László I. Simándi

Catalytic activation of dioxygen by metal complexes

Preface. 1. Dioxygen Complexes. 2. Catalytic Oxidation of Saturated Hydrocarbons with O2. 3. Catalytic Oxidation of Alkenes with O2. 4. Catalytic Hydroxylation of Aromatic Hydrocarbons with O2. 5. Catalytic Oxidation of Phenols. 6. Catalytic Oxidation of Catechols. 7. Catalytic Oxidation of Alcohols with O2. 8. Catalytic Oxidation of Diols and Polyols with O2. 9. Catalytic Oxidation of Aldehydes and Ketones with O2. 10. Catalytic Oxidation of Nitrogen Compounds with O2. 11. Oxidation and Co-Oxidation of Tertiary Phosphines. 12. Oxidation of Sulfur Compounds. Index.

Catalytic activation of dioxygen by oximatocobalt(II) and oximatoiron(II) complexes for catecholase-mimetic oxidations of o-substituted phenols

Bis(dimethylglyoximato)cobalt(II) and -iron(II) complexes, referred to as cobaloxime(II) and ferroxime(II), respectively, containing rigid equatorial macrocycles stabilized by hydrogen bonding, are functional catecholase and phenoxazinone synthase models, but show no catechol dioxygenase type activity. We have studied the oxidation of 3,5-di-tert-butylcatechol (DBCatH2) and 2-aminophenol (AP) as model substrates with the objective of elucidating the mechanisms of these reactions, using a combination of techniques including identification of free-radical intermediates by ESR spectroscopy and UV–vis spectrophotometry of semiquinone anion radical adducts of cobaloxime and ferroxime species. Detailed kinetic studies of the catecholase-mimetic oxidations reveal a general mechanistic pattern involving reversible formation of ternary catalyst–substrate–dioxygen complexes, which are key intermediates capable of H-atom abstraction from the substrates. The resulting semiquinone anion-radical intermediate and its adducts with the catalyst complexes have been detected by ESR spectroscopy. They contain a unidentate catecholato ligand as shown by the adduct of cobaloxime(II), which has been isolated and characterized by X-ray diffraction. Our work has led to the conclusion that the lack of catechol dioxygenase activity in the case of ferroxime(II) is due to the rigid equatorial macrocycle, which prevents bidentate catechol coordination. To further test this hypothesis, we have synthesized and studied analogous iron(II) complexes with flexible quadridentate and quinquedentate dioximato Schiff-base ligands. In line with expectations, these new complexes exhibit both catecholase and catechol dioxygenase activity.

Catalytic oxidation of 2-aminophenol to questiomycin a by dioxygen in the presence of cobaloxime derivatives. Free radical intermediates

The multistep oxidative dehydrogenation of 2-aminophenol to Questiomycin A, catalyzed by cobaloxime(II) derivatives, involves ESR-detectable 2-aminophenoxyl type free radical intermediates.

Homogeneous catalytic oxidation of o-phenylenediamine by dioxygen in the presence of cobaloxime(II) derivatives: synthesis of substituted 2H-benzimidazoles

Abstract The cobaloxime(II) derivatives Co(Hdmg) 2 (Ph 3 P) 2 and [Co(Hdmg) 2 -py] 2 have been found to catalyze the oxidation of o -phenylenediamine by atmospheric oxygen at room temperature. The reaction requires carbonyl type solvents such as acetone, methyl ethyl ketone or cyclohexanone. The oxidation products have been identified as 2,2-disubstituted-2H-benzimidazoles, formed via catalytic dehydrogenation of 2,2-disubstituted dihydrobenzimidazole intermediates, which are cyclization products of o -phenylenediamine with the ketone solvent. A similar reaction is observed for acetaldehyde in THF.

A new method for the preparation of (arylsulfonyliminoiodo)benzenes

Abstract (Arylsulfonyliminoiodo)benzenes ( 3 ) can be prepared in 52–92% yield via the reaction of dimethoxyiodobenzene ( 1 ) with the corresponding arylsulfonamide in MeOH or CH 2 Cl 2 at room temperature.

Kinetics of the oxidation of 2-aminophenol by dioxygen in the presence of tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)-dodecachlorophthalocyaninatocobalt(II)

Abstract The kinetics and mechanism of the O2 oxidation of 2-aminophenol (H2AP) to 2-aminophenoxazin-3-one (APX) under ambient conditions, catalyzed by the recently synthesized tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)-dodecachlorophthalocyaninatocobalt(II), (R4PcCo), have been studied by spectrophotometry. The rate of APX formation is first-order in [R4PcCo] and obeys Michaelis—Menten type kinetics with respect to [H2AP]. The suggested mechanism involves rate-determining inner-sphere electron transfer from coordinated H2AP to coordinated O2 in the superoxo complex.

Kinetics and mechanism of the cobaloxime(II)-catalysed oxidation of 2-aminophenol by dioxygen. A phenoxazinone synthase model involving free-radical intermediates

Cobaloxime(II) derivatives catalysed the oxidative dehydrogenation of 2-aminophenol (ap) to 2-amino-3H-phenoxazin-3-one by dioxygen under ambient conditions. The 2-aminophenoxyl radical (ap˙) and a cobalt-bound dimeric radical have been detected as intermediates. The kinetics was followed by monitoring dioxygen uptake. The cobaloxime(II) concentration is very low during the reaction as evidenced by ESR spectroscopy. According to the proposed mechanism, in the rate-determining step superoxocobaloxime abstracts an H atom from ap via a hydrogen-bonded intermediate, affording the ap˙ radical. In the steady state cobaloxime(III) predominates and the active catalyst is generated in low concentration via its reduction by the ap˙ radical. The system studied serves as a model of phenoxazinone synthase and calls attention to the possible involvement of radical intermediates in the enzymatic reaction.

Cobalt(II) ion catalyzed oxidative cyclization of o-Phenylenediamine in the presence of dioxygen synthesis of substituted 2H-benzimidazoles and 2,3-diaminophenazine

Abstract o-Phenylenediamine (OPD) has been found to undergo facile catalytic oxidation in the presence of cobalt(II) perchlorate, nitrate and chloride at room temperature and atmospheric oxygen pressure. The oxidation product depends on the type of solvent used. In acetone the exclusive product is 2,2-dimethyl-2H-benzimidazole (IIIa) formed via oxidation of 2,2-dimethyl-dihydrobenzimidazole (IIa), the condenation product of OPD with the solvent. In MeOH and THF OPD is converted to 2,3-diaminophenazine (VI) with 100% selectivity.

Mechanism of the permanganate oxidation of unsaturated compounds. Part III. Intermediates in the oxidation of maleic and fumaric acids

The intermediates in the permanganate oxidation of maleic and fumaric acids have been studied in acidic solutions. The accumulation and decay of manganese(III) has been demonstrated by the stopped-flow technique. The concomitant four-electron oxidation of the substrates leads to the formation of formyl(hydroxy)acetic acid. The subsequent reactions reveal a complex pattern in which hydroxymalonic, glyoxylic, and oxalic acid are further intermediates. The product distribution has been determined as a function of the pH and the mole ratio of the reactants. A reaction scheme is suggested which rationalises the observed behaviour.

Hydrogen atom vs electron transfer in catecholase-mimetic oxidations by superoxometal complexes. Deuterium kinetic isotope effects

Dioximato-cobalt(II), -iron(II) and -manganese(II) complexes (1)-(6), acting as functional catecholase and phenoxazinone synthase models, exhibit a deuterium kinetic isotope effect predicted by theory (k4H/k4D < or = 3) in the catalytic oxidative dehydrogenation of 3,5-di-tert-butylcatechol and 2-aminophenol by O2. KIEs in the range of (k4H/k4D approximately 1.79-3.51) are observed with (1) and (2) as catalysts, pointing to hydrogen atom transfer in the rate-determining step from the substrate hydroxy group to the metal-bound superoxo ligand. Less significant KIEs (1.06-1.20) are exhibited by catalysts systems (3)-(6), indicating that proton-coupled electron transfer is the preferred route in those cases.