Alan R. Bassindale

The interaction of electrophilic silanes (Me3SiX, X = ClO4, I, CF3SO3, Br, Cl) with nucleophiles. The nature of silylation mixtures in solution.

Abstract Silanes, Me 3 SiX, and Nucleophiles, Nu, form 1:1 salts, M 3e SiNu + X − in solution. The effect of both nucleophile and silane on salt formation has been studied, and the implications for the design of more efficient silylation mixtures are discussed.

Siloxane and hydrocarbon octopus molecules with silsesquioxane cores

Hydrogen silsesquioxane, (HSiO)8, T8, has been synthesized and isolated in synthetically useful quantities. The T8 cage-like structure allows eight pendant groups to be placed symmetrically about the surface of a sphere to form an ‘octopus’ molecule. Using T8 to hydrosilylate compounds with the formula CH2CH(X)nY, octopus molecules of the formula (SiO)8(CH2CH2(X)nY)8 have been synthesized, including octopus molecules with organic ‘arms’ as well as those with silicone ‘arms’ of various lengths. The regioselectivity of addition in the above two cases was found to differ. The synthesis of these novel octapus molecules and their characterisation by 29Si NMR, FTIR, and gel-permeation chromatography (GPC) is described.

The synthesis of functionalised silyltriflates

Abstract The cleavage of R 2 SiXY (R = alkyl, phenyl; X = H, Cl, Y = α-Np, Ph, Cl, H) by triflic acid is selective, leading to new difunctional silyltriflates, R 2 SiXOTf, with the relative ease of cleavage of Si-Y being, α-Np>Ph>Cl>H⪢Me, Et, Bu t .

Enzyme-catalysed siloxane bond formation

Biosilicification occurs on a globally vast scale under mild conditions. Although research has progressed in the area of silica biosynthesis, the molecular mechanisms of these interactions are effectively unknown. The natural production of silica in the Tethya aurantia marine sponge, Cylindrotheca fusiformis diatom, and Equisetum telmateia plant appear to be similar. However, the studies were complicated mechanistic queries due to the use of silicic acid analogues. Given these complications, a carefully chosen model study was carried out to test the ability of enzymes to catalyse the formation of molecules with a single siloxane bond during the in vitro hydrolysis and condensation of alkoxysilanes. Our data suggest that homologous lipase and protease enzymes catalyse the formation of siloxane bonds under mild conditions. Non-specific interactions with trypsin promoted the in vitro hydrolysis of alkoxysilanes, while the active site was determined to selectively catalyse the condensation of silanols.

The Structure of Silylated Amides: N-Methyl-N-Trimethylsilyltrifluoroacetamide, a Reassignment of structure

Abstract The 29 Si and 14 N nmr spectra of some bissilylamides and monosilylamides are reported. The results are compared with those for model silylesters, amines and imines. The compound of formula CF 3 CONCH 3 Si(CH 3 ) 3 (MSTFA) is shown to exist as a tautemeric mixture of a silylamide and a silylimidate. This assignment is confirmed by 13 C nmr. A rationale is proposed for the structure directing effect of substituents on the amide carbon atom.

Modelling SN2 nucleophilic substitution at silicon by structural correlation with X-ray crystallography and NMR spectroscopy

Abstract 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O bond formation being obtained from the 13 C chemical shifts of the quinolinone carbons and the extent of pentacoordination from the 29 Si 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.

Derivatisation of octasilsesquioxane with alcohols and silanols

The T8 cage in (HSiO32)8 has been successfully derivatised to give (ROSiO32)8 and (R3SiOSiO32)8 by the reaction with alcohols and silanols in the presence of catalytic amounts of amines. The most effective catalyst is Et2NOH.

Pentacoordinate silicon complexes with dynamic motion resembling a pendulum on the SN2 reaction pathway

A series of glutarimide derivatives which has two carbonyl coordination sites for intramolecular pentacoordination at silicon with a X(1+n)SiC(3-n)O moiety have been synthesised and characterized. The substituent (leaving group) effects on the Si-O bond exchange between the two coordination sites (resembling a pendulum) have been studied by comparison of the differently substituted (X = F, Cl, OTf, Br and I) structures. The activation parameters for the Si-O bond exchange process were measured by NMR and separately computed and are consistent with the strength of Si-O bond coordination and the nature of the leaving group, X. The temperature-dependent (29)Si NMR spectroscopy is supported by X-ray crystallography and shows that the tetrahedral reactant is converted into pentacoordinate intermediates by intramolecular O-Si association followed by reversion to a tetrahedral geometry by Si-X dissociation. The two association/dissociation patterns offer a model for nucleophilic substitution at a silicon atom. A continuum of structures on the S(N)2 reaction profile from the glutarimide derivatives correlates reasonably well with the structural data obtained from derivatives of lactams, diketopiperazines and quinolones.

A higher yielding route for T8 silsesquioxane cages and X-ray crystal structures of some novel spherosilicates

The synthesis of T8 silsesquioxane cages from trialkoxysilanes using tetra n-butylammonium fluoride is described. The yields are in the range 20–95%, which is a great improvement on other literature routes. This methodology enables a wide range of functionalised T8 cages to be prepared. The X-ray crystal structures of three new T8 cages, octacyclopentylsilsesquioxane, octaisobutylsilsesquioxane and octa(4-carboxymethyl-3,3-dimethylbutyl)silsesquioxane are also reported.

Modelling nucleophilic substitution at silicon using hypervalent silicon compounds based on urea ligands

Abstract The X-ray crystal structure of a pentacoordinate silicon compound, 3, with a urea ligand shows partial Si–O bond formation and Si–Cl cleavage. The solution NMR parameters for the related series of compounds with different leaving groups provides structural information and this can be used to model substitution at silicon. The corresponding thioureas exhibit N–Si bond formation with the silicon remaining tetracoordinate.