Esters, silicate preparation

The synthetic methods used for the preparation of bonded phase materials are illustrated in Figure 6.35. One of the first reported bonded phases, the alkoxy silanes (1) also referred to as silicate esters, was prepared by the direct esterification of silanol groups with alcohols. The major disadvantage of this packing material was its hmited hydrolytic stability, as it is readily hydrolysed by aqueous alcohol eluants. 

Bonded phases have been prepared by other general methods besides those indicated so far [64-66]. Reaction of silica with an alcohol or isocyanate resulted in the formation of silicate esters (estersils), but these phases were too hydrolytically unstable to be generally useful. Bonded phases with an Sl-R or Sl-NHR structure are more hydrolytically stable than the estersils but... 

Synthetic esters, (a) primarily esters of dibasic acids such as adipic and sebacic, though in Europe some monobasic acid esters have been prepared and used (h) organic esiers of phosphoric and silicic acid, which have some advantage of being more lire-resislant than Ihe other organic compounds bm which are subject to hydrolysis on exposure to water,... 

Boronic esters of (R,R)-l,2-dicyclohexyl-l,2-ethanediol or pinanediol react with thionyl chloride and excess imidazole in acetonitrile on a borosilicate glass surface to form the corresponding cyclic sulfites of diols <20010M2920>. Similarly, cyclic sulfites and sulfates have been prepared from silicates of diols <1997TL4841>. 

A variation of the method was described in 1884 by Pape, who used a mixture of silicon tetrachloride and propyl iodide with metallic zinc instead of preparing the zinc dipropyl separately. Another variation, exploited by Friedel and Ladenburg, eliminated the sealed tube by introducing metallic sodium along with the zinc alkyl and silicate ester ... 

The ability of fluoro-2 -phosphanes to transform silyl ethers into fluorides was first observed during a study of the reactions of phosphorus pentafluoride and its derivatives R PF5 (n = 1, 2, 3 R = hydrocarbon group) with trimethylsilyl ethers. Subsequently, this reaction was proposed as a new method for the preparation of C-F compounds from silyl ethers or silicic acid esters with fluoro-A -phosphanes. Pentafluorophenyl-substituted fluoro-A -phos-phanes were found to react similarily, Other workers found that tctrafluoro(phenyl)-A -phos-phane. which was chosen as the most convenient reagent with regard to reactivity and stability, gave considerable amounts of elimination products, especially with primary and cyclic alcohols. Good yields of fluorinated products are obtained when stable carbocations can be formed at the site of substitution, such as in tertiary alcohols, but 2-phcnylethanol. benzyl alcohol and diphcnylmethanol, on the other hand, give only poor yields of fluorinated products ethers and polymers are the main products. ... 

The stereoselective allylation of aldehydes was reported to proceed with allyltrifluorosilanes in the presence of (S)-proline. The reaction involves pentacoordinate silicate intermediates. Optical yields up to 30% are achieved in the copper-catalyzed ally lie ace-toxylation of cyclohexene with (S)-proline as a chiral ligand. The intramolecular asymmetric palladium-catalyzed allylation of aldehydes, including allylating functionality in the molecules, via chiral enamines prepared from (5)-proline esters has been reported (eq 15). The most promising result was reached with the (S)-proline allyl ester derivative (36). Upon treatment with Tetrakis(triphenylphosphine)palladium(0) and PPh3 in THF, the chiral enamine (36) undergoes an intramolecular allylation to afford an a-allyl hemiacetal (37). After an oxidation step the optically active lactones (38) with up to 84% ee were isolated in high chemical yields. The same authors have also reported sucessful palladium-catalyzed asymmetric allylations of chiral allylic (S)-proline ester enamines" and amides with enantiomeric excesses up to 100%. 

Compounds 3 and 18 - 20 have been prepared to combine luminescent semiconductors with curable units. The transformation of the monomeric compounds to siloxanes of higher molecular weight was performed by hydrolysis of the silicic esters in chloroform/ethanol solution, catalyzed by traces of hydrochloric acid for 2 days (Scheme 4). 

Finally, sol-gel processing is widely used in the exploration of methods to prepare biomaterials for example, hydroxyapatite prepared from triethyl phosphite and calcium salts.237,238 Phosphate sol-gel synthesis is more difficult than for silicates since phosphoric esters are very difficult to hydrolyze 239... 

Silicon tetrachloride is also used to prepare silicate esters, eg, tetraethylorthosilicate ... 

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