Caesium—silicon

Many other heterogeneous electrodes have been developed based on, e.g., calcium oxalate or stearate in paraffin, barium sulphate in paraffin or silicone-rubber, bismuth phosphate or iron(III) phosphate in silicone-rubber, caesium dodecamolybdophosphate in silicone-rubber and amminenickel nitrate in phenol-formaldehyde resin39 these permit the determination, respectively, of Ca and oxalate, Ba and sulphate, Bi or Fe(HI) and phosphate, Cs, Ni and nitrate, etc. 

See Xenon hexafluoride Silicon dioxide Xenon hexafluoride Caesium nitrate... 

Figure 8.10 Representation of the crystal structure of [Cs((if-PhMe)3(p-Fln N(SiMe3)2 3)]. ° Indium, caesium, nitrogen and silicon atoms are shown as black spheres, fluorine atoms are grey and carbon atoms are white. Selected bond lengths Inl-NI 2.121(3), lnl-N2 2.128(3), In1-N3 2.127(3), Inl-FI 2.065(2), Cs1-F1 2.777(2)A...

T. Gilson, M. Webster, and G. P. McQuillan The Stereochemistry of Addition Compounds of Silicon Tetrahalides, Studied by Their Spectra in the Caesium Bromide Region. J. chem. Soc. 1964, 238—44. 

Radius ratio values relative to O2 are given in Table 4.3. The table shows that silicon (Si) exists in four-fold (tetrahedral) coordination with oxygen (O), i.e. it will fit into a tetrahedral site. This explains the existence of the Si04 tetrahedron. Octahedral sites, being larger than tetrahedral sites, accommodate cations of larger radius. However, some cations, for example strontium (Sr2+) and caesium (Cs+) (radius ratio >0.732), are too big to fit into octahedral sites. They exist in eight-fold or 12-fold coordination and usually require minerals to have an open, often cubic, structure. 

Where the catalyst is less nucleophilic, e.g. potassium fluoride or solid caesium fluoride, only one fluoride ion is likely to coordinate at all firmly to the silane. The nucleophilic reactant will then also be able to coordinate to the electrophilic silicon atom, itself receiving further activation in the process, and reaction ensues by intramolecular transfer about the hexacoordinate silicon atom as demonstrated in the GTP process. Less nucleophilic substrates such as alkyl halides are unreactive in these circumstances. 

The trimethylsilylated silicic acids formed in this instance are soluble in conventional organic solvents, and their volatility is sufficiently high for them to be analysed by gas chromatography. Carzo and Hoebbel [411] carried out a comprehensive study of the chromatographic retention of various trimethylsilylated silicic acids on different stationary phases Apiezon L and silicone OV-1 and OV-17. The analysis of metals in the form of volatile complexes continues to attract attention, and have been described for analysing sodium [412], potassium [412], radium [413], caesium [413], barium [414], calcium [414], strontium [415], beryllium [416, 417], magnesium [418], zinc [419, 420], nickel [419], mercury [421], copper [422, 423], silver [424, 425], cadmium [421], indium [426, 427], g ium [428], scandium [217], cobalt [421], thallium [426], hafnium [429, 430], lead [431, 432], titanium [430], vanadium [433], chromium [434-436], manganese [426], iron [437], yttrium [438], platinum [439,440], palladium [439, 441, 442], zirconium [430], molybdenum [443], ruthenium [444], rhodium [445], rare earths [446—449], thorium [221, 450, 451] and uranium [221, 452]. The literature on GC analysis of metal chelates was reviewed by Sokolov [458]. 

Nucleophilic intramolecular addition of a carbon-silicon bond onto a carbon-nitrogen iminium ion is the key step of a berbine synthesis <82CC769,83H(20)417>. Thus, treatment of the isoquinolinium salt (235) with caesium fluoride in ethanol afforded (+)-xylopinine (237), presumably via the betaine (236) (Scheme 45). A silicon-directed intramolecular cyclization of a A-acyliminium ion (238) was also the key step of Speckamp s synthesis of (+)-epilupinine as shown in Scheme 46 <85JOC40I4>. 

The reaction of 1 -trimethylsilylnaphthalene with pivaldehyde in the presence of 20 mol% Bu-P4 base proceeded smoothly at room temperature to give the alcohol in 91% yield. Other phosphazene bases with weaker basicities, such as Bu-P2 base and BEMP, showed no catalytic activity. As one of the conventional strong organic bases, DBU was found to be inactive. Caesium fluoride (CsF) was then examined as a fluoride anion donor, but no carbon-silicon bond cleavage was observed. Reactions with other aldehydes have been examined that with benzaldehyde was found to proceed somewhat slowly at room temperature. Other aryl aldehydes with electron-donating groups were also employed as electrophiles and the reactions proceeded smoothly at room temperature [57] (Table 5.5). 

Generation of the silicon stabilized benzyl carbanion 153 from 149 required the presence of hexamethylphosphotriamide. In its absence lithiation occurred at least in part in the aromatic ring. Alkylation of 153 with 152 produced 154 in 94% yield as a 2 1 isomer mixture at the benzylic site. Methiodide formation, addition of caesium fluoride, and refluxing in acetonitrile for 1.5 hours then afforded ( )-estrone methyl ether 28 (containing 7-8% of the C-9 p-H isomer) in 86% yield. Pure 28 was obtained on recrystallization. 

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