Catalysis acidic-basic

Using crystalline cytochrome-c peroxidase [38] and catalase [39], it is shown that intermediates are formed according to the general mechanism of acidic-basic catalysis ... 

Aramendia. MA Borau. V Jimenez, C Marinas, JM Porras. A Urbano, I J. Chemoselcctive and rcgioscicctive reduction of citral (3.7-dimeihyl-2.6-ociadicnal) by ga.s-phase hydrogen transfer over acid-basic catalysis. Applied Catalysis. -f. General. 1998 172,31-40. 

This review clearly illustrates the recent advances in the use of ILs as solvent in heterocyclic synthesis along with the advantages, such as commercial availability, increased reaction rates, high product yield, easy workup procedures, their recyclability, and reuse, etc. In this review more than 80% of the articles involved the use of neutral reactants and neutral reaction conditions. The ILs used in most of cyclocondensation reactions appreciably improved the yield and shortened the reaction time. Despite, in few cases IL Bronsfed acid cafalysis (e.g., NH or CH of imid-azolium-based ILs) or Lewis acidic/basic catalysis (interaction of IL cation or anion with substrates) have also been reported, we believe the most important benefit of ILs in organic reactions can be explained by the general concepts of solvent effects, where the ability of stabilizing the charged activated complex via solvent-solute-type interactions, enhances the reaction rates and improves yield in comparison to same reaction when performed in molecular solvents. 

Useful thermosetting resins are obtained by interaction of furfural with phenol. The reaction occurs under both acidic and basic catalysis. Other large uses of furfural together with phenol are in the manufacture of resin-bonded grinding wheels and coated abrasives (5). 

High Carbon Yield. Furfuryl alcohol and furfural are reactive solvents (monomers) and are effective in producing high carbon yield (heat induced carbonization in a reducing atmosphere). They function as binders for refractory materials or carbon bodies. Furfuryl alcohol usually requires acidic catalysis and furfural basic catalysis. Mixtures of furfuryl alcohol and furfural are generally catalyzed with acid although some systems may be catalyzed with base. 

Furfuryl alcohol alone, or in combination with other cross-linkable binders such as phenoHc reins, chemical by-products and pitch, catalyzed with acid, gives carbon yields of 35—56%. Furfural together with cyclohexanone, pitch, or phenoHc resins gives, under acid catalysis, yields of 35—55% carbon under basic catalysis yields of 5—50% are achieved. FurfuryHdeneacetone resins (13 and 14), catalyzed by acid or base, give carbon yields of 48—56 and... 

Reactions with Aldehydes and Ketones. An important use for alkylphenols is ia phenol—formaldehyde resias. These resias are classified as resoles or aovolaks (see Phenolic resins). Resoles are produced whea oae or more moles of formaldehyde react with oae mole of pheaol uader basic catalysis. These resias are thermosets. Novolaks are thermoplastic resias formed whea an excess of phenol reacts with formaldehyde under acidic conditions. The acid protonates formaldehyde to generate the alkylating electrophile (17). 

Hydrogen Sulfide andMercaptans. Hydrogen sulfide and propylene oxide react to produce l-mercapto-2-propanol and bis(2-hydroxypropyl) sulfide (69,70). Reaction of the epoxide with mercaptans yields 1-aLkylthio- or l-arylthio-2-propanol when basic catalysis is used (71). Acid catalysts produce a mixture of primary and secondary hydroxy products, but ia low yield (72). Suitable catalysts iaclude sodium hydroxide, sodium salts of the mercaptan, tetraaLkylammonium hydroxide, acidic 2eohtes, and sodium salts of an alkoxylated alcohol or mercaptan (26,69,70,73,74). 

Relative hydrolysis and condensation rate studies of multifunctional silanes, Si(OR), under acidic and basic catalysis showed that the first (OR) group hydroly2es much more readily than subsequent groups (195). Sdanol—sdanol condensation is much slower than sdanol—alkoxysilane condensation, even if the alkoxysilane is monofunctional, thus suggesting that chain extension is insignificant ia the presence of a cross-linker (196—199). 

The product described here, 4-(4-chlorophenyl)butan-2-one, was previously prepared in the following ways a) by reduction of the corresponding benzalacetone, b) by catalyzed decarbonylation of 4-chlorophenylacetaldehyde by HFeiCO) in the presence of 2,4-pentanedione, - c) by reaction of 4-chlorobenzyl chloride with 2,4-pentanedione under basic catalysis (K2CO3 in EtOH), d) by reaction of 4-chlorobenzyl chloride with ethyl 3-oxobutanoate under basic catalysis (LiOH), - and e) by reaction of 3-(4-chlorophenyl )-propanoic acid with methyl lithium. - ... 

Interpretation of Measurements in Experimental Catalysis P. B. Weisz and C. D. Prater Commercial Isomerization B. L. Evering Acidic and Basic Catalysis Martin Kilpatrick Industrial Catalytic Cracking Rodney V. Shankland... 

The same framework of eight possible mechanisms that was discussed for ester hydrolysis can also be applied to amide hydrolysis. Both the acid- and base-catalyzed hydrolyses are essentially irreversible, since salts are formed in both cases. For basic catalysis the mechanism is Bac2-... 

Sulfonyl chlorides as well as esters and amides of sulfonic acids can be hydrolyzed to the corresponding acids. Sulfonyl chlorides can by hydrolyzed with water or with an alcohol in the absence of acid or base. Basic catalysis is also used, though of course the salt is the product obtained. Esters are readily hydrolyzed, many with water or dilute alkali. This is the same reaction as 10-4, and usually involves R —0 cleavage, except when R is aryl. However, in some cases retention of configuration... 

The reaction is subject to both general-acid and general-base catalysis the following mechanisms can be written for basic (B) and acidic (BH) catalysis,... 

Compounds containing carbon-nitrogen double bonds can be hydrolyzed to the corresponding aldehydes or ketones. For imines (W = R or H) the hydrolysis is easy and can be carried out with water. When W = H, the imine is seldom stable enough for isolation, and hydrolysis usually occurs in situ, without isolation. The hydrolysis of Schiff bases (W = Ar) is more difficult and requires acid or basic catalysis. Oximes (W = OH), arylhydrazones (W = NHAr), and, most easily, semicarbazones (W = NHCONH2) can also be hydrolyzed. Often a reactive aldehyde (e.g., formaldehyde) is added to combine with the liberated amine. 

It is thus an example of reaction type A (p. 1175). The sequence shown is genera-lized." In specific cases there are variations in the sequence of the steps, depending on acid or basic catalysis or other conditions. Which step is rate determining also depends on acidity and on the nature of W and of the groups connected to the carbonyl. ... 

Under acidic conditions in a two-phase system the rate of hydrolysis is in the order MeOSiMe3 13a>(MeO)2SiMe2>(MeO)3SiMe. Under the action of basic catalysis this order of reactivity is reversed [10]. 

Scheme 1. Switchover of the reaction path by acidic molecules to basic catalysis.

By analogy with acids above, specific basic catalysis is found to be characteristic of reactions in which there is rapid, reversible proton-removal from the substrate before the slow, rate-limiting step. 

Some indolo[2..w/]c iiinolixidines undergo easy acid-catalyzed epimerization <1998H(50)243>. For instance, the alkaloid reserpine equilibrates to a mixture of starting material and its 3-epimer, isoreserpine, under acid or basic catalysis (Equation 6). A controlled epimerization of this type has been employed as the key step in a total synthesis of ( )-tacamonine <1998T157>. 

The most important heuristics relate to reaction conditions, in particular, to acid-base catalysis. Depending on whether acidic or basic conditions are specified, the reactivity of certain bonds is changed. As an example, under basic conditions the breakability of H-X bonds is increased in comparison with other bonds. In fact, the relative acidities of all H-X bonds (X = any other element) can be rapidly calculated in EROS, and this allows further distinction within this class of bonds. 

Oximes generally demonstrate good stability in the solid state when stored at low temperature. Simple oximes show reasonable stability in neutral aqueous solution but hydrolyze to hydroxylamine and the parent ketone under acidic or basic catalysis [2], As noted, nitro-containing oximes, such as FK409 (9), spontaneously decompose... 

Earlier pioneering work by Eigen (56) showed that the exchange process of a proton in aqueous acidic/basic medium between species MOH and MO - can in general be represented by Scheme 4 (which can also be adapted to include the proton transfer between a [MOH2] and a [MOH] species). This mechanism, involving protolysis and hydrolysis (acid and base catalysis) and direct proton transfer, can be... 

As discussed in the previous section, metal oxides have both acidic and basic properties. The acid-base properties of metal oxides have led to many interesting catalytic reactions. Catalytic reactions such as H2-D2 exchange, hydrogenation, isomerization, dehydrogenation, dehydrohalo-genation, and benzylation can be considered as examples of acid-base catalysis reactions.31-36 These reactions will be briefly discussed in the following section. The remarkable properties of MgO as a catalyst have been well documented in the literature and we shall discuss some of these unique catalytic properties. 

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

One of the central mechanistic questions regarding ubiquitination has been whether the reaction utilizes general acid/base catalysis, possibly in a manner analogous to the catalysis of peptide-bond cleavage. For example, an acidic catalytic residue could deprotonate the substrate lysine and make it a better nucleophile for attacking the ubiquitin thioester bond. In addition, a basic catalytic residue could polarize the thioester bond making the carbonyl carbon a better electrophile, and... 

It appears that all these possibilities can be excluded. If reactions (a) or (gf) were rate-limiting the reaction velocity would be independent of the concentration of the substrate, while reaction (e) (identical with (Z)) would predict no catalysis by acids or bases. If reactions (b), (d) or (h) determined the rate the reaction would show specific catalysis by hydrogen or hydroxide ions, in place of the general acid-base catalysis actually observed. Reactions (c), (f) and (m) are unacceptable as rate-limiting processes, since they involve simple proton transfers to and from oxygen. Reactions (j) and (k) might well be slow, but their rates would depend upon the nucleophilic reactivity of the catalyst towards carbon rather than on its basic strength towards a proton as shown in Section IV,D it is the latter quantity which correlates closely with the observed rates. 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

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