DTBC complex

The kinetic experiments were not performed under true catalytic conditions, i.e. the pre-prepared [FeL(DTBC)] complexes were introduced into the reaction mixtures as reactants and excess substrate was not used. Nevertheless, the results are important in exploring the intimate details of the activation mechanisms of the metal ion catalyzed autoxida-tion reactions of catechols. In excess oxygen the reaction was first-order in the complex concentration and the first-order dependence in dioxygen concentration was also confirmed with the BPG complex. As shown in Table II, the rate constants clearly correlate with the Lewis character of the complex, i.e. the rate of the oxidation reaction increases by increasing the Lewis acidity of the metal center. 

The scission product from the diamagnetic [Ga(BPG)(DTBC)] complex is considerably less (4%) than that from the corresponding Fe complex (97%) (29). Gallium(III) is a diamagnetic d10 ion with an ionic radius very similar to that of iron(III) and expected to possess similar Lewis properties to that of Fe(III). Thus, the results clearly demonstrate that the ability of the metal center to transfer paramagnetic spin density to... 

Kinetic Parameters for the Reactions of [FeL(DTBC)] Complexes with Dioxygen in Different Solvents (29)a... 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

The spectroscopic features of the DTBC complex with [Cu2(L55)], which has been detected at low temperature, suggests that catecholate binds as bridging ligand to the dicopper(II) complex in a r r fashion, as proposed for nitrocatecholate-copper(II) complexes (38). In the complex formulated as [CU2/DTBC] in reaction 17 the catechol is supposed to bind to the dicopper(I) complex in a symmetric r r bridging mode (757). A kinetic treatment of the two steps of the catalytic reaction yielded the parameters kj, Kj)j, and Kj)2 The activation and thermodynamic parameters were obtained from the temperature dependence of kj, and Kd2- The constants... 

Recently, experimental supports for the oxygen activation process (B) were reported to explain the extradiol oxygenation of the Fe" -(DTBC) complex. The process is favored by the complex having facial tridentate ligands rather than tetradentate ligands in the former 02 and substrate can occupy the opposite face and form an intermediate that leads to the extradiol products (Fig. 9). The A process was proposed for the intradiol oxygenations for the complexes with meridional... 

In our ongoing efforts to develop oxidation catalysts that are functional in water as environmentally berrign solvent, we synthesized a water-soluble pentadentate salen ligand with polyethylene glycol side chairts (8). After coordination of copper(II) ions to the salen ligand, a dinuclear copper(II) complex is obtained that is soluble in water, methanol and mixtures of both solvents. The aerobic oxidation of 3,5-di-tert.-butylcatechol (DTBC) into 3,5-di-terr.-butylqitinone (DTBQ) was used as a model reaction to determine the catalytically active species and initial data on its catalytic activity in 80% methanol. 

For dinuclear Cu complexes, several pathways are possible as summarized in Scheme 15 [182]. In addition, plausible alternatives involve mixed-valent Cu Cu species where only one of the Cu ions is directly involved in the electron transfer. The latter seems most hkely in cases where the substrate binds to only one of the two copper ions, and H2O2 may then form upon oxidation of the Cu Cu -semiquinone intermediate [195]. Different coordination modes of the DTBC substrate appear to be indeed possible, depending on the particular dicopper scaffold [133,196,197]. Unfortunately, detailed mechanistic studies are still quite scarce [198-203] and most proposed catalytic pathways are rather speculative. 

Pyrazolate-based dinucleating hgands have proven useful to control crucial characteristics of the dicopper core, such as the Cu - Cu separation and the electronic properties of the metal ions, by variation of the chelate side arms attached to the heterocycle (31). This leads to greatly differing activities in the catalytic oxidation of DTBC mediated by those dicopper complexes [133,135]. While most of the pyrazolate-derived complexes 31 display an enzyme-hke Michaehs-Menten type kinetic behavior, it is apparent that both the Cu - Cu separation as well as the redox potential play an important... 

Dinudear copper complexes with aliphatic tripodal amino alcohol ligands were tested in the oxidation of DTBC (Eq. 17) [221]. It was shown that... 

Although the uncharged tris(3,5-di-t-butylcatecholate) complex of iron [Fe(DTBC)3] has been extensively studied, " the proposed bonding in these reports is unclear. The most common formulation is as an ionic salt between iron(lll) and three semiquinone anion radicals, Fe +(DTBSQ 03 However, the magnetic moment is 2.9 BM (consistent with an 5 = 2/2 spin state) and the electrochemistry indicates a ligand-centered reduction. Both of these characteristics are analogous to ferrate dianion. 

V(N3S2)L(L )] (L = bipy or phen L = cat or dtbc) have been synthesized. Complexes of the general type [V(dtbc)2L ][SbF6], prepared by oxidation [V(dtbc)2L ] with Ag[Sbp6], have also been investigated. ... 

As compared to the oxygenation reaction of phenols to catechols (phenolase reaction), dehydrogenation of catechols to the corresponding o-quinones (catecholase reaction) proceeds more readily. Thus, the catalytic activity of several tyrosinase and catechol oxidase models have been examined using 2,4-di-tert-butylcatechol (DTBC) as a substrate.Direct reactions between the (/r-77 77 -peroxo)dicopper(II) complexes and DTBC also have been studied at a low tempera-and a semiquinone-copper(II) complex has been isolated and structurally characterized... 

In an attempt at characterizing catalytic activities of different copper(II) complexes in the oxidation of 3,5-DTBC [13], the... 

Catalytic activity was characterized by the initial slope of the absorbance vs. time curves at 400 nm (o-quinone absorption). For 3,5-DTBC a factor of about 40 was observed between the slowest (Cu2[7-8]) and fastest (Cu[5-10]) reaction. Complexes of ligands 5, 6,... 

Of importance in this respect are the structure and properties of copper(II) 3,5-DTBC and related complexes. For example. Sawyer et al. 

A different kind of transformation occurs when ternary copper(II) catecholato complexes [Cu(3,5-DTBC)(phen)] and [Cu(3,5-DTBC)(bpy)] react with 0 in DMSO [55], affording the intradiol cleavage product... 

Weller and Weser s iron(III)-NTA complex shows catechol cleavage activity [96], converting 3,5-DTBC to 3,5-di-t-butyl-5-carboxymethyl-... 

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