae lipase (ROL), lysozyme and phytase are reported to catalyse the condensation of the model compound, trimethylsilanol, formed in situ from trimethylethoxysilane, to produce hexamethyldisiloxane in aqueous media at 25 °C and pH 7. Thermal denaturation and reactant inhibition experiments were conducted to better understand the catalytic role of these enzyme candidates. It was found that enzyme activities were significantly reduced following thermal treatment, suggesting a potential key-role of the enzyme active sites in the catalysis. Similarly, residue-specific modification of the key-amino acids believed to participate in the ROL catalysis also had a significant effect on the silicon bio-catalysis, indicating that the catalytic triad of the lipase may be involved during the enzyme-mediated formation of the silicon-oxygen bond. E. coli phytase was found to be particularly effective at catalysing the condensation of trimethylsilanol in a predominantly organic medium consisting of 95% acetonitrile and 5% water. Whereas the use of enzymes in silicon chemistry is still very much a developing and frontier activity, the results presented herein give some grounds for optimism that the variety of enzyme mediated reactions will continue to increase and may one day become a routine element in the portfolio of the synthetic silicon chemist.

A new pentacoordinate silicon species containing two chelating ligands has been synthesized. The structures of four independent cations of the same compound correspond to different points on the Berry pseudorotation pathway. The percentage of square planar character varies between 19% and 40%. [ABSTRACT FROM AUTHOR]

The X-ray crystal structures of four 1-(halodimethylsilylmethyl)-2-quinolinones have been measured and used to model the reaction profile for nucleophilic substitution at silicon. Similar structural correlations have been performed in solution, the percentage Si&z.sbnd;O bond formation being obtained from the 13C chemical shifts of the quinolinone carbons and the extent of pentacoordination from the 29Si chemical shift of the silicon. Excellent agreement is obtained between the two methods confirming the validity of the NMR technique for structural correlation in solution. [Copyright &y& Elsevier]

Addition of acid to the pentacoordinate fluorosilane 8, leads to enhanced exchange of fluorine and loss of diastereotopicity of the silicon methyl groups. A DNMR study of the 1H-NMR spectra suggests a dissociative mechanism involving protonation of the fluorine leaving group. Variable temperature studies suggest that at lower temperatures the tetracoordinate form is favoured. [Copyright &y& Elsevier]

Solution 1JSi&z.sbnd;F and 2JC&z.sbnd;F NMR coupling constant and 19F-NMR chemical shift data have been analysed in a series of pentacoordinate silyl monofluoride complexes used to model nucleophilic substitution at silicon. Patterns in the data reveal strong correlations between both coupling constants and 19F-NMR chemical shifts and the degree of substitution displayed by each. Excellent correlation is obtained between the new data and our previous 13C- and 29Si-NMR studies to further confirm the validity of the NMR technique for structural correlation in solution. By pooling the X-ray crystal structures of a large number of compounds of this class from both our own results and the literature we are able to provide a possible explanation for some of the trends in the NMR data we observe in solution. [Copyright &y& Elsevier]