- Biomimetic iron(iii) complexes of N3O and N3O 2 donor ligands: Protonation of coordinated ethanolate donor enhances dioxygenase activity
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A series of iron(iii) complexes 1-4 of the tripodal tetradentate ligands N,N-bis(pyrid-2-ylmethyl)-N-(2-hydroxyethyl)amine H(L1), N,N-bis(pyrid-2- ylmethyl)-N-(2-hydroxy- propyl)amine H(L2), N,N-bis(pyrid-2-ylmethyl)-N- ethoxyethanolamine H(L3), and N-((pyrid-2-ylmethyl)(1-methylimidazol-2-ylmethyl) )-N-(2-hydroxyethyl)amine H(L4), have been isolated, characterized and studied as functional models for intradiol-cleaving catechol dioxygenases. In the X-ray crystal structure of [Fe(L1)Cl2] 1, the tertiary amine nitrogen and two pyridine nitrogen atoms of H(L1) are coordinated meridionally to iron(iii) and the deprotonated ethanolate oxygen is coordinated axially. In contrast, [Fe(HL3)Cl3] 3 contains the tertiary amine nitrogen and two pyridine nitrogen atoms coordinated facially to iron(iii) with the ligand ethoxyethanol moiety remaining uncoordinated. The X-ray structure of the bis(μ-alkoxo) dimer [{Fe(L5)Cl}2](ClO4)25, where HL is the tetradentate N3O donor ligand N,N-bis(1-methylimidazol-2-ylmethyl)-N- (2-hydroxyethyl)amine H(L5), contains the ethanolate oxygen donors coordinated to iron(iii). Interestingly, the [Fe(HL)(DBC)]+ and [Fe(HL3)(HDBC)X] adducts, generated by adding ~1 equivalent of piperidine to solutions containing equimolar quantities of iron(iii) complexes 1-5 and H2DBC (3,5-di-tert-butylcatechol), display two DBC2- → iron(iii) LMCT bands (λmax: 1, 577, 905; 2, 575,915; 3, 586, 920; 4, 563, 870; 5, 557, 856 nm; Δλmax, 299-340 nm); however, the bands are blue-shifted (λmax: 1, 443, 700; 2, 425, 702; 3, 424, 684; 4, 431, 687; 5, 434, 685 nm; Δλmax, 251-277 nm) on adding 1 more equivalent of piperidine to form the adducts [Fe(L)(DBC)] and [Fe(HL3)(HDBC)X]. Electronic spectral and pH-metric titration studies in methanol disclose that the ligand in [Fe(HL)(DBC)]+ is protonated. The [Fe(L)(DBC)] adducts of iron(iii) complexes of bis(pyridyl)-based ligands (1,2) afford higher amounts of intradiol-cleavage products, whereas those of mono/bis(imidazole)-based ligands (4,5) yield mainly the auto-oxidation product benzoquinone. It is remarkable that the adducts [Fe(HL)(DBC)] +/[Fe(HL3)(DBC)X] exhibit higher rates of oxygenation affording larger amounts of intradiol-cleavage products and lower amounts of benzoquinone. The Royal Society of Chemistry 2011.
- Sundaravel, Karuppasamy,Sankaralingam, Muniyandi,Suresh, Eringathodi,Palaniandavar, Mallayan
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experimental part
p. 8444 - 8458
(2011/10/10)
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- Oxidative Ring Cleavage of o-Benzoquinone by Potassium Peroxomonosulphate
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Oxidation of 3,5-di-t-butyl-1,2-benzoquinone by potassium peroxomonosulphate gave both extra- and intra-diol cleavage products.
- Ando, Wataru,Miyazaki, Hajime,Akasaka, Takeshi
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p. 518 - 519
(2007/10/02)
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- Mechanism of C-C Cleavage of Cyclic 1,2-Diketones with Alkaline Hydrogen Peroxide. The Acyclic Mechanism and Its Application to the Basic Autooxidation of Pyrogallol
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The reaction of 3,5-di-tert-butyl-o-benzoquinone (3) with alkaline hydrogen peroxide was found to give a considerable amount of methyl ester 5 when H2O2 was added dropwise.In contrast, the corresponding diacids were not obtained from o-benzoquinone or 1,2-naphthoquinone on reaction with alkaline H2O2.An 18O-tracer study of the reaction of 3 and 9,10-phenanthrenequinone indicated that the C-C cleavage reaction proceeds via the acyclic Baeyer-Villiger type mechanism and clearly eliminated possible dioxetane or epoxide mechanisms.A similar study of the base-catalyzed autooxidation of 4,6-di-ter-butylpyrogallol revealed that the C-C bond is cleaved in a similar way by hydrogen peroxide formed from O2 and the polyphenol.
- Sawaki, Yasuhiko,Foote, Christopher S.
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p. 5035 - 5040
(2007/10/02)
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