Esters, silicate reactions

Acid derivatives that can be converted to amides include thiol acids (RCOSH), thiol esters (RCOSR), ° acyloxyboranes [RCOB(OR )2]. silicic esters [(RCOO)4Si], 1,1,1-trihalo ketones (RCOCXa), a-keto nitriles, acyl azides, and non-enolizable ketones (see the Haller-Bauer reaction 12-31). A polymer-bound acyl derivative was converted to an amide using tributylvinyl tin, trifluoroacetic acid, AsPh3, and a palladium catalyst. The source of amine in this reaction was the polymer itself, which was an amide resin. 

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... 

A convenient method to introduce organic functions into inorganic networks is the use of substituted siliceous acid esters and the sol-gel route. The basic reactions, therefore, are given in Equations 1 and 2. Equation 1 describes the sol-gel reaction to a borosilicate glass. 

The chemistry of silicone halides was recently reviewed by Collins.13 The primary use for SiCU is in the manufacturing of fumed silica, but it is also used in the manufacture of polycrystalline silicon for the semiconductor industry. It is also commonly used in the synthesis of silicate esters. T richlorosilane (another important product of the reaction of silicon or silicon alloys with chlorine) is primarily used in the manufacture of semiconductor-grade silicon, and in the synthesis of organotrichlorosilane by the hydrosilylation reactions. The silicon halohydrides are particularly useful intermediate chemicals because of their ability to add to alkenes, allowing the production of a broad range of alkyl- and functional alkyltrihalosilanes. These alkylsilanes have important commercial value as monomers, and are also used in the production of silicon fluids and resins. On the other hand, trichlorosilane is a basic precursor to the synthesis of functional silsesquioxanes and other highly branched siloxane structures. 

Mowery and DeShong used the commercially available hypervalent silicate complex TBAT as a phenylating agent for the cross-coupling reaction with allylic esters. They later reported on the use of the same organosilane for the coupling with aryl iodides and triflates and electron-deficient aryl bromides. The reactions were catalyzed by either Pd(dba)2 or [Pd(allyl)Cl]2 without the need of added phosphine ligands. 

Acid derivatives that can be converted to amides include thiol acids RCOSH, thiol esters RCOSR,911 acyloxyboranes RCOB(OR )2,912 silicic esters (RCOO)4Si, 1,1,1-trihalo ketones RCOCX3,913 a-keto nitriles, acyl azides, and nonenolizable ketones (see the Haller-Bauer reaction 2-33). 

This reaction is called esterification and the alkoxysilanes produced are called esters , due to the close resemblance to silicic acid esters. 

Fig. 13.22. O-Silylation of an ester enolate to give a silyl ketene acetal. (The formation of the silicate complex in step 1 of the reaction is plausible but has not yet been proven.)...

TMS esters are the derivatives of oxygen-containing anions that have been applied most frequently to GC analysis. They were first applied to the analysis of silicates [582], Further, the method was developed for the analysis of five silicates in siliceous rocks [583], as follows. A 20-g amount of powdered sample was added to a mixture of 125 g of ice, 150 ml of concentrated HC1, 300 ml of 2-propanol and 200 ml of hexamethyl-disilazane, which had been stirred for 1 h. The complete reaction mixture was then stirred for 16 h at room temperature. After filtration the silazane (upper) layer was separated, washed with water, mixed with 2.5 g of Amberlyst 15 for 1 h, filtered and stripped at 115°C. The liquid residue in the distillation flask was analysed on a column packed with SE-30 with temperature programming. 

Much of the early work on the synthesis of organosilicon compounds was based on reactions between Grignard reagents and silicon halides (and to a lesser extent on silicate esters), and much effort was devoted to optimizing stepwise reactions ... 

The tendency to hydrolyze by a relatively slow and controllable stepwise reaction is characteristic of all the silicon orthoesters. When ethyl silicate, for example, is poured into water it remains immiscible and hydrolyzes at the interface over a period of many days or even months. If both the ester and the water are dissolved in alcohol, however, a much faster reaction in the homogeneous medium becomes possible. If a drop of strong acid then is added to the solution, the hydrolysis is further accelerated to such an extent that the solution becomes warm and silica is precipitated as solid or gel. If less than the equivalent amount of water is used, there is formed a viscous nonvolatile liquid which may be stored for long periods. The addition of further water (as in mixing an ethyl silicate paint) then... 

Besides hydrolysis, the silicic esters show many more of the reactions which have been described for the halides. For example, the alkoxy group reacts with the Grignard reagent to attach organic groups directly to the silicon atom in stepwise fashion ... 

Of particular interest to the user are the physiological reactions of the silicon esters within the human body. Ethyl silicate apparently is readily absorbed,38 and has a hemolytic effect such as is produced by ether, chloroform, or the higher alcohols. Inhalation of air containing 9 to 20 mg. per liter or absorption of comparable quantities by other methods is fatal to experimental animals because of this... 

Cymene or isopropyltoluene is produced via alkylation of toluene with propylene. Cymene is an important intermediate in the production of cresol, and it is also used as an industrial solvent. Again, for both environmental and economic reasons, the use of zeolitic materials for this conversion has been studied. For example, Flockhart et al. have used zeolite Y to effect this reaction (7). They observed that the state of the zeolite, including its degree of ion-exchange and the temperature at which it was calcined, strongly affected the distribution of cymene isomers obtained. In order to enhance the selectivity to para-cymene, the direct precursor to para-cresol, various studies have focused on the use of surface modified zeolites, for example, ZSM-5 materials, including those produced by chemical vapor deposition (CVD) of silicate esters. These species serve to reduce surface acidity and change limit diffusion within the crystal. 

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. ... 

Hypervalent silicon componnds have found wide utility in organic synthesis. In general, pentacoordinated anionic silicates are more reactive toward nucleophiles than are tetracoordinated silanes. For example, Mes2SiF2 is nmeactive toward water, while (the 18-crown-6 potassium salt of) Mes2SiF3 is completely hydrolyzed within minutes. Similarly, the pentacoordinate anion HSi(OEt)4 is an effective reducing agent for aldehydes, ketones, and esters at or below room temperature (Scheme 2) no snch reaction occurs with HSi(OEt)3. The difference in relative reactivities of hypervalent and nonhypervalent species is relevant to the intermediates proposed in Section 7.6. 

Acetals and ketals having a second junctional group ate made by these procedures. For example, acrolein reacts with ethyl orthoformate in alcohol solution with ammonium nitrate as catalyst to give acrolein diethyl acetal (73%). On the other hand, it reacts with ethyl ortho silicate with anhydrous hydrogen chloride as catalyst to furnish (i-ethoxypropionaldehyde diethyl acetal (76%). p-Bromoacetophenone and ethyl orthoformate give the corresponding ketal in 65% yield. p-Methoxy- and m-amino-benzaldehyde diethyl acetals are made in a similar way in 96% and 85% yields, respectively. a-Keto esters like ethyl a-keto-n-butytate and ethyl a-keto-tr-valetate are converted to their diethyl ketals in excellent yields by the action of orthoformic ester in ethanol-hydrochloric acid solution. If the reaction is carried out in the presence of ethylene glycol instead of ethanol and, in addition, the volatile products are removed by distillation, then the ethylene ketal is formed in almost quantitative yield (cf. method 133). 

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%. 

Heating a phenol ester in the presence of Nafion-H gave good yields of the hydroxy phenyl ketone.This Fries rearrangement gave a mixture of the ortho and para hydroxy compounds in a 1 2-2.5 ratio (Eqn. 22.19). A study of the rearrangement of phenyl benzoate over a number of solid acid catalysts indicated that Nafion-H was more effective than montmorilIonite or amorphous aluminum silicate for this reaction. " ... 

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