Silylated carbon analysis

A solution of Pd(OAc)2 (0.05 mmol) and bis(diphenylphosphino)ethane (0.05 mmol) in acetonitrile (1 ml) was heated gently to reflux, at which time a solution of the silyl enol ether (1 mmol) and diallyl carbonate (2 mmol) in MeCN (4 ml) was added in one portion. The mixture was heated under reflux for 1-3 h, the course of reaction being monitored by t.l.c. or g.l.c. analysis. On completion, the cooled reaction solution was filtered through fluorosil. The pure a/ -unsaturated compound was isolated by column chromatography on silica gel (70-95%). 

The disilanickela compound 21 is not effective in the nickel-catalyzed double silylation reaction with styrene. However, the stoichiometric reaction of 21 with styrene afforded 4,5-carboranylene-l,l,3,3-tetramethyl-2-phenylmethylene-1,3-disilacyclopentane 29. A key feature in the 111 NMR spectmm of 29 includes a singlet at 7.71 ppm assigned to the vinyl proton. A characteristic low-frequency 13C NMR resonance at 139.75 ppm provides evidence for a tethered carbon atom of the two silicon moieties. Unambiguous confirmation was provided by X-ray crystallographic analysis of 29. 

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For a long time, this knowledge on carbon-centred radicals has driven the analysis of spectroscopic data obtained for silicon-centred (or silyl) radicals, often erroneously. The principal difference between carbon-centred and silyl radicals arises from the fact that the former can use only 2s and 2p atomic orbitals to accommodate the valence electrons, whereas silyl radicals can use 3s, 3p and 3d. The topic of this section deals mainly with the shape of silyl radicals, which are normally considered to be strongly bent out of the plane (a-type structure 2) [1]. In recent years, it has been shown that a-substituents have had a profound influence on the geometry of silyl radicals and the rationalization of the experimental data is not at all an extrapolation of the knowledge on alkyl radicals. Structural information may be deduced by using chemical, physical or theoretical methods. For better comprehension, this section is divided in subsections describing the results of these methods. 

In a novel method of forming cyclobutanols, titanium(IV) chloride is found to promote the condensation of substituted bisketene silyl acetals 23 with more reactive carbon electrophiles such as acetic anhydride or benzoic anhydride. In each experiment, it is apparent that only one diastercomer of the substituted 2,4-dicarbomethoxycyclobutanols 24 is furnished, as can be vindicated by NMR spectral analysis.33... 

Two transition structures with a retention (192, TSret) and an inversion (193, TSinv) configuration (Figure 2) were optimized for 1,3-silyl migration in allylsilane at HF/6-31G, MP2/6-31G and DFT/6-31G levels. The TSjnV 193 was found to be a distorted trigonal bipyramid (TBP) around the silicon with the two allylic carbons at the equatorial positions different from the TS illustrated by Kwart and Slutsky297,302, while 192 has a distorted square pyramid (SP) structure around silicon. Analysis of the orbital interaction in the transition states showed that the major stabilization of 193 was caused by the MO interaction as predicted by the Woodward-Hoffmann rules, while the major stabilization in 192 was ascribed to the subjacent orbital control. 192 was more stable than 193 at... 

Laik Ali [167] applied BSTFA and MSTFA to the silylation of trace amounts of salicylic acid in acetylsalicylic acid. A 0.5—1-ml volume of the reagent and 50—100 mg of acetylsalicylic acid were heated at 70—80°C in a closed vessel for 30—60 min. As the TMS derivative of acetylsalicylic acid decomposes, an exact evaluation of the analysis is difficult and therefore the author recommended stable methyl derivatives prepared by reaction with methyl iodide and potassium carbonate. 

An attractive, although tentative, alternative would be an alkyl-substituted silylsilylene formed from the polymer chain. Two thermodynamically reasonable routes to such intermediates are possible. The first route (equation 4) involves 1,1-elimination to produce the silylsilylene directly. This route has a precedent in organosilane thermal processes (78, 79). The second route (equations 5a and 5b) involves rearrangement from a silene produced by the disproportionation (46, 80, 81) of two silyl radicals caused by bond homolysis. This type of rearrangement has also been described in the literature (82). The postulated silylsilylenes are also attractive intermediates to explain the rebonding of silicon to carbon atoms other than those in the original a positions (CH insertion), which is obvious from the mass spectral analysis of gaseous products from the laser ablation of isotopically labeled poly(di-n-hexylsilane). 

The following examples further illustrate the selectivity that can emerge from this analysis (Scheme 6). Divinylcyclopropane (30) underwent exclusively the cleavage of the siloxy-substituted ring with activation enhancement of ca. 5 kcal mol", whereas the silyl-substituted counterpart (31) preferred the cleavage of the carbon-substituted ring due to the decelerating effect of silicon. ... 

The synthesis and the structural analysis of silicon unsaturated compounds impressively demonstrate that tricoordinate silicon species are able to exist under normal conditions. Since more than 50 years organosilicon research tries to answer the question if this might be true for silyl cations RsSi, in which silicon also is surrounded by three neighbors. Those compounds seem to meet the requirements because silicon is more electropositive than the carbon homologue and therefore should accommodate a positive charge more efficiently. And in fact, calculations confirm that [HsSi] is thermodynamically much more stable than [HsC]. ... 

Summary Methoxy-bis[tris(trimethylsilyl)silyl]methane (4), the first geminal di(hyper-silyl) compound with a central carbon atom, was prepared by the reaction of tris(tri-methylsilyl)silyl lithium with dichloromethyl methyl ether. The structure of 4, which is characterized by considerable distortions due to the spatial demand of the two (MesS aSi groups, is discussed on the basis of an X-ray crystal structure analysis. 

Upon reaction with di- and trisilylated silyl anions, the tri-fert-butylcyclopropenylium ion 14 was also converted into the 3-silylated cyclopropene derivatives 16, which exhibited remarkable thermal stability owing to the large steric hindrance around the three-membered ring. The single bond between the cyclopropenyl carbon and the substituted silicon was shown by X-ray analysis to be elongated by 0.05... 

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