Solvent catalytic effects

Alternatively the alkylated aromatic products may rearrange. -Butylbenzene [104-57-8] is readily isomerized to isobutylbenzene [538-93-2] and j Abutyl-benzene [135-98-8] under the catalytic effect of Friedel-Crafts catalysts. The tendency toward rearrangement depends on the alkylatiag ageat and the reaction conditions (catalyst, solvent, temperature, etc). 

Stability toward reduction makes hydrogen fluoride a good medium for different hydrogenation processes [1, 2] It is a useful solvent for the hydrogenation of benzene in the presence of Lewis acids [f ] Anhydrous hydrofluonc acid has pronounced catalytic effect on the hydrogenations of various aromatic compounds, aliphatic ketones, acids, esters, and anhydrides in the presence of platinum dioxide [2] (equations 1-3)... 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

Other salts, especially fluoride salts, (e.g., KF) can be used to perform nucleophilic substitution. As is well known, halides, and particularly the fluoride anions, are rather powerful Lewis bases and can exert a catalytic effect on aromatic nucleophilic substitutions in dipolar aprotic solvents. Phenols can be alkylated in the presence of KF (or CsF) absorbed on Celite64,65 or Et4NF.66 Taking advantage of this reaction, halophenols and dihalides with bisphenols have been successfully polymerized in sulfolane at 220-280°C by using KF as the base. 

Engberts [3e, f, 9, 29] investigated the influence of metal ions (Co, Ni, Cu +, Zn +) on the reaction rate and diastereoselectivity of Diels-Alder reaction of dienophile 31 (Table 6.5, R = NO2) with cyclopentadiene (32) in water and organic solvents. Relative reaction rates in different media and the catalytic effect of Cu are reported in Table 6.5. 10 m Cu(N03)2 accelerates the reaction in water by 808 times, and when compared with the uncatalyzed reaction in MeCN by a factor of 232 000. 

Amines (primary and secondary aromatic) / -Chloranil The reaction depends on the catalytic effect of silica gel. Monochlorobenzene, as solvent for the reagent, also contributes. There is no reaction on cellulose layers. [17, 22]... 

Catalytic Effect of the Liquid-Liquid Interface in Solvent Extraction Kinetics Hitoshi Watarai... 

One of the most attractive roles of liquid liquid interfaces that we found in solvent extraction kinetics of metal ions is a catalytic effect. Shaking or stirring of the solvent extraction system generates a wide interfacial area or a large specific interfacial area defined as the interfacial area divided by a bulk phase volume. Metal extractants have a molecular structure which has both hydrophilic and hydrophobic groups. Therefore, they have a property of interfacial adsorptivity much like surfactant molecules. Adsorption of extractant at the liquid liquid interface can dramatically facilitate the interfacial com-plexation which has been exploited from our research. 

In the ONIOM(QM MM) scheme as described in Section 2.2, the protein is divided into two subsystems. The QM region (or model system ) contains the active-site selection and is treated by quantum mechanics (here most commonly the density functional B3LYP [31-34]). The MM region (referred to as the real system ) is treated with an empirical force field (here most commonly Amber 96 [35]). The real system contains the surrounding protein (or selected parts of it) and some solvent molecules. To analyze the effects of the protein on the catalytic reactions, we have in general compared the results from ONIOM QM MM models with active-site QM-only calculations. Such comparisons make it possible to isolate catalytic effects originating from e.g. the metal center itself from effects of the surrounding protein matrix. 

This and other work indicates that HC1 is largely undissociated in nitromethane for [HC1]>- 0.015 M and that there is little association either. There is evidence that a corresponding addition occurs to olefins in theimally degraded PVC. Results carried out in a variety of solvents (26) are consistent with elimination of HC1 occurring by a/3 -elimination of the Ex type favored by polar solvents. The same authors showed that at least in nitrobenzene containing dissolved HC1, the reverse reaction, i.e. addition of HC1, takes place. The fact that this may be interpreted as a retardation of the degradation process may have contributed to the confusion which has arisen and emphasizes the care which must be taken to disentangle the possible catalytic effect of HC1 when concurrent addition of HC1 to the polyenes is possible. 

For both catalytic and stoichiometric reactions, each step of the process taking place on the metal can be influenced by the nature of ligands, cations, anions, or solvent. The effects of these factors on reaction rate, selectivity, stereoselectivity, etc. cannot be easily predicted, because each step can be influenced in different ways. The reader is referred to the literature cited below. 

Nozaki-Hiyama-Kishi (NHK) reactions215,216 are well known and often employed as a useful method for the synthesis of natural products by coupling of allyl, alkenyl, alkynyl, and aryl halides or triflates with aldehydes. The organochromium reagents are prepared from the corresponding halides or triflates and chromium(ll) chloride, and are employed in polar aprotic solvents (THF, DMF, DMSO, etc.). Subsequently, it was found that nickel salts exhibited a significant catalytic effect on the formation of the C-Cr bond217,218 (Equation (19)). 

A mechanistic study of acid and metal ion (Ni2+, Cu2+, Zn2+) promoted hydrolysis of [N-(2-carboxyphenyl)iminodiacetate](picolinato)chromate (III) indicated parallel H+- or M2+-dependent and -independent pathways. Solvent isotope effects indicate that the H+-dependent path involves rapid pre-equilibrium protonation followed by rate-limiting ring opening. Similarly, the M2+-dependent path involves rate-determining Cr-0 bond breaking in a rapidly formed binuclear intermediate. The relative catalytic efficiencies of the three metal ions reflect the Irving-Williams stability order (88). 

In other systems, similar kinetic laws were observed when studying the effect of added pyridine, although differentiation with the dimer nucleophile mechanism is made in the interpretation of the experimental results (see below). Rationalizations of the involved phenomena are based on the strong hydrogen-bond interactions between the nucleophile and the pyridine, and on the catalytic effect of a third amine molecule in the decomposition of the zwitterionic intermediate in non-polar solvents. 

Hydrogen-bond donors have the ability to enhance the selectivities and rates of organic reactions. Examples of catalytic active hydrogen-bond donor additives are urea derivatives, thiourea derivatives (Scheme 10, Tables 12 and 13) as well as diols (Table 14). The urea derivative 7 (Scheme 9) increases the stereoselectivity in radical allylation reactions of several sulphoxides (Scheme 10)171. The modest increase in selectivity was comparable to the effects exerted by protic solvents (such as CF3CH2OH) or traditional Lewis acids like ZnBr2172. It was mentioned that the major component of the catalytic effect may be the steric shielding of one face of the intermediate radical by the complex-bound urea derivative. 

As expected, the reaction is fastest in water due to its hydrogen-bonding ability and high dielectric constant. Addition of 1 mol% of the thiourea catalyst 10 increases the yields after 1 h in cyclohexane and chloroform by about 60% a 40 mol% catalyst doubles the yield. A sizeable catalytic effect of the m-trifluoromethyl-substi luted thiourea was also found in water. Explanations for the surprising fact that this hydrogen-bond donor is catalytically active even in a highly competitive solvent such as water will be given in Section III.D.3. 

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