Resting state, catalytic

Brown and coworkers [128] have studied the exchange process for the enamide complex 103, using magnetization transfer techniques (Figure 1.28). Compound 103 represents the catalytic resting state in the asymmetric homogeneous hydroge-... 

Spectral investigations of the catalytic resting states in both asymmetric hydroformylation and hydrocyanation using bisphosphite 1 were performed. Reaction of Rh(CO)2(acac) with 1 equiv of i under 1 1 CO/H2 led to clean formation of Rh(l)(CO)2H. Infrared spectra of the reaction product exhibited bands at 2069, 2015 and 1984 cm. Extensive characterization of this complex by NMR has been published by van Leeuwen.(4) Under 50 psi l.T CO/H2 in the presence of styrene, identical bands were observed by in situ IR using the ReactIR system. 

Brown, J. M. Chaloner, P. A. The catalytic resting state of asymmetric homogeneous hydrogenation. Exchange processes delineated by nuclear magnetic resonance saturation-transfer (DANTE) techniques. /. Chem. Soc., Perkin Trans. II1987,1583-1588. 

Figure 18.11 Plausible catalytic cycle for the ORR by simple Fe porphyrins adsorbed on the electrode surface and side Reactions (18.15)-(18.18). At pH < 3, the resting state of the catalyst is assumed to be ferric-aqua.

Only three steps of the proposed mechanism (Fig. 18.20) could not be carried out individually under stoichiometric conditions. At pH 7 and the potential-dependent part of the catalytic wave (>150 mV vs. NHE), the —30 mV/pH dependence of the turnover frequency was observed for both Ee/Cu and Cu-free (Fe-only) forms of catalysts 2, and therefore it requires two reversible electron transfer steps prior to the turnover-determining step (TDS) and one proton transfer step either prior to the TDS or as the TDS. Under these conditions, the resting state of the catalyst was determined to be ferric-aqua/Cu which was in a rapid equilibrium with the fully reduced ferrous-aqua/Cu form (the Fe - and potentials were measured to be within < 20 mV of each other, as they are in cytochrome c oxidase, resulting in a two-electron redox equilibrium). This first redox equilibrium is biased toward the catalytically inactive fully oxidized state at potentials >0.1 V, and therefore it controls the molar fraction of the catalytically active metalloporphyrin. The fully reduced ferrous-aqua/Cu form is also in a rapid equilibrium with the catalytically active 5-coordinate ferrous porphyrin. As a result of these two equilibria, at 150 mV (vs. NHE), only <0.1%... 

The classical peroxidative catalytic cycle involves the formation of a so-called compound I intermediate product of the binding of the hydrogen peroxide to the heme group of the enzyme and the subsequent release of a water molecule. The cycle operates through a second intermediate, compound II, to the resting state enzyme by two individual one-electron withdrawals from the reducing substrates [70],... 

Extensive studies have established that the catalytic cycle for the reduction of hydroperoxides by horseradish peroxidase is the one depicted in Figure 6 (38). The resting enzyme interacts with the peroxide to form an enzyme-substrate complex that decomposes to alcohol and an iron-oxo complex that is two oxidizing equivalents above the resting state of the enzyme. For catalytic turnover to occur the iron-oxo complex must be reduced. The two electrons are furnished by reducing substrates either by electron transfer from substrate to enzyme or by oxygen transfer from enzyme to substrate. Substrate oxidation by the iron-oxo complex supports continuous hydroperoxide reduction. When either reducing substrate or hydroperoxide is exhausted, the catalytic cycle stops. 

The first step in the catalytic cycle is the reaction between H2O2 and the Fe(ni) resting state of the enzyme to generate compound I, a high-oxidation-state intermediate comprising an Fe(IV) oxoferryl center and a porphyrin-based cation radical... 

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