Haruo Tomita

Model Catalytic oxygenations with Co(II)—Schiff base complexes and the role of cobalt-oxygen complexes in the oxygenation process

Abstract Catalytic activity of Co(II)—Schiff base complexes toward selective oxygenation of organic molecules related to biological systems has been reviewed. The complexes display (i) dioxygenase-type activity (selective oxygenolyses of 3-substituted indoles and flavonols, and regioselective dioxygen incoporation into 2,6-di-butylphenols yielding peroxyquinolato Co(III) complexes), (ii) monooxygenase- and/or phenolase-type activity (selective formation of p -quinols and benzoquinones from phenols), and (iii) peroxidase-type activity (one electron oxidation of phenols and oxidative cleavage of ether bond of p -phenoxyphenols). Preliminary kinetic studies on the regioselective peroxyquinolato Co(III) complex formation show that the reaction involves a rate-determining hydrogen abstraction from the phenol substrate by a superoxo Co(III) complex initially formed, followed by rapid reduction of the resulting phenoxy radical with Co(II) species leading to the formation of a phenolato Co(III) complex, to which dioxygen is incorporated. This mechanism may be supported by the reaction of phenols and phenoxy radicals with other superoxo Co(III) complexes, [(CN) 5 Co(III) (O 2 )] 3− and [(3-MeOSalen)Co(III)(O 2 )]. In all the selective oxidations, the role of cobalt—dioxygen complexes is to activate substrates, and the selectivity results from the coordination of thus activated substrates to Co(III) species to form key intermediates, into which dioxygen is incorporated.

Mechanism of selective p-quinol formation in the cobalt schiff base complex-catalyzed oxygenation of 4-alkyl-2,6-di-t-butylphenols

Abstract The selective formation of p-quinols in the Co(Salpr)-catalyzed oxygenation of 4-alkyl-2,6-di-t-butylphenols in MeOH has been found to involve the rate determining reduction of peroxy-p-quinolato Co(III) complex formed in the initial fast step. An ionic mechanism of the reduction of the O O bond in the peroxy complex by MeOH has been discussed based on kinetic studies. The reactive species in the catalytic cycle is found to be [Co(III)(Salpr)(OH)].

Base-catalysed oxygenolysis of 3-hydroxyflavones

3-Hydroxyflavones, except those containing a 7-hydroxy-group, undergo base-catalysed oxygenolysis under mild conditions leading to oxidative cleavage of the heterocyclic ring to give the corresponding depsides and carbon monoxide in excellent yields. The same result is obtained in the reaction of quercetinase, a dioxygenase. The oxygenation of 3,4′,7-trihydroxyflavone in aqueous solution gave p-hydroxyphenylglyoxylic acid and 2,4-dihydroxybenzoic acid; in absolute methanol containing sodium methoxide methyl 4-hydroxyphenylglyoxylate and methyl 2,4-dihydroxybenzoate as well as 4-hydroxyphenylglyoxylic acid were obtained. The formation of these products is rationalized by assuming that 2-hydroperoxy-4′,7-dihydroxyflavan-3,4-dione is formed first and is reduced with the intervention of the 7-hydroxy-group to give 2,4′,7-trihydroxyflavan-3,4-dione, which is solvated and oxidized under the reaction conditions. 2′-Substituted-3-hydroxyflavones were not susceptible to oxygenolysis.

Oxygenation of p-nitrophenylhydrazones with Co(II)-schiff base complexes

p-Nitrophenylhydrazones, unsusceptible to autoxidation, are readily oxygenated in the presence of a five-coordinate cobalt(II)-Schiff base complex, Co(II)(MeOSalen) (Py) leading to quantitative formation of novel 1-(p-nitrophenylazo)-1-peroxy Co(III) complexes 2, which were isolated as crystals. A plausible mechanism involving hydrogen abstraction by Co(III)(O2−.) from the substrate followed by formation of a substrate anion Co(III) complex intermediate is proposed.

Reaction of superoxo Co(III) complex with stable phenoxy radicals

A typical superoxo complex [Co(CN)5O2][Ph3PNPPh3]3 combines with stable phenoxy radicals in CH2Cl2 leading to selective formation of peroxy-p-quinols except for 2,4,6-tri-t-butylphenoxy radical, representing radical reactivity of the complex.

Regioselective formation of peroxyquinolatocobalt(III) complexes in the oxygenation of 2,6-di-t-butylphenols with cobalt(II) Schiff-base complexes

The oxygenation of 2,6-di-t-butylphenols with five-co-ordinate cobalt(II) Schiff-base complexes in aprotic solvents, such as CH2Cl2, thf, and dmf (thf = tetrahydrofuran, dmf = dimethylformamide), has been found to result in regioselective formation of peroxyquinolatocobalt(III) complexes. The regioselectivity depends on the nature of the substituent at the 4-position of the phenol used: 4-alkyl-2,6-di-t-butylphenols (1) afford peroxy-p-quinolatocobalt(III) complexes, whereas peroxy-o-quinolato-complexes are formed from 4-aryl-2,6-di-t-butyl phenols (4). The initiation of the oxygenation is hydrogen abstraction by superoxocobalt(III) species from the phenols to give the corresponding phenoxy-radicals (10). Rapid reduction of (10) with cobalt(II) species follows giving rise to a phenolatocobalt(III) complex intermediate, within which dioxygen is incorporated. The regioselectivity of the oxygenation is attributable to the formation of the phenolatocobalt(III) complex intermediate. Crystals of the peroxy-p-quinolatocobalt(III) complex (2a) are orthorhombic, space group P212121, with a= 33.749(11), b= 11.844(5), c= 9.329(4)A, and Z= 4. The crystal structure has been refined from 3 018 diffractometer data to R= 0.067.

Reaction of superoxo Co(III) complex with quinones formation of a semiquinone Co(III) complex

Abstract The superoxo complex [Co(CN) 5 O 2 ] 3- was found to act as a reducing agent towards quinones. One-electron reduction took place with o -quinones whereas two-electrons reduction with p -quinones. 3,5-Di- t -butyl- o -benzoquinone gave the corresponding semiquinone Co(III) complex quantitatively.

Selective formation of peroxy-p-quinolato Co(III) complexes in the oxygenation of 4-alkyl-2,6-di-t-butylphenols with Co(II)-Schiff's base complexes

Abstract The oxygenation of 4-alkyl-2,6-di- t -butylphenols ( 2 ) with Co(II)-Schiffs base complexes in aprotic solvents such as CH2Cl2, THF, Py, and DMF leads to highly selective formation of the corresponding peroxy- p -quinolato Co(III) complexes. The reaction proceeds by mechanism involving a rate determining hydrogen abstraction by superoxo Co(III) species from 2 giving phenoxy radical, rapid step of electron transfer from Co(II) complex to the phenoxy radical, and dioxygen incorporation into phenolato Co(III) complex thus formed.

THE ROLE OF DIOXYGEN SPECIES IN THE BASE‐ AND COBALT‐CATALYZED OXYGENATION OF HINDERED PHENOLS

Abstract— The mechanisms by which 4‐substituted 2,6‐di‐t‐butylphenols are oxygenated by base‐ and Co(II) Schiff base complex‐catalysis into o‐ or p‐peroxyquinols and their Co(III) complexes, respectively, have been investigated. For the base‐catalyzed oxygenation, a one‐step ionic mechanism involving no radical species is suggested to be the most probable one. For the formation of the peroxycobalt(III) complexes, the following stoichiometry is concluded: ArOH + Co(II) + 5/4 O2→ peroxycobalt(III) complex + 1/2 H2O. A mechanism involving an electron transfer between the phenols and the Co(II)‐O2 complex followed by further electron transfer between the formed phenoxy radicals and the Co(II) complex to give the corresponding phenolate anions is proposed.

Reaction of superoxo Co(III) complexes with 2,6-di-t-butyl-p-benzoquinone methides

Abstract Superoxo Co(III) complexes derived from Co(Salpr) and [Co(CN)5]3− reacted with 2,6-di- t -butyl- p -benzoquinone methides to give 2,6-di- t -butyl- p -benzoquinone and 2,6-di- t -butyl-2,5-cyclohexadienonespirooxiranes as the main products, which are considered to result from nucleophilic attack by the superoxo species on the exo double bond of the quinone methides.