Aromatization acid-base catalysis

Acid anhydrides, mixed s. Carboxylic acid anhydrides, mixed Acid-base catalysis, aromatization by - 26, 946 Acid chlorides... 

Catalog of Teratogenic Agents, 25 209 Catalysis 5 200-254. See also Acid-base catalysis Catalyst entries Catalytic entries Heterogeneous catalysis Homogeneous catalysis Photocatalysis of aromatic reactions, 16 844 cerium applications, 5 685-688... 

Bovine pancreatic chymotrypsin (Mr 25,191) is a protease, an enzyme that catalyzes the hydrolytic cleavage of peptide bonds. This protease is specific for peptide bonds adjacent to aromatic amino acid residues (Trp, Phe, Tyr). The three-dimensional structure of chymotrypsin is shown in Figure 6-18, with functional groups in the active site emphasized. The reaction catalyzed by this enzyme illustrates the principle of transition-state stabilization and also provides a classic example of general acid-base catalysis and covalent catalysis. 

Section 5.2 aromatic Friedel-Crafts-type alkylations C. Perego and P. Ingallina, Catal. Today 2002, 73, 3 A. Corma, Chem. Rev. 1995, 95, 559 alkane cracking and isomerization Y. Ono, Catal. Today 2003,81, 3 A. Feller and J. A. Lercher, Adv. Catal. 2004, 48, 229 isoparaffin-olefin alkylation A. Corma and A. Martinez, Catal. Rev. Sci. Eng. 1993, 35, 483 acid-base catalysis with metal oxides K. Tanabe and W. F. Floelderich, Appl. Catal. A Gen. 1999, 181, 399. 

Several examples in previous sections fall under the heading of acid-base catalysis. Nitration of aromatics with catalyst acid HB and its conjugate base B is one of these (see reaction 4.6 in Section 4.1). Also, acid-catalyzed hydrolysis and hydration reactions such as 8.3 and 8.7 can be viewed as belonging to this category because the original catalyst actually is H30+ rather than H+ and is reconstituted in a step that involves its conjugate base, H20, as co-reactant. 

The reactivity, fate, and distribution of bound solutes are certainly changed by association with stream humic substances. The rate of photolysis of certain organic compounds (Zepp et al., 1981a,b), the rate of volatilization of polychlorinated biphenyls (Griffin and Chian, 1980), the bioaccumulation of polynuclear aromatic hydrocarbons in fish (Leversee, 1981), the rate of humic acid induced acid-base catalysis (Perdue, 1983), and the rate of microbiological decomposition are some specific examples. The octyl ester of 2,4-D (2,4 DOE) was predicted by theoretical and mathematical models and found by experimentation to be resistant to base hydrolysis when bound to humic substances (Perdue, 1983). The same model predicted the humic acid catalyzed hydrolysis of atrazine as demonstrated by Li and Felbeck 11972). 

Mercuric chloride piperidine Aromatization by acid-base catalysis 946. AcOCHj OAc... 

Stout DM, Meyers AI (1982) Recent advances in the chemistry of dihydropyridines. Chem Rev 82 223-243 Siidi J (1976) Chiral recognition of prochiral centers and general acid-base catalysis. Biochem J 153 491-493 Sugimoto T, Matsumura Y, Tanimoto S, Okano M (1978) Asymmetric reduction of aromatic ketones by sodium borohydride in the presence of bovine serum albumin. J Chem Soc Chem Commun 926-927 Sugimoto T, Kokubo T, Miyazaki J, Tanimoto S, Okano M (1979a)... 

Carboxyhc acids react with aryl isocyanates, at elevated temperatures to yield anhydrides. The anhydrides subsequently evolve carbon dioxide to yield amines at elevated temperatures (70—72). The aromatic amines are further converted into amides by reaction with excess anhydride. Ortho diacids, such as phthahc acid [88-99-3J, react with aryl isocyanates to yield the corresponding A/-aryl phthalimides (73). Reactions with carboxyhc acids are irreversible and commercially used to prepare polyamides and polyimides, two classes of high performance polymers for high temperature appHcations where chemical resistance is important. Base catalysis is recommended to reduce the formation of substituted urea by-products (74). 

Schultz and coworkers (Jackson et a ., 1988) have generated an antibody which exhibits behaviour similar to the enzyme chorismate mutase. The enzyme catalyses the conversion of chorismate [49] to prephenate [50] as part of the shikimate pathway for the biosynthesis of aromatic amino acids in plants and micro-organisms (Haslam, 1974 Dixon and Webb, 1979). It is unusual for an enzyme in that it does not seem to employ acid-base chemistry, nucleophilic or electrophilic catalysis, metal ions, or redox chemistry. Rather, it binds the substrate and forces it into the appropriate conformation for reaction and stabilizes the transition state, without using distinct catalytic groups. 

One aspect of asymmetric catalysis has become clear. Every part of the molecule seems to fulfill a role in the process, just as in enzymic catalysis. Whereas many of us have been used to simple acid or base catalysis, in which protonation or proton abstraction is the key step, bifunctional or even multifunctional catalysis is the rule in the processes discussed in this chapter.Thus it is not only the increase in nucleophilicity of the nucleophile by the quinine base (see Figures 6 and 19), nor only the increase in the electrophilicity of the electrophile caused by hydrogen bonding to the secondary alcohol function of the quinine, but also the many steric (i.e., van der Waals) interactions between the quinoline and quinuclidine portions of the molecule that exert the overall powerful guidance needed to effect high stereoselection. Important charge-transfer interactions between the quinoline portion of the molecule and aromatic substrates cannot be excluded. 

In general, the product ratio of a mixed aldol condensation will depend upon the individual reaction rates. Most ketones show a pattern similar to butanone in reactions with aromatic aldehydes. Base catalysis favors reaction at a methyl position over a methylene group, whereas acid catalysis gives the opposite preference. 

Polyquinolines (PQ) are obtained by the Friedlander reaction of a bis-o-aminoaromatic aldehyde (or ketone) with an aromatic hisketomethylene reactant [Concilio et al., 2001 Stille, 1981]. The quinoline ring is formed hy a combination of an aldol condensation and imine formation (Eq. 2-221). Polymerization is carried out at 135°C in m-cresol with poly (phosphoric acid) as the catalyst. The reaction also proceeds under base catalysis, but there... 

The bacterial D-hydantoinase has been isolated as crystals from cells of Pseudomonas putida (= P. striata) (Table 1) [5]. Because the purified enzyme showed the highest activity and affinity toward dihydrouracil, the enzyme was identified as dihydropyrimidinase (EC. 3.5.2.2). Interestingly, the enzyme also attacked a variety of aliphatic and aromatic D-5-mono-substituted hydantoins, yielding the corresponding D-form of N-carbamoyl-a-amino acids. Thus, the enzyme can be used for the preparation of various D-amino acids. Under the conditions used for the enzymatic hydrolysis of hydantoin at pH 8 to 10, the L-isomers of the remaining hydantoins are racemized through base catalysis. Therefore, the racemic hydantoins can be converted quantitatively into N-carbamoyl-D-amino acids through this step. 

Cyclization occurs directly through catalysis by the acid liberated when a pyridine-2(lH)-thione is heated with an a-halo acid ester. The most convenient method for preparing the thiazole, however, seems to be the cyclization of (2-pyridinethio)acetic acids in acetic anhydride in the presence of pyridine. Without base catalysis the reaction is slow, which suggests a mixed anhydride intermediate. Mixed anhydride formation with ethyl chlorofor-mate in pyridine, or carboxyl activation by DCC in pyridine, gives the mesoionic product. The cyclization reaction and the chemical stability of the thiazole are adversely affected by a pyridine 6-substituent. The initially formed acylpyridinium salt (407) undergoes rapid tautomerization to the aromatic thiazole form equilibrium between the forms (407) and (408) is verified by rapid deuteration at C-2 (R1 = H) in AcOH-d (81H(15)1349). 

Both acid and base catalysis have been used extensively to catalyze exchange in aromatic, and to a lesser extent, heterocyclic molecules. In acid exchange, the most widely used catalysts are sulfuric acid,122,129, 131 phosphoric acid,132 trifluoroacetic acid5133 perchloric acid,134 aluminum chloride,135 and the phosphoric acid-boron trifluoride complex.132 These reactions constitute the simplest electrophilic substitution. The mechanism for such substitution in benzenoid compounds is now comparatively well understood 122 however, the problem of heteroaromatic electrophilic substitution is still being clarified and has led to renewed interest in acid-catalyzed exchange in heterocyclic compounds.122... 

F. Special Topics — Resonance, molecular orbital theory, catalysis, acid-base theory, carbon acidity, aromaticity, antiaromaticity, macromolecules, lipids, amino acids, peptides, carbohydrates, nucleic acids, terpenes, asymmetric synthesis, orbital symmetry, polymers... 

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