Efficient radical scavenging ability of artepillin C,a major component of Brazilian propolis,and the mechanism

Hydrogen transfer from artepillin C to cumylperoxyl radical proceeds via one-step hydrogen atom transfer rather than via electron transfer, the rate constant of which is comparable to that of (+)-catechin, indicating that artepillin C can act as an efficient antioxidant.     

Electron-transfer mechanism in radical-scavenging reactions by a vitamin E model in a protic medium

The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH.) or galvinoxyl radical (GO.) by a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly accelerated by the presence of Mg(ClO4)2 in de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH proceeds via an electron transfer from 1H to the radical, followed by a proton transfer, rather than the one-step hydrogen atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN (0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media.     

Mechanism of scandium ion catalyzed Diels-Alder reaction of anthracenes with methyl vinyl ketone

Rates of Diels-Alder cycloadditions of anthracenes with methyl vinyl ketone (MVK) are accelerated significantly by the presence of scandium triflate [Sc(OTf)3]. Sc(OTf)3 also promotes photoinduced electron-transfer reactions from various electron donors to MVK significantly. Comparison of the promoting effect of Sc(OTf)3 in photoinduced electron-transfer reactions of MVK with the catalytic effect of Sc(OTf)3 in the Diels-Alder reaction of 9,10-dimethylanthracene with MVK has revealed that the MVK-Sc(OTf)3 complex is a reactive intermediate in both the Diels-Alder and photoinduced electron-transfer reactions. The observed second-order rate constants of the Sc(OTf)3-catalyzed Diels-Alder reactions of anthracenes with MVK are by far larger than those expected from the observed linear Gibbs energy relation for the Diels-Alder reactions of anthracenes with stronger electron acceptors than MVK, which are known to proceed via electron transfer. This indicates that the Sc(OTf)3-catalyzed Diels-Alder reactions of anthracenes with MVK does not proceed via an electron-transfer process from anthracences to the MVK-Sc(OTf)3 complex.     

Proton-coupled electron shuttling in a covalently linked ruthenium-copper heterodinuclear complex

A heterodinuclear complex based on a Ru(II)-TPA [TPA = tris(2-pyridylmethyl)amine] complex having a peripheral Cu(II)(bpy)(2) (bpy = 2,2'-bipyridine) group bonded through an amide linkage displayed reversible intramolecular electron transfer between the Ru and Cu complex units that can be controlled by protonation and deprotonation of the bridging amide moiety.     

Formation of a ruthenium(IV)-oxo complex by electron-transfer oxidation of a coordinatively saturated ruthenium(II) complex and detection of oxygen-rebound intermediates in C-H bond oxygenation

A coordinatively saturated ruthenium(II) complex having tetradentate tris(2-pyridylmethyl)amine (TPA) and bidentate 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)](2+) (1), was oxidized by a Ce(IV) ion in H(2)O to afford a Ru(IV)-oxo complex, [Ru(O)(H(+)TPA)(bpy)](3+) (2). The crystal structure of the Ru(IV)-oxo complex 2 was determined by X-ray crystallography. In 2, the TPA ligand partially dissociates to be in a facial tridentate fashion and the uncoordinated pyridine moiety is protonated. The spin state of 2, which showed paramagnetically shifted NMR signals in the range of 60 to -20 ppm, was determined to be an intermediate spin (S = 1) by the Evans' method with (1)H NMR spectroscopy in acetone-d(6). The reaction of 2 with various oraganic substrates in acetonitrile at room temperature afforded oxidized and oxygenated products and a solvent-bound complex, [Ru(H(+)TPA)(bpy)(CH(3)CN)], which is intact in the presence of alcohols. The oxygenation reaction of saturated C-H bonds with 2 proceeds by two-step processes: the hydrogen abstraction with 2, followed by the dissociation of the alcohol products from the oxygen-rebound complexes, Ru(III)-alkoxo complexes, which were successfully detected by ESI-MS spectrometry. The kinetic isotope effects in the first step for the reaction of dihydroanthrathene (DHA) and cumene with 2 were determined to be 49 and 12, respectively. The second-order rate constants of C-H oxygenation in the first step exhibited a linear correlation with bond dissociation energies of the C-H bond cleavage.     

Structural and spectroscopic features of a cis (hydroxo)-Fe(III)-(carboxylato) configuration as an active site model for lipoxygenases

In our preliminary communication (Ogo, S.; Wada, S.; Watanabe, Y.; Iwase, M.; Wada, A.; Harata, M.; Jitsukawa, K.; Masuda, H.; Einaga, H. Angew. Chem., Int. Ed. 1998, 37, 2102-2104), we reported the first example of X-ray analysis of a mononuclear six-coordinate (hydroxo)iron(III) non-heme complex, [Fe(III)(tnpa)(OH)(RCO(2))]ClO(4) [tnpa = tris(6-neopentylamino-2-pyridylmethyl)amine; for 1, R = C(6)H(5)], which has a characteristic cis (hydroxo)-Fe(III)-(carboxylato) configuration that models the cis (hydroxo)-Fe(III)-(carboxylato) moiety of the proposed (hydroxo)iron(III) species of lipoxygenases. In this full account, we report structural and spectroscopic characterization of the cis (hydroxo)-Fe(III)-(carboxylato) configuration by extending the model complexes from 1 to [Fe(III)(tnpa)(OH)(RCO(2))]ClO(4) (2, R = CH(3); 3, R = H) whose cis (hydroxo)-Fe(III)-(carboxylato) moieties are isotopically labeled by (18)OH(-), (16)OD(-), (18)OD(-), (12)CH(3)(12)C(18)O(2)(-), (12)CH(3)(13)C(16)O(2)(-), (13)CH(3)(12)C(16)O(2)(-), (13)CH(3)(13)C(16)O(2)(-), and H(13)C(16)O(2)(-). Complexes 1-3 are characterized by X-ray analysis, IR, EPR, and UV-vis spectroscopy, and electrospray ionization mass spectrometry (ESI-MS).     

Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins

The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co(2)(DPX)]. The other cofacial dicobalt porphyrins [Co(2)(DPA), Co(2)(DPB), and Co(2)(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co(2)(DPX). The micro-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting micro-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the micro-superoxo species. The superhyperfine coupling constant of the micro-superoxo species of Co(2)(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co(2)(DPX) for the four-electron reduction of dioxygen by Fe(C(5)H(4)Me)(2) results from the strong binding of the reduced oxygen with Co(2)(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP)(+), whereas the turnover-determining step in the Co(2)(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species of Co(2)(DPX), depending on the electron donor ability of ferrocene derivatives.     

Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

Superoxide ion (O2˙-) forms a stable 1 : 1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)(3)(3+)], which can be detected in solution by ESR spectroscopy. Electron transfer from O2˙- -Sc(HMPA)(3)(3+) complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)(3)(3+) to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)(3)(3+) complexes. The 1 : 1 and 1 : 2 complexes between semiquinone radical anions and Sc(HMPA)(3)(3+) depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)(3)(3+)-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)(3)(3+)-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA)(3)(3+) to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)(3)(3+). The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA)(3)(3+) occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)(3)(3+) depending the number of Sc(HMPA)(3)(3+) (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.     

Brønsted base-assisted boronic acid catalysis for the dehydrative intramolecular condensation of dicarboxylic acids

Br?nsted base-assisted boronic acid catalysis for the dehydrative self-condensation of carboxylic acids is described. Arylboronic acid bearing bulky (N,N-dialkylamino)methyl groups at the 2,6-positions can catalyze the intramolecular dehydrative condensation of di- and tetracarboxylic acids. This is the first successful method for the catalytic dehydrative self-condensation of carboxylic acids.     

Kinetic studies of retinol addition radicals

Retinol neutral radicals (RS-retinol˙), generated from the reaction of retinol with 4-pyridylthiyl and 2-pyridylthiyl radicals in argon-saturated methanol, undergo β-elimination, which can be monitored via the slow secondary absorption rise at 380 nm attributed to the rearrangement of the unstable retinol neutral addition radicals to the more stable addition radicals. Rate constants for the β-elimination reactions (k(β)) of 4-PyrS-retinol˙ were measured at different temperatures and the Arrhenius equation for the reaction is described by log (k(β)/s(-1)) = (12.7 ± 0.2) - (54.3 ± 1.3)/θ, where θ = 2.3RT kJ mol(-1). The reactivities of retinol addition radicals (RS-retinol˙), generated from the reaction of retinol with various thiyl radicals, towards oxygen have also been investigated in methanol. In the presence of oxygen, the decay of RS-retinol˙ fits to biexponential kinetics and both observed rate constants for the RS-retinol˙ decay are oxygen-concentration dependent. This suggests that at least two thiyl addition radicals, formed from the reaction of RS˙ with retinol, undergo oxygen addition reactions. In light of the estimated rate constants for oxygen addition to RS-retinol˙ and RS-CAR˙ (CAR: carotenoid), the antioxidant-prooxidant properties of retinol are discussed.