Reaction media employed

Water has been illustrated as an efficient medium for the Wittig reaction employing stabilized ylides and aldehydes.63 It has been demonstrated that the solubility of the reagents and substrates is relatively unimportant, even though pronounced hydrophobic entities are present. 

Alkylation reactions employing alkyl halides are carried out in an acidic medium. For example, hydrobromic acid is formed when methyl bromide is used in the alkylation leading, and for such reactions an autoclave with a replaceable enameled liner and a lead-coated cover is suitable. 

A,7V-polyazolylmethanes can be prepared by reaction of azoles with methylene chloride in an autoclave at 150 °C (Scheme ll).20 At 200 °C this reaction leads to 4,4 -dipyrazolylmethane, which upon reaction with boranes gives a pyrazaboles polymer. Some derivatives have been prepared from the reaction of potassium salts of the azole and methylene iodide. The ligand (tetrakis-(l-pyrazolyl)methane was prepared analogously. In selected cases a basic medium was employed.21... 

The decomposition of hydrogen peroxide in an aqueous medium at pH 7 was investigated. This reaction employed the catalysts, Fe - and Co -2,9,16,23-tetra-carboxyphthalocyanine, and their quatemized derivatives immobilized on 2-vinyl-pyridine and styrene copolymers by the Friedel-Kraft reaction [158J. The kinetics of H2O2 decomposition at pH 7 in the presence of immobilized catalysts, including the evaluation of the initial and maximum reaction rates, Vq and F,nax> studied. It was shown that for the flxed iron complex, times higher than... 

The employment of suitable organic solvents, such as acetonitrile and acetic acid, with oxidation-resistant supporting electrolytes permits the anodic formation of reactive radical cations from many organic materials. Most aromatic compounds and olefins, as well as those alkanes which have particularly weak C—H bonds, are oxidised in acetonitrile containing fluoroborate or hexafluorophosphate electro-lytes. °" 2 Some aromatic radical cations can be further oxidised to dications within the available potential range. Radical cations in general either deprotonate or attack nucleophiles present in the medium reactions with pyridine, methanol, water, cyanide ion, acetate ion or acetonitrile itself produce addition or substitution products. The complete reactions involve a second electron transfer and coupled chemical... 

These reactions employ microwave heating and water as the reaction medium at 100 °C for 7-9 min. The mechanism for diastereoselectivity was confirmed by density functional theory (DFT) (B3LYP) calculations. Subsequently, new multicomponent domino reactions of Meldrum s acid 159, aromatic aldehydes 69, and electron-rich heteroaryl-amines 80 have been established for the synthesis of spiro pyrazolo[l,3]dioxanes-pyridine -4,6-diones and spirojisoxazolo [l,3]dioxanes-pyridine -4,6-diones 160 in aqueous solution under microwave irradiation (Scheme 12.64) [88]. A total of 26 examples were examined to show the broad substrate scope and high overall yields (76-93%). A new mechanism was proposed to explain the reaction process and the resulting chemo-, regio-, and stereoselectivity. 

Chemical imidization is not widely used because it employs additional reagents. However, it has the advantage of low temperature imidization and can he nsed to directly form fine polyimide molding powder (16,50). A t5 ical chemical imidization reaction employs a 20-30% solids polyamic acid in an amide solvent with a slight molar excess of acetic anhydride and a molar equivalent of a triamine (triethyl amine, pyridine, or -picoline) (51-54). The percent conversion for chemical imidization is a function of polyimide solnhiUty. If the polymer crystallizes and/or precipitates from the reaction medium, imidization will he incomplete (16,50). Those systems that remain soluble must imdergo thermal treatment to convert any isoimide, and remove residual solvent. The mechanistic routes of chemical imidization are shown in Figure 4, and involve the use of a triamine... 

Many polar solvents, e g., ethers, possess lone pairs and are therefore capable of acting as dative ligands to RE centers. Many organometallic RE complexes are either prepared in ethers to aid the salt-elimination reactions employed in their synthesis through provision of a polar reaction medium or utilize RE halides which incorporate ethers to aid then-solubility. Thus, if coordinated organometallic ligands are not... 

Dambacher, J., Zhao, W., El-Batta, A., Anness, R., Jiang, C. and Bergdahl, M., Water is an efficient medium for Wittig reactions employing stabilized ylides and aldehydes. Tetrahedron Lett., 2005, 46, 4473-4477. 

Under neutral reaction conditions, the C3-alkenylated product 109 was obtained predominantly. Regioselectivity favored the C2-alkenylated product 108 when acidic medium was employed, presumably due to retardation of the deprot-onation step, thereby allowing the palladium to migrate from the C3 position to C2. 

For the Michael addition between 3-pentanone and nitrostyrene a report by Patil and Sunoj points out a key limitation of the standard enamine derived transition state model when a polar protic reaction medium is employed (Scheme 17.10) [42]. The unassisted transition state inclusive of continuum solvent effects failed to predict the correct stereochemical outcome of the reaction as compared to the experimental observations. The predicted lowest energy transition state has been identified as leading to an incorrect configuration of the newly formed chiral centers as well as the wrong diastereomer. Further refinements to the transition state models were carried out with inclusion of explicit solvent molecules in view of the fact that the reaction being modeled has been conducted in methanol as the solvent. After examining several microsolvated transition states with varying... 

A monomer can be in either the hquid phase or the gas phase at polymerization conditions. If monomer is a gas, a solvent medium is employed in which the Ziegjer-Natta catalyst is dispersed and polymerization starts as soon as the gaseous monomer is introduced (see Fig. 5.10a for the setup). In the case of liquid monomers, a solvent is not necessary, but it is preferred because it facilitates temperature control of the reaction. 

The reaction is exothermic, and multitubular reactors are employed with indirect cooling of the reactor via a heat transfer medium. A number of heat transfer media have been proposed to carry out the reactor cooling, such as hot oil circuits, water, sulfur, mercury, etc. However, the favored heat transfer medium is usually a molten heat transfer salt which is a eutectic mixture of sodium-potassium nitrate-nitrite. 

To hydrolyse an ester of a phenol (e.g., phenyl acetate), proceed as above but cool the alkaline reaction mixture and treat it with carbon dioxide until saturated (sohd carbon dioxide may also be used). Whether a solid phenol separates or not, remove it by extraction with ether. Acidify the aqueous bicarbonate solution with dilute sulphuric acid and isolate the acid as detailed for the ester of an alcohol. An alternative method, which is not so time-consuming, may be employed. Cool the alkaline reaction mixture in ice water, and add dilute sulphuric acid with stirring until the solution is acidic to Congo red paper and the acid, if aromatic or otherwise insoluble in the medium, commences to separate as a faint but permanent precipitate. Now add 5 per cent, sodium carbonate solution with vigorous stirring until the solution is alkaline to litmus paper and the precipitate redissolves completely. Remove the phenol by extraction with ether. Acidify the residual aqueous solution and investigate the organic acid as above. 

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. 

According to Le Chatelier s principle, a system at equilibrium adjusts so as to mini mize any stress applied to it When the concentration of water is increased the system responds by consuming water This means that proportionally more alkene is converted to alcohol the position of equilibrium shifts to the right Thus when we wish to pre pare an alcohol from an alkene we employ a reaction medium m which the molar con centration of water is high—dilute sulfuric acid for example... 

In general ketones are more stable than their enol precursors and are the products actually isolated when alkynes undergo acid catalyzed hydration The standard method for alkyne hydration employs aqueous sulfuric acid as the reaction medium and mer cury(II) sulfate or mercury(II) oxide as a catalyst... 

Direct titrations are commonly carried out using disodium dihydrogen ethylenediaminetetraace-tate, NajHjY, which is available in pure form. The reaction of the chelon with the indicator must be rapid for a practical, direct titration. Where it is slow, heating of the titration medium is often expedient, or another indicator is employed. 

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