Trimethylsilylation anion structure

An X-ray crystal structure analysis was obtained from the 3-(trimethylsilyl)-aUyllithium-(—)-sparteine complex (5 )-302b. It reveals the monomeric structure of these aUyllithium compounds and a -coordination of the ally lie anion to the lithium cation. The latter is tetracoordinated and takes advantage of the chelating 0x0 group. The fixation of the lithium at the a-carbon atom is supposed to be the origin of the high regioselectivity of several substitution reactions. 

The reduction of l,2,4,5-tetrakis(trimethylsilyl)benzene under aprotic conditions yielded the sodium salt of the radical anion, and the red needles obtained from DME were shown by an X-ray investigation to have an ionic structure with no short Na-C contacts. There is a small but significant change from the neutral molecule ArH to the radical anion and the dianion.125 As electrons are taken up by the aromatic system, the bond lengths between the substituted carbons are increased from 141 pm in ArH126 to 147 pm in ArH and 155 pm in ArH2. The reduction of 9,10-bistrimethylsilylanthracene is similar. The radical anion has been character-... 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an allylsilane structure as intermediates. Therefore, they are further oxidized to give l,l,4-trimethoxy-2-butene derivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, /t-unsaUirated aldehydes. Since 1-trimethylsilyl-l,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trhnethylsilyl)propene offers a, /i-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15)16. 

Similarly, l-[bis(trimethylsilyl)methyl]-3,5-bis(trimethylsilyl)-l,2,4-triphosphole 54b, whose structure was determined by X-ray diffraction (see Figure 3), was synthesized by the reaction of 3,5-bis(trimethylsilyl)-l,2,4-triphos-pholide anion 135 with (Me3Si)2CHBr (Equation 15) <1998AGE1083>. 

Analytical Procedure. The structures of silicate anions in the solutions and solids have been examined with the trimethylsilylation technique combined with gas-liquid chromatography and "Si NMR. The molecular weight distribution was measured by applying gel permeation chromatography to the trimethylsilylated derivatives. 

Because C-S-H gel is nearly amorphous. X-ray diffraction has given only very general indications of its structure. The nature of the silicate anions has been determined from the kinetics of the reaction with molybdate (S43), and, in greater detail, by trimethylsilylation (TMS) and Si NMR. In TMS methods, the sample is treated with a reagent that converts the silicate anions into the corresponding silicic acids, which then react further with replacement of SiOH by SiOSi(CH3)3. The resulting TMS derivatives can be identified and semiquantitatively determined by various procedures, of which the most widely used have been differential evaporation to isolate the... 

Through a rather complicated sequence of meanwhile fully understood reaction steps ethyl benzoate and lithium bis(trimethylsilyl)phosphanide form tris(l,2-dimethoxyethane-0,0 )-lithium 3-phenyl-l,3-bis(trimethylsilyl)-l,2-diphosphapropenide and 3,5-diphenyl-l,2-bis(tri-methylsilyl)-l,2,4-triphospholide. X-ray structure determinations on orange or green, metallically lustrous, crystals show the compoxmds to be ionic in the solid and to contain a 1,2-diphosphaallyl and a 2-phosphaallyl anion, respectively. Dark red tetrakis(tetra-... 

Recently, detailed P NMR studies of the formation of the 1,2-diphosphaallyl anion were carried out in order to increase its unacceptably low yield. This led to the very surprising discovery that an unstable ElZ- (5 P-Si +41.4 5 P-C +454.2 ppm jpp 460.6 Hz) as well as the finally isolated EIE- sova.er (8 P-Si +8.0 8 P-C 411.1 ppm jpp 461.2 Hz l,2-dimethoxyethane/[D5]-benzene solution) are present in the reaction mixture. An X-ray structure determination shows the solid compound to be built up of discrete tris(l,2-dimethoxyethane-0,0 )lithium cations and / -isomeric 3-phenyl-l,3-bis(trimethylsilyl)-l,2-diphosphapropenide anions (Fig. 6). 

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