Catalytic cyde

Ogliaro, F., de Visser, S.P., Cohen, S., Sharma, P.K. and Shaik, S. (2002) Searching for the second oxidant in the catalytic cyde of cytochrome P450 a theoretical investigation of the iron (Ill)-hydroperoxo spedes and its epoxidation pathways. Journal of the American Chemical Society, 124,2806-2816. 

A quantitative model for the electrocatalyitic response of enzymatic amperometric biosensors requires consideration of the diffusion of all the involved species and the kinetics of the redox-enzyme catalytic cyde, as is depicted in Figure 2.27. 

In common with many catalysed reactions, the important features of carbonylation process chemistry may be associated with different aspects of the catalytic cyde. Broadly, process activity may vary either because (i) more of the catalyst is present in the active form, (ii) the activity of the catalyst in the active form is enhanced or inhibited or, less commonly, (iii) the rate controlling step does not involve the catalyst. The process selectivity may vary because of side reactions (i) occurring through the active catalyst cycle, (ii) involving inactive catalyst, or (iii) taking place because of the organic chemistry of the systems. Examples of all these contributions to overall process effidency are found in the various commerdal carbonylation processes. 

As shown in Scheme 3.8, the catalytic oxidation reactions can be rationalized by assuming the formation of oxo-ruthenium species by the reaction of low-valent ruthenium complexes with peroxides. The C-H activation at the a-position of amines and the subsequent electron transfer gives iminium ion ruthenium complex 55. Trapping 55 with f-BuOOH would afford the corresponding a-ferf-butylhydroxy-amines, water, and low-valent ruthenium complex to complete the catalytic cyde. 

Catalyst initiation involves the formation of a metathesis-active ruthenium species from the starting precatalyst and its entry into the catalytic cyde. For both Ru-2 and Ru-4, the initiation event consists of phosphine (PCys) dissodation to produce the 14-electron intermediate [(L)(Cl)2Rr CHR ], where L= PCys for Ru-2 and L = H2lMes for Ru-4) (Figure 6.4). Although this proposed spedes has not been observed in solution, it has been identified in the gas phase [7], and the ligand dissociation step has been studied by NMR magnetization transfer experiments. 

AC Cu bond is a stable covalent bond and is difficult to deave by itsdf [93]. After charge transfer from cuprate(I) to substrate however deavage of the resulting R Cu bond becomes easy. The reductive elimination reaction regenerates RCu j which may take part in further catalytic cydes. Thus in copper-... 

The ate complexes [Li(THF)4][Ln (R) 1,1 CioH6N(R) 2 2] (R) 32 Ln = Sm, Yb, Y R = alkyl) [53 57, 60] are unusual hydroamination catalysts as they lack an obvious leaving amido or alkyl group that is replaced during the initiation step by the substrate. It is very likely that at least one of the amido groups is protonated during the catalytic cyde, analogous to the mechanism proposed for Michael additions and aldol reactions catalyzed by rare earth metal alkali metal BINOL heterobime tallic complexes [69, 70]. The observation of similar selectivities for rare earth metal... 

Dihydrofolate Reductase during the Catalytic Cyde, Biochemistry 43, 16046-16055. 

Both dasses of reaction in Scheme 18.2 are shown as catalytic cydes to emphasize the often overlooked fact that hydrolase turnover depends on the exchange of products for substrate and water, and proton shuffling to prime the system for another round of catalysis. We will see that this step can be kinetically significant, leading to rate-limiting product release or protein- conformational isomerization. 

Heterogeneously catalyzed reactions are rather complex processes. Considering a two-phase system, either liquid-solid or gas-solid, several steps are needed to complete the catalytic cyde ... 

Sehested M, Jensen PB. Mapping of DNA topoisomerase 11 poisons (etoposide, clerocidin) and catalytic inhibitors (adarubicin. ICRF-187) to fourdistinci steps in the copoisomerase II catalytic cyde. Biochem Phamnacot 1996 51 879-886. 

The reaction was successfully carried out with various aryl(hetaryl) iodides and bromides involving different aryl thiols and alkyl thiols. A plausible catalytic cyde includes reduction of Co(II) complexes to Co(I), substitution of iodide ligand by SR leading to the formation of ArSCo(I), oxidative addition of ArX to this cobalt complex with formation of Co(III) derivative, followed by reductive elimination (Scheme 3.57) resulted in the product formation and regeneration of Co(I) catalyst. 

Although much interest has been shown recently in the possible application of various iron catalysts, care should be taken to understand the nature of the active species in the catalytic cyde. A recent study by Buchwald and Bolm raised a question on the role of trace quantities of other metals, particularly Cu 98]. 

The reaction can also be catalyzed by Cu(I) ions supplied by elemental copper, thus further simplifying the experimental procedure - a small piece of copper metal (wire or turning) is all that is added to the reaction mixture, followed by shaking or stirring for 12 8h [3, 21, 56]. Aqueous alcohols (methanol, ethanol, tert-butanol), tetrahy-drofuran, and dimethylsulfoxide can be used as solvents in this procedure. Cu(II) sulfate may be added to accelerate the reaction however, this is not necessary in most cases, as copper oxides and carbonates, the patina on the metal surface, are sufficient to initiate the catalytic cyde. Although the procedure based on copper metal requires longer reaction times when performed at ambient temperature, it usually provides access to very pure triazole products with low levels of copper contamination. Alternatively, the reaction can be performed under microwave irradiation at elevated temperature, reducing the reaction time to 10-30 min [56-62]. 

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