Solvents atmospheric reaction

Quite recently this reaction was revisited by Kappe [91] et al. who reinvestigated the Biginelli synthesis under the action of microwave irradiation under a variety of different conditions. At atmospheric pressure in ethanol solution there is no difference from conventional heating. Under pressure the yield is reduced and byproducts are formed. In an open system rate and yield enhancements are significant and this is rationalized by the rapid evaporation of the solvent which means that this is in fact a solvent-free reaction. This was confirmed by running the reaction without solvent under the action of microwaves and with thermal heating. (Scheme 8.65)... 

Laity JL, Burstain IG, Appel BR. 1973. Photochemical smog and the atmospheric reactions of solvents. In Tess RW, ed. Solvents theory and practice. Advances in Chemistry Series 124, Washington, DC American Chemical Society, 95-112. 

Selenium tetrafluoride is a liquid (bp 106 C) which is soluble in halogenated solvents enabling reactions to be carried out at atmospheric pressure. It is has been reported to fluorinate alcohols to their corresponding fluorides, carbonyl compounds to gem-difluorides, e.g. 1—2, and carboxylic acids to acid fluorides. 

Applications under solvent-free conditions have been the subject of the greatest activity and have been reviewed extensively7,10 16 17. Besides apparent potential benefits in minimising solvent usage, reactions can be conducted conveniently and rapidly, without temperature measurement in domestic microwave ovens. A dedicated commercial reactor for applications at atmospheric pressure has also been used7. 

It was thought38 that the mechanism was intermediate between SE1 and SE2. Other workers39 have suggested that the kinetic results of Reutov et al 1 suffer from irregularities arising from atmospheric oxidation of dibenzylmercury. When conducted under a nitrogen atmosphere, reaction (26) followed second-order kinetics in solvent aqueous acetonitrile39. 

General Method. All solvents used were of analytical reagent grade. Toluene and THF were refluxed under N2, and over sodium and benzophenone. Pyridine was distilled and stored over KOH pellets. Water was doubly distilled before use. All the silanes were vacuum-distilled and all reactions were performed under N2 atmosphere. Reactions on polypropylene films were normally carried out with pieces of 5 x 5 cm or for industrial applications with foils of 35 x 24 cm. 

As you should have gathered by now, THF is an important organic solvent in which many low-temperature, inert-atmosphere reactions are conducted, it has a drawback, however it is quite hygroscopic, and often the reactions for which it is used as a solvent must be kept absolutely free of water. It is therefore always distilled immediately... 

Several factors such as the structure of the substrate, the catalyst, the solvent, the reaction temperature, the pressure of hydrogen and other reaction conditions determine the stereochemistry of the catalytic hydrogenation of cyclic ketones, and it is sometimes difficult to predict the major pn uct of catalytic hydrogenation. One reason for the complexity of the stereochemistry of the hydrogenation of cyclic ketones, at least in part, is related to the isomerization of the products under the reaction conditions. Some cyclohexanols were isomerized in the presence of platinum or nickel catalysts at room temperature or at higher temperature under a hydrogen atmosphere, and the isomerization reached a cis-trans equili-brium. For example, rranj-3,3,5-trimethylcyclohexanol isomerized in the presence of a nickel catalyst. 

Drying solvents - the tlrying.agent used for solutions or pure liquids is not usually suitable for drying solvents for inert atmosphere reactions. 

All chemicals and solvents used in inert atmosphere reactions must be dry. Most of these materials provided by supphers are not dry enough, even solvents which you consider to be immiscible with water, and may contain enough moisture to hinder the reaction or reduce the yield of your product. Therefore you must ensure that all chemicals to be used in the process have been dried to the appropriate levels, as described below. 

Some manufacturers supply dry solvents in 2.5 L quantities for inert atmosphere reactions and HPLC (p. 218). These solvents are relatively expensive but may be economic in terms of time and expense if one-off reactions are required. However, such solvents should be treated with suspicion if the containers are less than half-full, since air and moisture may have been allowed into the container by previous users. If you have any doubts, dry the solvent. 

Polymeric precursors have been developed which are stable at room temperature and when polymerized convert to ceramics in high yield. Such precursors may be synthesized by reaction of a vinylsilane, vinylmethylsilane, acetylene silane, or acetylene alkyl silane with a borane or a borane amine derivative The reactants are mixed in an inert atmosphere, either neat or in an aprotic solvent like acetonitrile, tetrahydrofuran, or a hydrocarbon, or in a mixture of such solvents. The reaction mixture is heated for 0.1 to 120 h at 90-170°C. The solvent, if any, is then removed. The polymer is pyrolyzed in argon or N2 at SOO-ISOO C for 1 h. BN and other ceramics such as B4C, or SipB QNs compounds can be made, where p, q, r, and s have various numerical values. To date, no measurements of the crystallographic form of the resultant BN compound have been made. Other precursors convertible to BN include poly (2-vinylpentaborane) oligomers. ... 

Though the reaction temperature, of course, generally depends on the employed solvent, it is within a temperature range of from—20 ° C. to near the boiling point of the solvent. The reaction is preferably carried on in an atmosphere of such an inactive gas as nitrogen, helium and the like. Thus, the amide derivatives of the objective products are obtained under relatively mild conditions in high yields and simplicity without oxidation of the reactant. 

In a bimolecular reaction in solution reactants diffuse through the assembly of solute and solvent molecules (Scheme 1) and collide to form an encounter complex within a solvent cage. Reaction is not possible until any necessary changes occur in the ionic atmosphere to form an active complex and in solvation (such as desolvation of lone pairs) to form a reaction complex in which bonding changes take place. The encounter complex remains essentially intact for the time period of several collisions because of the protecting effect of the solvent surrounding the molecules once they have collided. The products of the subsequent reaction could either return to reactants or diffuse into the bulk solvent. 

Cyclooctadiene (Aldrich Chemicals) was distilled from sodium and stored under argon. ra-Butyl bromide and 1.6 M ra-butyl lithium solution were purchased from Aldrich and used as received. Solvents used in the syntheses were dried with appropriate drying agents16 and freshly distilled under inert gas before use. All procedures are performed in an anhydrous, oxygen-free atmosphere using standard techniques for bench-top inert atmosphere reactions.17,18... 

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