Wet silica

The pyrogenic flame hydrolyzed silica Aerosil 200, a commercial product from Degussa, was used as a dispersion in doubly distilled water (1). The precipitated silica was prepared by hydrolysis of orthosilicic acid tetraethylester in ammoniacal solution according to the method of Stober, Fink and Bohn (11). The prepared suspension was purified by repeated centrifugation, separation from solvent and redispersion of the sediment in fresh water. Finally, the water was evaporated and the wet silica dried at 150°C for about half an hour. 

Interesting variations in product distributions could be observed when ozonolysis was carried out with alkenes adsorbed on silica gel.628 The low-temperature ozonolysis on dry silica gel of cyclopentene led to the formation of the normal, monomeric ozonide in high yield [Eq. (9.114)]. This corresponds to the product formed in aprotic and nonparticipating solvents. In contrast, reaction of cyclopentene on wet silica gel resulted in the formation of the corresponding oxo acid... 

No cross ozonide was formed from unsymmetrical alkenes. The authors theorized628 that the carbonyl oxide zwitterionic species formed on wet silica gel immediately adds water followed by rapid decomposition of the intermediate hydroxyalkyl hydroperoxide to carboxylic acid and water. It means that water on silica gel acts as participating solvent. In the absence of adsorbed water, rapid recombination of the adsorbed aldehyde and carbonyl oxide due to a favorable proximity effect gives normal ozonide. The low mobility of adsorbed species on the silica surface accounts for the absence of cross ozonides. 

The metallation of 3-methyl-4//-5,6-dihydro-l,2-oxazine has been shown to take place at the methyl group with hindered bases and at the methylene group with unhindered bases (81JA5916). Deprotonation of (753) with lithium dimethylamide at -65 °C followed by reaction with benzyl bromide gave (754) in 85% yield. This product was converted to enone (755) by reaction first with triethyloxonium tetrafluoroborate to produce an oxoiminium salt. The salt was stirred with trimethylamine and the resulting a,/3-unsaturated imine hydrolyzed with wet silica gel to the enone (Scheme 174). The lithiated derivative of (753) serves as a synthon for the unknown a-anion of methyl vinyl ketone. 

Keywords sulfide, sodium periodate, wet silica, microwave irradiation, sulfoxide... 

Treatment of the oxathiolan 1 with two equivalents of sulfuryl chloride in carbon tetrachloride at 0°C gives the 1,4-oxathian 2 which, on treatment with two equivalents of triethylamine followed by hydrolysis with wet silica gel, is converted into the oxathiolan 3 in 95% yield. The latter is unstable to acid, and when heated in benzene with p-toluenesulfonic acid is slowly converted into 4 in 83% yield. 

OTS does react slightly with wet silica at room temperature, containing multilayers of water on the surface. The broad band at 3650 cm 1 decreases slightly, and a band at 3350 cm 1 arises, attributed to trisilanols.79,80 Tripp and Hair78 state that the first layer of water is strongly bonded to the surface and does not participate in the hydrolysis of the chlorosilane headgroup of the OTS molecule. 

The process of oligo- and polymerisation can occur either on the surface of the wet silica or in the toluene solution. The formation of polylayered coatings was investigated in detail by Gorski et al.,91 reporting on the chemisorption of 3-methylacryloxypropyltrimethoxysilane from acetone solutions, containing various amounts of water. 

Fig. 4.21k. Representation of the steps involved in the column fabrication processes (A) introducing with wet silica material into the end of the capillary, (B) the silica material is ready to fabricate the temporary frit, (C) fabrication of the temporary frit with a heating element, and (D) excess of silica material is flushed out after temporary frit is made.

S/L interactions, which are both metallic. No wetting is observed in the CU/AI2O3 system (0 = 128°, Table 6.1) which features strong L/L but weak S/L interactions. However, many organic liquids, whose cohesion is due to weak, physical, interactions, wet silica well despite the fact that their S/L interactions are rather weak (Johnson and Dettre 1993). 

The reaction of an azadiene RIN = CH—CH = CHR2 (R = Ph, Me R2 = rc-Pr, Bn, i-Pr, n-Pr) with a (l-alkynyl)carbene chromium complex la (R = Ph, SiMe3, f-Bu) quite unexpectedly affords [4+3] cycloaddition products 133 in 57-93% yield, from which azepinones 132 are obtained on contact with wet silica (Scheme 52).141 Reaction of azadienes R N=CH — CH = CHR2 (R = Ph, Me R2 = n-Pr, Bn, i-Pr, n-Pr) with a (l-alkynyl)car-bene tungsten complex lb (R = Ph) was shown to produce 1,4-dihydropyri-dine complexes (of the type shown in Scheme 50) as the major products.142... 

Acylmetallates lp,q generated by addition of lithio prop-2-ynylic ethers to hexacarbonyl chromium or tungsten afford oxacyclopenten-2-ylidene complexes 173 on contact with wet silica gel (Scheme 73).194 The chromium complex 173 (M = Cr) undergoes a condensation with tolane to give a 1,4-dioxy aromatic compound 174. Addition of cyclopentadiene to 173 (M = W) affords a Diels-Alder adduct 176 in a 1 3 exo/endo ratio,195 while addition of dimethylamine quite unexpectedly does not lead to production of an aminocarbene complex but to formation of 5,5 -diphenyl-2,2 -bifuran 175 (Scheme 73).196... 

This I04 -5i02 reagent is not very elective in the oxidation of sulfides into sulfoxides. However, sulfuryl chloride adsorbed wet silica gel is an excellent reagent for this transformatim (equation 5). High yields of methyl aryl, diaryl, allylic. benzylic and dialkyl sulfoxides are thus obtainable. The procedure commends itself by its simplicity and its extension to thioacetals provides a good, quantitative re-... 

UV irradiation at 260-320 nm in a Rayonette reactor in methanol of Chloralkylene-12 led to formation of reductive dechlorination products at a significant rate. This result indicated a greater overall ease of photodegradation of the alkyl substituted PCBs relative to the parent compound [66]. By irradiation at over 290 nm wavelength UV both in water and on a wet silica gel column in air flow, Chloralkylene-9 was transferred mainly to hydroxylation and reductive dechlorination products by similar mechanisms to PCBs [67]. 

The interaction between silica surface and nitrogen compounds was studied by using mainly solid state NMR. The quinoline as basic nitrogen compound and carbazole as non-basic nitrogen compound were adsorbed on the dry or wet silica. Both of them made a hydrogen bonded hydroxyl proton on the surface of silica. Surface water on the silica might effect on the interaction between silica surface and nitrogen compounds. 

Before the adsorption of nitrogen compounds, the silica was exposed in the moistured air to have different water contents. Figure 1 shows the H MAS NMR spectra of silica used in this study. Figure 1(a) is the NMR spectrum of dry silica powder. The narrow peak at 5 = 1.8 ppm is assigned the isolated siianol. It showed general spectrum of dry silica as reported in many studies[9-l 1]. After the exposure in wet condition, the resonance at 3.5 ppm which means the physically adsorbed water was appeared. The water content of silica was increased by exposure in wet air up to 0.11 lg/g.silica. These dry and wet silica were used for adsorption of nitrogen compounds. 

Figure 3. MAS NMR spectra of wet silica (a) before the adsorption of quinoline and (b) after the adsorption of quinoline.

Most aromatic compounds, whether of high or low reactivity, can be nitrated, because a wide variety of nitrating agents is available. For benzene, the simple alkylbenzenes, and less reactive compounds, the most common reagent is a mixture of concentrated nitric and sulfuric acids,but for active substrates, the reaction can be carried out with nitric acid alone, or in water, acetic acid, acetic anhydride, or chloroform.Nitric acid in acetic anhydride/trifluoroacetic anhydride on zeolite H-(3 was used to convert toluene to 2,4-dinitrotoluene, and AcONOi on clay converted ethylbenzene to ortho-para nitro ethylbenzene. " In fact, these milder conditions are necessary for active compounds, such as amines, phenols, and pyrroles, since reaction with mixed nitric and sulfuric acids would oxidize these substrates. With active substrates, such as amines and phenols, nitration can be accomplished by nitrosation under oxidizing conditions with a mixture of dilute nitrous and nitric acids.A mixture of N02/02/Fe(acac)3 can be used for active compounds, as can NaN02 with trichloroisocyanuric acid on wet silica gel, or N2O4 and silica acetate. Trimethoxybenzenes were nitrated easily with ceric ammonium nitrate on silica gel, and mesitylene was nitrated in an... 

Stirring an ester in an aqueous suspension of Dowex-50 is an efficient method for hydrolyzing the ester to the acid. Similar results have been obtained by refluxing an ester in an aqueous suspension of H-beta zeolite. Stirring acetals in an acetone suspension of wet silica gel or moistened Amberlyst-15 0 is a convenient method for converting an acetal to the aldehyde or ketone without having side reactions such as double bond isomerizations also take place (Eqn. 22.40). 9... 

Sulfoxides can be obtained from sulfides without sulfone formation by either acyl nitrates (0COC1 + HN03 or Ac20 + HNO3)72 or sulfuryl chloride and wet silica gel.73 Both methods are clean, mild and rapid. 

---