Catechol catalytic reaction mechanism

For the studied catechol methylation reaction the catalyst structure and surface properties can explain the catalytic behaviour As mentioned above, the reaction at 260-350°C has to be performed over the acid catalysts. Porchet et al. [2] have shown, by FTIR experiments, the strong adsorption of catechol on Lewis acid/basic sites of the Y-AI2O3 surface. These sites control the reaction mechanism. 

As part of a theoretical examination of the factors controlling the catalytic efficiency of a transmethylation enzyme (catechol (9-mcthyltransferase), the reaction mechanism of the non-enzymic transmethylation of catechol by, -adcnosylmethionine (AdoMet, as modelled by sulfonium ion) has been elucidated by using ab initio and semiempirical quantum mechanical methods.97 The gas-phase reaction between catecholate and sulfonium is extremely fast, involving no overall barrier, and the reaction profile to some extent resembles that of a typical gas-phase, S N 2 reaction. However, in aqueous solution, this reaction is very slow, with a predicted barrier of 37.3 kealmol-1. Good agreement between calculated KIEs for the model reaction and measured KIEs for the enzymic reaction suggests that the transition states are similar. 

The mechanism of the catalytic reaction proved indeed to be very different from that found for [Cu2([22]py4pz)( r-0H)](C104)3 H20. Thus, in the first step of the reaction, a stoichiometric oxidation of catechol by the dicopper(II) complex takes place however, only one electron is transferred in this stoichiometric reaction, resulting in the formation of a semiquinone radical and a mixed-valence Cu"Cu species. Interestingly, the dicopper(II) complex was found to be essentially dinuclear in solution nevertheless, only one of the two copper(II) ions was found to participate in the redox process, whereas the second one played a purely structural... 

The fact that Weiss and Downs have been aide to isolate phenol in the products of their reactions with solid catalysts indicates a hydroxylation mechanism similar to that postulated in the case of vapor phase catalysis, in whidi the formation of the monohydroxylated derivative is the first step. The presence of the hydroxyl group as a substituent in the benzene molecule activates the para and ortho positions so that the introduction of a second oxygen molecule would be expected to result in the formation of quinol (C6H4(OH)2l 4) and catechol (C0H4(OH)21 2) with a preponderance of the former. Quinone which would result from the further oxidation of quinol has been found in the oxidation products from benzene for the case of the homogeneous catalytic reaction. 

The kinetic experiments were not performed under true catalytic conditions, i.e. the pre-prepared [FeL(DTBC)] complexes were introduced into the reaction mixtures as reactants and excess substrate was not used. Nevertheless, the results are important in exploring the intimate details of the activation mechanisms of the metal ion catalyzed autoxida-tion reactions of catechols. In excess oxygen the reaction was first-order in the complex concentration and the first-order dependence in dioxygen concentration was also confirmed with the BPG complex. As shown in Table II, the rate constants clearly correlate with the Lewis character of the complex, i.e. the rate of the oxidation reaction increases by increasing the Lewis acidity of the metal center. 

The kinetic mechanism of the methylation reaction of human COMT has been studied exhaustively using recombinant enzymes [19]. The mechanism is sequential ordered AdoMet binding first, then Mg2+ and the catechol substrate as the last ligand. Human S-COMT and MB-COMT have similar kinetic properties. The main difference is the one-order lower Km value of MB-COMT for dopamine as substrate (S-COMT 207 pMand MB-COMT 15 pAT). The COMT enzyme is a rather slow enzyme with a low catalytic number. At saturating substrate levels S-COMT has a double efficiency compared with MB-COMT (kcat=37 and kcat =17, respectively). At low substrate concentrations (<10 iM) the MB-COMT seems to methylate catecholamines more rapidly than S-COMT. 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

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