Catalysis solvent

The bulk polycondensation of (10) is normally carried out in evacuated, sealed vessels such as glass ampules or stainless steel Parr reactors, at temperatures between 160 and 220°C for 2—12 d (67). Two monomers with different substituents on each can be cocondensed to yield random copolymers. The by-product sdyl ether is readily removed under reduced pressure, and the polymer purified by precipitation from appropriate solvents. Catalysis of the polycondensation of (10) by phenoxide ion in particular, as well as by other species, has been reported to bring about complete polymerisation in 24—48 h at 150°C (68). Catalysis of the polycondensation of phosphoranimines that are similar to (10), but which yield P—O-substituted polymers (1), has also been described and appears promising for the synthesis of (1) with controlled stmctures (69,70). 

This reaction type has been intensely studied °. The application of highly polar solvents, catalysis with tertiary amines" or with acids mesomeric stabilization of intermediate carbenium ions " (allylic and benzylic systems propargylic systems" ) as well as derivatives of sulfinic acids with increasing acidity - usually indicate an ionic pathway (intra- and/or inter-molecular) ... 

In halogenated solvents, catalysis by a second bromine molecule, which assists the Br—Br bond heterolysis, is the main driving force. The role of the solvent is electrostatic, but the absence of an extensive Kirkwood relationship suggests that there is some other kind of contribution (Bellucci et al, 1985b). 

The value of k has been determined for a very large number of bases. There is a reasonable correlation of log/ B S P-A iBH+ agreement with (2.162). The value of p is — 0.25, which indicates that general base catalysis holds. For specific base catalysis, P would equal 1, while for reactions in which solvent catalysis predominates p would be zero, since only the A , [H2O] term would then be important. 

In comparison to traditional biphasic catalysis using water, fluorous phases or polar organic solvents, catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications the use of a defined transition metal complex immobilised on a liquid ionic support has already shown its unique potential. 

Determine graphically (a) the catalytic coefficient for base-catalysis, k -, and also (b) the coefficient for solvent catalysis, k. ... 

The evidence for diazonium-ion formation in neutral or basic solutions is strong. Nonetheless, a number of serious problems remain. One difficulty is the high reactivity that must be attributed to the diazocompounds. Although aliphatic diazoalkanes can be expected to be particularly reactive towards protonation, the difference between, on the one hand, diazomethane, which requires the presence of a carboxylic acid for the observation of proton exchange at room temperature (van der Merwe et al., 1964) and, on the other hand, diazobutane, which undergoes protonation in methanolic sodium methoxide at —64° (Kirmse and Rinkler, 1962) is somewhat surprising. One would wish to see the acidic character of the solvent catalysis corroborated by a Bronsted relation within which the rate constant for the solvent reaction is compared with that for other molecular acids. 

Nguyen, M. T., Raspoet, G., Vanquickenborne, L. G. Mechanism of the Beckmann rearrangement ab initio calculations suggest an active solvent catalysis. Trends in Organic Chemistry 997, 6, 169-180. 

Ramirez-Verduzco, L.F., Torres-Garcia, E., Gomez-Quintana, R., Gonzalez-Pena, V., and Murrieta-Guevara, F. Desulfurization of diesel by oxidation/extraction scheme influence of the extraction solvent. Catalysis Today, 2004, 98, 289. 

A second, related aspect of the medium effect is also likely to be important. Decarboxylation converts a polar, highly solvated -C02 group into a nonpolar, nearly unsolvated CO2. For the case of benzisoxazole-3-carboxylic acids, carboxyl desolvation has been demonstrated to be an important component of solvent catalysis of decarboxylation 113). Desolvation of the carboxyl group on binding to an enzyme may likewise be an important factor. Carbon isotope effects are consistent with this possibility (777). Oxygen isotope effects on formate dehydrogenase are also consistent with this mechanism 114). 

A series of quantitative data for solvent effect on the aminolysis of nitrophenyl esters attached to polyacrylamides have also been reported [41b]. These data are in broad agreement with the above-mentioned observations. However, the apparent solvent effects in chemical transformation of polymers must be interpreted in terms of a dual function, i.e. polymer solvation and solvent catalysis . For example, DMSO is a poor solvent for copol3 AOTq)-styrene), but a good solvent for polymers carrying amide residues. It should also be noted that alcohols and water are not usually suitable as solvent for chemical transformation of activated esters, because they may them lves enter the reaction as nucleophiles. 

A review has illustrated the importance of atomic-level DFT studies in elucidation of the function of hydrogen bonds in organocatalytic reactions through influence on the mechanism of substrate activation and orientation, and the stabilization of transition states and intermediates. Examples discussed include stereoselective catalysis by bifunctional thioureas, solvent catalysis by fluorinated alcohols in epoxidation by hydrogen peroxide, and intra-molecular cooperative hydrogen bonding in trans-a,a -(dimethyl-l,3-dioxolane-4,5-diyl)bis(diphenyl methanol) (TADDOL) (7)-type catalysts. ... 

The uncatalysed pathway k in the rate law (8) could represent either a purely uncatalysed reaction of amine and cephalosporin, or solvent catalysis with water acting as a general base to remove a proton from the attacking amine, or as a general acid to donate a proton to the 3 lnctam nitrogen. The k pathway cannot represent rate-limiting formation of the tetrahedral... 

Baeyer-Villiger Oxidation of Ketones in Fluorinated Alcohol Solvents - Catalysis by Arsonic and Seleninic Adds... 

---