Copper(II)-induced oxidations of aromatic substrates: catalytic conversion of catechols to o-benzoquinones. Copper phenoxides as intermediates in the oxidation of phenol, and a single-step conversion of phenol, ammonia, and oxygen into muconic acid mononitrile

Abstract

Cupric chloride or bis( 1 -phenyl-l,3,5-hexanetrionato)dicopper(II) complex in the presence of triethylamine or copper(I1) species produced by reaction of cuprous chloride with oxygen in an aprotic solvent in the absence of added base all catalyze conversion of 3,5-di-tert-butylcatechol and 4-tert-butylcatechol with oxygen to the corresponding 1,2-benzoquinones. It appears that the catalytic sequence involves (a) formation of the dicopper(I1) catecholate intermediate, (b) electron transfer from the aromatic ring to two copper(I1) centers in the intermediate providing o-benzoquinone and two copper(1) centers, (c) irreversible reaction of the generated copper(1) species with oxygen to give the active copper(I1) reagent, and (d) reaction of this reagent with catechol leading to regeneration of the dicopper(I1) catecholate intermediate and formation of the water byproduct. Under anaerobic conditions the extent of the electron transfer from the substrate is determined by the thermodynamic stabilities of the copper(II)/copper(I) pairs. Copper(I1) species produced by oxidation of cuprous chloride in pyridine reacted with phenol to give a copper(I1) reagent which converted both o-benzoquinones and catechols under anaerobic as well as under aerobic conditions to monophenyl esters of muconic acid; the same reaction with phenol itself occurred only in the presence of oxygen. It was concluded that the latter reaction involves copper phenoxides as intermediates and subsequently demonstrated that this was indeed the case by using bona fide copper phenoxides. Using anhydrous ammonia as a nonhydroxylic nucleophile instead of alcohol or phenol nucleophiles in the reaction with copper-oxygen species resulting from the oxidation of cuprous chloride in pyridine provided a new copper(II)/ammonia reagent. This "CuO/NH," reagent reacts with o-benzoquinones and catechols under anaerobic and aerobic conditions to give muconic acid mononitriles, and with n-heptaldehyde, benzaldehyde, and cinnamaldehyde it provides the corresponding nitriles. Conversion of phenol to muconic acid mononitriles required oxygen. A different copper reagent, obtained by the addition of anhydrous ammonia to pyridine cupric methoxy chloride in pyridine, converts 4-tert-butylcatechol, 3,5-di-tert-butyl-1,2-benzoquinone, and 3-methoxy-4-tert-butylcatechol into the corresponding substituted muconic acid imides. Possible mechanisms for these transformations were discussed. and the conclusions about the role of oxygen were presented. Activation"; Hayaishi, O., Ed.; Academic Press: New York, 1974, Chapter 1 and references therein. Schoot-Viterkamp, A. J. M.; Mason, H. S. Proc. Natl. Acad. Sci. (I.S.A. 1973, 70, 993. Jolley, R. L.; Evans, L. H.; Makino, N.; Mason, H. S. J. Bioi. Chem. 1974, 249, 335. Makino, N.; McMahill, P.; Mason, H. S.; Moss, T. H. Ibid. 1974, 219, 6062. Ochiai, E. I. "Bioinorganic Chemistry"; Allyn and Bacon: Boston, 1977; Chapter 9, pp 218-262; Chapter 10, pp 263-278.(9) A common feature of multicopper enzymes is that they contain socalled type 3 co...

Formation and Biological Targets of Quinones: Cytotoxic versus Cytoprotective Effects

“…3,25−28 p -Quinones are formed from oxidation of t -butylhydroquinone, hydroquinone, naphthohydroquinone, and thymohydroquinone. 25,29−32 Oxidation of catechols in natural products [caffeic acid, carnosic acid, catechin, ellagic acid, epigallocatechin (ECGC), hydroxychavicol, luteolin, procyanidin, quercetin, taxifolin, and urushiols] gives o -quinones with a variety of biological effects (Table 2).…”

Synthetic models of the active site of catechol oxidase: mechanistic studies

“…53 In their study of the kinetics of the reaction of 3,5-DTBCH 2 with dioxygen catalyzed by bis(1-phenyl-1,3,5-hexanetrionato)dicopper(II) complex, the authors showed that the overall reaction is first order in the substrate 53 Furthermore, these authors 55 reported a stoichiometric oxidation of 3,5-DTBCH 2 in anaerobic conditions to the respective quinone by a number of mononuclear and dinuclear copper(II) complexes, which was consistent with the first step of the mechanism proposed by Demmin, Swerdloff and Rogić. 54 They also made an interesting observation that mononuclear planar copper(II) complexes could not be reduced by 3,5-DTBCH 2 and showed very little catecholase activity in comparison to the readily reducible complexes. Thus, the catalytic activity of the complexes appeared to correlate with their reduction potentials.…”

Knocking on Wood: Base Metal Complexes as Catalysts for Selective Oxidation of Lignin Models and Extracts

“…44,45 Alternatively, Rogićand co-workers have proposed a single electron transfer step in the aerobic oxidation of catechols using catalytic CuCl/pyridine. 46,47 Hydrogen atom abstraction has also been invoked as a pathway in the oxidation of alcohols by several copper complexes that model the enzyme galactose oxidase. 48,49 In the aerobic oxidation of the simple lignin model compounds, the copper catalytic system exhibits quite different selectivity from the vanadium one.…”