Decomposition catalytic model

The diazo function in compound 4 can be regarded as a latent carbene. Transition metal catalyzed decomposition of a diazo keto ester, such as 4, could conceivably lead to the formation of an electron-deficient carbene (see intermediate 3) which could then insert into the proximal N-H bond. If successful, this attractive transition metal induced ring closure would accomplish the formation of the targeted carbapenem bicyclic nucleus. Support for this idea came from a model study12 in which the Merck group found that rhodi-um(n) acetate is particularly well suited as a catalyst for the carbe-noid-mediated cyclization of a diazo azetidinone closely related to 4. Indeed, when a solution of intermediate 4 in either benzene or toluene is heated to 80 °C in the presence of a catalytic amount of rhodium(n) acetate (substrate catalyst, ca. 1000 1), the processes... 

Johanek, V., Schauermann, S., Laurin, M. et al. (2004) On the role of different adsorption and reaction sites on supported nanoparticles during a catalytic reaction NO decomposition on a Pd/alumina model catalyst , J. Phys. Chem. B, 108, 14244. 

Despite several decades of studies devoted to the characterization of Fe-ZSM-5 zeolite materials, the nature of the active sites in N20 direct decomposition (Fe species nuclearity, coordination, etc.) is still a matter of debate [1], The difficulty in understanding the Fe-ZSM-5 reactivity justifies a quantum chemical approach. Apart from mononuclear models which have been extensively investigated [2-5], there are very few results on binuclear iron sites in Fe-ZSM-5 [6-8], These DFT studies are essentially devoted to the investigation of oxygen-bridged binuclear iron structures [Fe-0-Fe]2+, while [FeII(p-0)(p-0H)FeII]+ di-iron core species have been proposed to be the active species from spectroscopic results [9]. We thus performed DFT based calculations to study the reactivity of these species exchanged in ZSM-5 zeolite and considered the whole nitrous oxide catalytic decomposition cycle [10],... 

Equilibrium studies under anaerobic conditions confirmed that [Cu(HA)]+ is the major species in the Cu(II)-ascorbic acid system. However, the existence of minor polymeric, presumably dimeric, species could also be proven. This lends support to the above kinetic model. Provided that the catalytically active complex is the dimer produced in reaction (26), the chain reaction is initiated by the formation and subsequent decomposition of [Cu2(HA)2(02)]2+ into [CuA(02H)] and A -. The chain carrier is the semi-quinone radical which is consumed and regenerated in the propagation steps, Eqs. (29) and (30). The chain is terminated in Eq. (31). Applying the steady-state approximation to the concentrations of the radicals, yields a rate law which is fully consistent with the experimental observations ... 

Although the above model was developed under non-catalytic conditions, some of the results may bear significance under natural conditions or in the presence of excess sulfite ions. Thus, the decomposition of the mono-sulfito complex was considered to be the rate-determining step in the catalytic cycle, but only estimates could be given for the rate constant in earlier studies. The comprehensive data treatment used by Lente and Fabian yielded a well established value for this parameter (106), which can then be used to improve previous kinetic models. Furthermore, the participation of reactions of the [Fe2(0H)(S03)]3+ complex was never considered in kinetic studies where excess sulfite ion was used over low iron(III) concentration in mildly acidic solution (pH 2.5-3.0). The above model predicts that in some cases the formation of the dimeric sulfito complex could make a substantial contribution to the spectral changes and omission of this species could lead to biased conclusions. Reevaluation of data sets reported earlier by including the reactions of [Fe2(0H)(S03)]3+ may resolve some of the controversies found in literature results. 

In addition to the universal concern for catalytic selectivity, the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize catalytic fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in electrolytic solutions can be closely modelled after analogous well-characterized molecular or cluster complexes (1-8). (v)... 

A similar catalytic effect of PbS on the decomposition of thiourea had been suggested previously by Norr [47]. Kinetic measurements by Rieke and Bentjen suggested that CdS likewise catalyzed thiourea decomposition [37], Ortega-Borges and Lincot also deduced such a mechanism based on kinetic measurements of the CdS deposition using a quartz crystal microbalance [48], In this case, the measurements were found to fit best with a complex-decomposition model. Both they and Rieke and Bentjen found optimum deposition to occur under conditions where Cd(OH)2 was formed as a surface species on the substrate but not in the bulk of the solution. Kinetic measurements also led Dona and Herrero to a similar conclusion of a complex-decomposition mechanism, but with the main difference that the initial adsorbed species is not Cd(OH)2 itself but an ammine-hydroxide [49] ... 

Moreover, the conductivity, and hence the catalytic decomposition of hydrogen peroxide, has been observed to influence the stability of the oxygen electrode. The stability of phthalocyanine catalysts is a decisive factor for the practical applicability of organic catalysts in fuel cells operating in an acid medium. This is therefore a very important observation. The observed disturbance of the delocalization of the n electrons (rubiconjugation) in Fe-polyphthalocyanines, in addition to the correlation between conductivity on the one hand, and electrocatalysis and catalytic decomposition of hydrogen peroxide on the other, leads to a special model of the electroreduction of oxygen on phthalocyanines. The model... 

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

These complexes (205) were found to be good models for the reactive intermediates involved in the catalytic decomposition of alkyl hydroperoxides (Haber Weiss mechanism), and in the catalytic hydroxylation of hydrocarbons by ROOH. 

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