Catalytic acidity

Treatment of 1,4-dicarbonyls (1) with catalytic acid yields substituted furans (2) and is called the Paal-Knorr furan synthesis. This method is used extensively to produce a variety of mono-, di-, tri-, and tetrasubstituted furans. ... 

Thioglycosynthase a double mutant of a retaining glycosidase, in which both the catalytic nucleophile and the catalytic acid-base residue at the active site have been substituted by non-nucleophilic residues. It... 

Aleshin and coworkers (49) have reported the X-ray crystal structure at 2.2-A resolution of a G2-type variant produced by Aspergillus awamori. Meanwhile, an attempt was made to determine the amino acid residues that participate in the substrate binding and catalysis provided by G2 of A. niger (52). The results of the chemical approach indicated that the Asp-176, Glu-179, and Glu-180 form an acidic cluster crucial to the functioning of the enzyme. This conclusion was then tested by site-specific mutagenesis of these amino acid residues, which were replaced, one at a time, with Asn, Gin, and Gin, respectively (53). The substitution at Glu-179 provided an inactive protein. The other two substitutions affected the kinetic parameters but were not of crucial importance to the maintenance of activity. The crystal structure (49) supports the conclusion that Glu-179 functions as the catalytic acid but Asp-17 6 does not appear to be a good candidate for provision of catalytic base. Thus, there still exists considerable uncertainty as to how the disaccharide is accepted into the combining site for hydrolysis. Nevertheless, the kind of scheme presented by Svensson and coworkers (52) almost surely prevails. 

Several review articles have been published on the catalytic functions of micelles and related systems (Fendler and Fendler, 1970, 1975 Menger, 1977 Berezin et al., 1973 Cordes and Dunlap, 1969 Cordes and Gitler, 1973 Kunitake, 1977 Kunitake and Okahata, 1976 Bunton, 1979). The conventional catalytic functions of micelles are, in most cases, related to (i) the concentration of reactants and catalytic acid-base species in the micellar phase due to electrostatic and/or hydrophobic forces and (it) the stabilization of transition states and/or destabilization of initial states by the micellar environments. The situation is more complex when one of the reagents is hydrophilic (Bunton et al., 1979). However, the last few years have witnessed several novel advances in this field especially in relation to enzymatic catalysis. 

A catalytic acid hydrolysis, AaCa mechanism, has been implicated in bromination of A-acetyl-A -arylurea, MeCONHCONHCeHtR-j (R = Me, Cl, NO2) in the presence of mercuric chloride. ... 

The 2-acetamido-2-deoxy derivative 146 (Scheme 39) of glucono-1,5-lactam 126 (Scheme 31) was another ligand (Kx 2 pM) probed with the bacterial O-GlcNAcase from B. thetaiotaomicron (GH 84).308 The lactam (PDB 2XM1) was found in the 4E conformation with its carbonyl group coordinated by the catalytic acid-base Asp 243, Tyr 282 as well as a water molecule. 

A recent patent application describes, in part, a multikilogram scale synthesis of quinapril (Jennings, 2004). The carboxyanhydride of 26 is prepared by treatment with phosgene (Scheme 10.8) (Youssefyeh et al., 1987). This is next coupled with tetrahydroisoquinoline subunit 27 in the presence of catalytic acid. Without isolation of the resultant quinapril t-butyl ester, the reaction solution is treated with acetic acid and anhydrous hydrogen chloride to deprotect the ester. Amorphous quinapril hydrochloride is obtained via treatment with acetonitrile. 

The compound in which the 3-keto group is reduced to a hydrocarbon interestingly still acts as an orally active progestin. The preparation of this compound starts with the hydrolysis of dihydrobenzene (13-2) to afford 19-nortestosterone (15-1). Reaction with ethane-1,2-thiol in the presence of catalytic acid leads to the cyclic thioacetal (15-2). Treatment of this intermediate with Raney nickel in the presence of alcohol leads to the reduced desulfurized derivative (15-3). The alcohol at 17 is then oxidized by any of several methods, such as chromic acid in acetone (Jones reagent), and the resulting ketone (15-4) treated with hthium acetylide. There is thus obtained the progestin lynestrol (15-5) [18]. 

In our previous paper we have reported that silica MCM-41 exhibits a large amount of reversible adsorption, high thermal and hydrothermal stabilities, and little catalytic acidity and is an excellent adsorbent in PSA process for recovery of 2-propanol and toluene vapors [1]. Here we present the results of PSA of butanone on silica MCM-41 and discuss the effect of pretreatment temperatures on adsorption properties of MCM-41. 

For these hydrolytic enzymes the glycosyl-enzyme would be attacked by a hydroxyl ion derived from H20, whose deprotonation would presumably be assisted by the conjugate base (—B in Eq. 12-5) of the catalytic acidic group. 

The inverting P-glucanases also have two catalytic acid base groups but they are 0. 9-1.0 nm apart rather than 0.6 nm for retaining enzymes. This allows space for a water molecule whose OH is the nucleophilic reactant (-OY in Eq. 12-5) and in which a carboxylate group assists in dissociating the water molecule.98 (This mechanism is illustrated for glucoamylase in Fig. 12-7). 

Figure 13-2 View of the active site of yeast enolase containing a bound molecule of 2-phospho-D-glycerate. The catalytic magnesium ion is at the left but the "conformational" metal is not visible here. The imidazole group of His 159 serves as the catalytic base and the -NH3+ of Lys 396 or Lys 34573b as the catalytic acid. From Vinarov and Nowak.69...

Figure 13-5 An S-mandelate ion in the active site of mandelate racemase. Only some of the polar groups surrounding the active site are shown. The enzyme has two catalytic acid-base groups. Lysine 166 is thought to deprotonate S-mandelate to form the aci anion, while His 297 deprotonates R-mandelate to form the same anion.106...

The peptidyltransferase reaction resembles that of the proteases (Chapter 12, pp. 649,650), with a tetrahedrally bonded intermediate probable (Fig 29-13A). As is shown on pp. 649-650, the catalytic acid has been proposed to be the N3 atom of adenosine 2486 (2451 in E. coli) in the H. marismortui 23S RNA. This is in the central loop of domain V (Fig. 29-14). However, replacement of A2451 with G, U, or C did... 

It was once thought that the rate of equilibrium of the catalytic acid and basic groups on an enzyme with the solvent limited the rates of acid- and base-catalyzed reactions to turnover numbers of 103 s 1 or less. This is because the rate constants for the transfer of a proton from the imidazolium ion to water and from water to imidazole are about 2 X 103 s 1. However, protons are transferred between imidazole or imidazolium ion and buffer species in solution with rate constants that are many times higher than this. For example, the rate constants with ATP, which has a pKa similar to imidazole s, are about I0 J s 1 M-1, and the ATP concentration is about 2 mM in the cell. Similarly, several other metabolites that are present at millimolar concentrations have acidic and basic groups that allow catalytic groups on an enzyme to equilibrate with the solvent at 107 to 108 s-1 or faster. Enzyme turnover numbers are usually considerably lower than this, in the range of 10 to 103 s-1, although carbonic anhydrase and catalase have turnover numbers of 106 and 4 X 107 s 1, respectively. 

When one deals with the use of nitric acid in conjunction with a solid acid, one has to keep in mind that the solid may play two fundamentally different roles, which frequently are strongly connected. One of being a CATALYTIC ACID, and the other of being a STOICHIOMETRIC DESSICANT. We tried to separate these two basic functions. 

The nature of a transformation obtained by analytical methods (see Chapter 2) provides the basis for mechanistic speculation, and devising one or more possible pathways is seldom problematical. Consider the reaction of Equation 1.1 where AH is a catalytic acid, X contains an electrophilic residue and B is a base/nucleophile - it could be the acid-catalysed addition of a nucleophile to a carbonyl compound, for example ... 

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

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