Tatiana L. Simándi

Kinetics and mechanism of the cobaloxime(II) catalyzed oxidative dehydrogenation and double bond cleavage of 3,3′,5,5′-tetra-tert-butyl-4,4′-dihydroxystilbene by O2

In the presence of the cobaloxime(II) catalyst [Co(Hdmg) 2 (Ph 3 P) 2 ], at room temperature and 1 atm O 2 or air, the dihydroxystilbene derivative referred to in the title (H 2 StQ) undergoes oxidative dehydrogenation to the corresponding stilbenequinone (StQ), and parallel oxidative cleavage at the C=C double bond to afford 2,6-di-tert-butyl-4-hydroxybe-nzaldehyde (Ald). The two products are formed via a common anion radical intermediate stable enough for detection by ESR spectroscopy. A detailed kinetic analysis has been carried out by volumetric, spectrophotometric and HPLC techniques. The observed kinetic behavior is consistent with the formation of a superoxocobaloxime(III) intermediate, which generates the free radical intermediate from H 2 StQ. In the steady state, the catalyst is present in a hydroxo-cobaloxime(III) precursor state, from which it is released via reduction to cobaloxime(II) by the anion radical.

Kinetics of radical polymerization—XLV. Steric effects in the radical reactivity of quinones

Abstract An attempt has been made to establish the dependence on structural parameters of the reactivity of substituted 1,4-benzoquinones as inhibitors in the radical polymerization of styrene. The relative reactivity depends on the redox potential of the quinone and on the size of the ortho -substituents. The steric factor has been separated from the overall reactivity and correlated with the extent of shielding of the reaction centre in the case of substituents of spherical symmetry.

SOLVENT EFFECT IN THE KINETICS OF COBALOXIME(II)-CATALYZED OXIDATIVE DEHYDROGENATION OF 3,5-DI-TERT-BUTYLCATECHOL BY O2

In the presence of triphenylphosphinecobaloxime(II), 3,5-di-tert-butylcatechol undergoes catalytic oxidative dehydrogenation to the corresponding 1,2-benzoquinone (DTBQ) at room temperature and atmospheric dioxygen pressure. The semiquinone anion radical (DBS•−) and its cobaloxime(III), complex CoIII(DBSQ)•−) have been detected as intermediates by ESR spectroscopy. The kinetics were followed in benzene by measuring the dioxygen uptake as a function of time. The reaction is somewhat faster in MeOH, which is due to the greater stability of the hydrogen-bonded intermediate (X) formed from superoxocobaloxime (CoIIIO2) and the catechol. H-atom abstraction occurs in the rate-determining decomposition of X. The system investigated is a functional model of catecholase (oxidase) activity, based on free-radical intermediates, a possibility recently demonstrated for certain oxidoreductases.

Addition of radicals to quinones—I. ESR study of addition products from chloroquinones and free radicals

Abstract The mechanism of addition reactions of radicals formed during thermal decomposition of benzoyl peroxide with various chloro-substituted p-benzoquinones has been studied by the ESR technique in various solvents. The ESR spectra of the intermediate radicals show that addition occurs at the carbonyl oxygen. The important role of charge-transfer complexes in the reaction has been established. For strong CT complexes, the quinone molecule reacts with radicals derived from the solvent.

Addition of radicals to quinones—IV. ESR study of some radical reactions of 2,6-di-t. butyl-1,4-benzoquinone☆

Abstract The radical reactions of 2,6-di-t. butyl-1,4-benzoquinone with various thermally decomposing azocompounds and peroxides have been studied by ESR. On the basis of the ESR spectra of the intermediate radicals, the mechanism of the reaction is suggested. It was found that, while primary alkyl radicals from azocompounds add to the carbonyl oxygen, the peroxides, in the first step, oxidise one of the methyl group of the t. butyl group of the guinone. The intermediates are formed by addition of these species to the initial quinone molecule.

Addition of radicals to quinones. ESR study of some radical reactions of 3,3′,5,5′-tetra-tert-butylstilbene-4,4′-quinone

Abstract Reactions of 3,3′,5,5′-tetra- tert -butylstilbene-4,4′-quinone with free radicals from the decomposition of azo compounds and peroxides (R . ) have been studied by ESR spectroscopy. Relatively stable phenoxyl-type free radicals are formed via addition of R . across the exo CC bond. Their coupling constants with the methine proton reflect steric effects. Further addition products lead to sterically crowded molecules which spontaneously dissociate into radicals via C(4)C(7) bond cleavage.

Addition of radicals to quinones—II. ESR spectra of radicals derived from fluoranil

Abstract In systems consisting of thermally decomposing benzoyl peroxide in alkylbenzenes, fluoranil behaves as a selective spin trap. As a result of charge transfer (CT) complex formation between alkylbenzenes and fluoranil, the latter traps secondary radicals formed via H-atom abstraction by phenyl radicals from the alkyl group of the aromatic solvents. Weaker CT interaction leads to the trapping of phenyl radicals. The kinetic isotope effect in H-atom abstraction leads to the same result.

Ferroxime(II)-catalysed oxidation of 3,5-di-tert-butylcatechol by O2. Kinetics and mechanism†

The complex [Fe(Hdmg)2(MeIm)2] 1, referred to as ferroxime(II), was found to be the precursor of a selective catalyst for the oxidative dehydrogenation of 3,5-di-tert-butylcatechol (H2dbcat) to the corresponding 1,2-benzoquinone (dtbq) at room temperature and atmospheric dioxygen pressure. The observed kinetic behaviour in MeOH is consistent with solvolysis of one MeIm ligand and binding of dioxygen to the five-co-ordinate intermediate to form a superoxo complex. The latter abstracts an H atom from H2dbcat via a hydrogen-bonded ternary active intermediate, affording the semiquinone anion radical dbsq˙– and [FeIII(Hdmg)2(MeIm)(O2H)]. Utilising an electron and a proton from H2dbcat, the latter undergoes heterolytic cleavage to yield a ferryl species, which rapidly oxidises a second H2dbcat. Complexation of dbsq˙– with [FeII(Hdmg)2(MeIm)] affords a strongly coloured paramagnetic species [FeII(Hdmg)2(MeIm)(dbsq˙–)], which persists throughout the catalytic reaction, acting as buffer for dbsq˙–. The proposed mechanism involves steps similar to those of the known cytochrome P450 cycle, with H2dbcat acting as both ancillary electron source and substrate.

Kinetics and mechanism of the cobaloxime(II)-catalysed oxidative dehydrogenation of 3,5-di-tert-butylcatechol by O2. A functional oxidase model

Triphenylphosphinecobaloxime(II) has been found to be a selective catalyst for the oxidative dehydrogenation of 3,5-di-tert-butylcatechol to the corresponding 1,2-benzoquinone at room temperature and atmospheric dioxygen pressure. In a rapid initial phase the catalyst is reversibly transformed to a previously characterised catecholatocobaloxime(III) species, which persists throughout the reaction, keeping CoII at a low level. The semiquinone anion radical (dbsq˙–) and its cobaloxime(III) complex CoIII(dbsq˙–) have been detected as intermediates by ESR spectroscopy. The kinetics was followed by volumetry. According to the proposed mechanism, in the rate-determining step superoxocobaloxime (CoIIIO2) abstracts an H atom from the cathechol via an hydrogen-bonded intermediate. The system investigated is a functional model of catecholase (oxidase) activity, based on free-radical intermediates, a possibility recently demonstrated for certain oxidoreductases.

Oxidation of styrene by 2,3-dichloro-5,6-dicyano-p-benzoquinone

Styrene is oxidized by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), affording hydroquinone mono(2-phenylethyl) ether. Kinetic studies (50°C in CHCl3) show that the reaction is faster under N2 than under air and takes placevia intramolecular H-atom transfer within the 1:1 and 1:2 DDQ-styrene charge-transfer complexes. The semiquinone radical intermediate is reoxidized to DDQ by O2 when the latter is present, therefore, the apparent rate of DDQ reduction is lower. Stability constants of the CT-complexes and kinetic parameters for the oxidation are reported.