Catechol production

With the benzene-derived Ugands 26a,b the Cu-catalyzed orfho-hydroxyl-ation of methyl-4-hydroxybenzoate was only highly selective at low temperature, as at room temperature bound catecholate was shown to undergo a formal Michael addition with unreacted phenolate to yield methyl 2-[4-(carbomethoxy)phenoxy]-3,4-dihydroxybenzoate [181,270]. With the amine-based ligand 27, no such reactivity was observed and the catechol product methyl 3,4-dihydroxybenzoate was formed selectively. Interestingly, this latter ligand system also displayed some selective reactivity with the neutral 4-hydroxybenzoic acid substrate. 

Fukuzumi and Itoh have jointly reported on a if-peroxo dicop-per(ll) complex that acts as a functional model for the phenolase activity of tyrosinase. lithium salts of para-substituted phenols were used as substrates, reaching yields between 60 and 90% with only the catechol product formed... 

Scheme 12) [178]. Isotope labeling experiments using confirmed that the origin of one of the 0 atoms of the catechol product is molecular oxygen. Deprotonation of the substrate is essential, since only the C - C coupling... 

Catechol Production at Optimum Conditions. It has already been demonstrated that the production of monomeric compounds reaches a maximum when glycol lignin is cooked in 2% NaOH at a treatment severity corresponding to a reaction temperature and time of 300°C and 1 h, respectively. It was then necessary to see if higher yields could be achieved when the severity of treatment (ii0) is varied. 

H-oxine-S03 = S-hydroxyquinoUne-5-sulfonic add. H-catechol = product of first add dissodation of catechol. 

A variety of 1,6-diyne substrates have been investigated for activity in the HSiR3/CO system. A ruthenium complex, Ru3(CO)12, with a tertiary phosphine additive catalyzes the formation of good yields of catechol products in which one molecule of diyne, two molecules of CO, and one or two molecules of silane are incorporated, as seen in Eq. (39).108... 

In the mechanism of Scheme 18.4, hydroxylation is carried out homolytically by the same species proposed for alkane hydroxylation [25]. The oxidation of the cyclohexadienyl intermediate by hydrogen peroxide produces the diphenol and regenerates the active species, closing the catalytic cycle. Scheme 18.5 illustrates the heterolytic routes proposed by Wilkenhoner and others for hydroquinone and catechol production, based on cationic peroxy intermediates [47]. Both types of mechanism, however, are little more than working hypotheses, needing validation by experimental evidence. 

The earliest reported demonstration of enzymatic activity in a supercritical fluid was for the reaction of disodium p-nitrophenyl phosphate to p-nitro-phenol, catalysed by alkaline phosphatase. Randolph et aL [26] detected the product in yields of up to 71% in carbon dioxide at 35°C and 100 atm, in the presence of 0.1% v/v water. Hammond et al. [33] found tyrosinase, a polyphenol oxidase, to be catalytically active for the oxidation of 4-methyl phenol in both supercritical carbon dioxide at (36 2)°C and supercritical trifluoro-methane at (34 2)°C, with oxygen, at a total pressure of 345 bar. Use of a flow reactor permitted isolation of greater quantities of the catecholic product (1,2-dihydroxy, 4-methylbenzene). Oxidative activity for 4-chlorophenol substrate was appreciably lower. 

For practical application in catechol production the oxygenase-catalysed reactions mentioned above suffer from several disadvantages. The need for expensive cofactor regeneration makes these conversions unfavourable for application on an industrial scale, particularly if cell-free systems are to be used. Furthermore, the toxicity of the oxygen-labile catechols will lead to severe problems in large scale production processes. Catechols easily autooxidize or polymerize in the presence of oxygen and the products from these reactions... 

Shirai, K. 1987. Catechol production from benzene through reaction with resting and immobilized cells of a mutant strain of Pseudomonas. Agric. Biol. Chem. 51 121-128. 

Ben2X>quinones exhibit redbx behavior at easily acoessiUe potentials and coordinate to metal ions. Catechols, products of die two-electron reduction of o-benzocpiinones, are also good ligands, giving rise to catecholate complexes. o-Benzoquinones have been used to obtain catecholate complexes via oxidative addition.203 For example, the reaction of fra s-[RhQ(CO)(PPh3)2] with tetrachloro-l,2-benzoquinone results in the conversion of the four-coordinate rhodium(1) complex to a six-coordinate rhodium(m) catecholate complex. Scheme 8.28 ... 

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