<i>N,N</i>-bis-(dimethylfluorosilylmethyl)amides of <i>N</i>-organosulfonylproline and sarcosine: synthesis, structure, stereodynamic behaviour and <i>in silico</i> studies

(O→Si)-Chelate difluorides R3R2NCH(R1)C(O)N(CH2SiMe2F)2 (9a–c, R1R2 = (CH2)3, R3 = Ms (a), Ts (b); R1 = H, R2 = Me, R3 = Ms (c)), containing one penta- and one tetracoordinate silicon atoms were synthesized by silylmethylation of amides R3R2NCH(R1)C(O)NH2, subsequent hydrolysis of unstable intermediates R3R2NCH(R1)C(O)N(CH2SiMe2Cl)2 (7a–c) into 4-acyl-2,6-disilamorpholines R3R2NCH(R1)C(O)N(CH2SiMe2O)2 (8a–c) and the reaction of the latter compounds with BF3·Et2O. The structures of disilamorpholines 8a,c and difluoride 9a were confirmed by an X-ray diffraction study. According to the IR and NMR data, the O→Si coordination in solutions of these compounds was weaker than that in the solid state due to effective solvation of the Si–F bond. A permutational isomerisation involving an exchange of equatorial Me groups at the pentacoordinate Si atom in complexes 9a–c was detected, and its activational parameters were determined by 1H DNMR. In silico estimation of possible pharmacological effects and acute rat toxicity by PASS Online and GUSAR Online services showed a potential for their further pharmacological study

Influence of the initial chemical conditions on the rational design of silica particles

The influence of the water content in the initial composition on the size of silica particles produced using the Stöber process is well known. We have shown that there are three morphological regimes defined by compositional boundaries. At low water levels (below stoichiometric ratio of water:tetraethoxysilane), very high surface area and aggregated structures are formed; at high water content (>40 wt%) similar structures are also seen. Between these two boundary conditions, discrete particles are formed whose size are dictated by the water content. Within the compositional regime that enables the classical Stöber silica, the structural evolution shows a more rapid attainment of final particle size than the rate of formation of silica supporting the monomer addition hypothesis. The clearer understanding of the role of the initial composition on the output of this synthesis method will be of considerable use for the establishment of reliable reproducible silica production for future industrial adoption

Synthesis of silsesquioxane cages from phenyl-cis-tetrol, 1,3-divinyltetraethoxydisiloxane and cyclopentyl resins

The synthesis of T-8, T-10 and T-12 silsesquioxane cages from a range of starting materials: phenyl-cis-tetrol, 1,3-divinyltetraethoxydisiloxane and cyclopentyl T resins by using tetra n-butylammonium fluoride (TBAF) as the catalyst is described in this paper. The reaction yields obtained via the current route are better compared to those via the literature routes. Some of the cage compounds have been characterized by X-ray crystallography

Biomimetic catalysis at silicon centre using molecularly imprinted polymers

The first example of biomimetic siloxane bond formation catalyzed by molecularly imprinted polymers (MIPs) is reported. MIPs were prepared by free-radical polymerization and characterized via a combination of solid-state NMR, infrared spectroscopy, and electron microscopy studies. Methacrylic acid-based polymers with high functional monomer content were observed to catalyze the hydrolysis and condensation of a model monoalkoxysilane to a greater extent when compared with the corresponding blank (non-imprinted) materials and buffer control solutions, as judged by quantitative gas chromatography investigations. Moreover, the polymers were used as biomimetic catalysts to synthesize silica-MIP hybrids with a certain degree of control of the microstructure of the resulting composites

An artificial organosilicon receptor

A biomimicking approach for the selective capture of dimethylcyclosiloxanes was developed. Inclusion complexes between cyclodextrins (CDs) and cyclosiloxanes were isolated and subsequently treated with toluene-2,4-diisocyanate (TDI) in DMSO to afford molecularly imprinted cyclodextrin (MICD) polymers. Following removal of the siloxane-based templates, the imprinted biomimetic polymers were characterized via scanning electron microscopy (SEM), cross-polarization magic angle spinning (CPMAS) NMR and elemental analysis. Substrate affinity and selectivity were evaluated via equilibrium batch-rebinding assays and quantitative gas-chromatographic analysis. The imprinting effect was assessed by comparing the binding of the synthetic receptors with blank (non-imprinted) polymers. Adsorption isotherms were measured and data fitted using several mathematical models and the dissociation constants (Kd) and the binding site densities (Bmax) were calculated. The study is believed to have delivered the first case of an artificial receptor for an organosilicon substrate, opening a new way for separation and purification in silicon chemistry

Proton catalysis of nucleophilic substitution at pentacoordinate silicon

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 H-1-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. (C) 2002 Elsevier Science B.V. All rights reserved

Simple and mild preparation of silica-enzyme composites from silicic acid solution

Silica-enzyme composite materials including nanoparticles are formed readily from silicic acid and some hydrolase enzymes under mild conditions when the enzyme pI is greater than about 10

A kinetic and mechanistic study of nucleophilic substitution at a pentacoordinated silicon atom

A model system for the exploration of nucleophilic substitution at a pentacoordinate silicon centre is reported. The degenerate nucleophilic exchange of free N-methylimidazole (NMI) with coordinated NMI in a chiral pentacoordinate silicon compound, 4, has been studied by dynamic NMR spectroscopy. In 4 the axial ligands are N-methylimidazole and an intramolecularly-coordinated and chelating oxygen atom of an amido group. The equatorial ligands are N-CH2, Ph and Me. Two competing mechanisms for the nucleophilic substitution at silicon have been distinguished from the data. One mechanism takes place with retention of configuration at silicon and is zeroth order in added nucleophile. This is interpreted as involving rate limiting dissociation of the N-methylimidazole followed by rapid re-association of another NMI molecule. The other mechanism takes place with both inversion and retention of configuration at silicon but is also of zeroth order in added nucleophile. A mechanism for this reaction in which the O-Si coordination is lost in the rate-determining step is consistent with the data. In this particular example there is no evidence for a mechanism in which pentacoordinate silicon is attacked to give a hexacoordinate intermediate or transition state