Reduction proton-catalyzed

The fraction n/m is equal to the exponent j in equation 6. It corresponds approximately to the oxidation number of the surface metal center = 2.1 for BeO (17), j = 3.25 for a-FeOOH (6), andj = 3.95 for Si02 (quartz) (19, 20). The following question arises What is the value ofj in reductive, proton-catalyzed dissolution of oxide minerals ... 

Chiang and coworkers synthesized a dimer of compound 26 in which two diiron subunits are linked by two azadithiolate ligands as a model of the active site for the [FeFeJ-hydrogenase [203]. Protonation of 26 afforded the p-hydride complex [26-2H 2H ] via the initially protonated spieces [26-2H ] (Scheme 62). These three complexes were also characterized by the X-ray diffraction analyses. H2-generation was observed by electrochemical reduction of protons catalyzed by 26 in the presence of HBF4 as a proton source. It was experimentally ascertained that [26-2H 2H ] was converted into 26 by four irreversible reduction steps in the absence of HBF4. 

It must, nonetheless, be emphasized that the products of reduction of pyrimidine have not been unequivocally identified, largely due to their instability in the presence of air (oxygen). Furthermore, the UV absorption spectra of the reduction products of waves I and II (kmax284 nm, smax 1.5 x 103) are suggestive of rapid conversion (proton-catalyzed hydration ) of the products, since both the dimer and the dihydro derivative possess a reduced system of aromatic bonds relative to the parent pyrimidine, as a result of which the UV absorption maximum should be shifted to the violet, whereas it is, in fact, shifted 44 nm to the red (from 240 nm to 284 nm) for both products. Of possible relevance to this is the fact that the reduced rings of 4-aminopyrimidine 102) and nicotinamide 103) undergo acidic hydration to form products absorbing at 280 to 290 nm. 

The kinetics of the photochemical reductive dissolution of lepidocrocite (y-FeOOH) with oxalate as the reductant depends strongly on pH both the rate and the overall rate constant, k> decrease with increasing pH. This behavior means that the pH dependence of the rate does not simply reflect the pH dependence of oxalate adsorption at the lepidocrocite surface. Between pH 3 and 5, the log k() values can be fitted with a straight line. The dependence of k on the concentration of surface protons, >FeOH2+, can be estimated from the slope of this line and from the protonation curve of lepidocrocite k0 >FeOHf I6. The value of 1.6, which can be considered only a rough estimate, is not too different from the theoretically expected value of 2 for the proton-catalyzed detachment of reduced surface iron centers (i.e., of surface metal centers with the formal oxidation state of II). 

Under our experimental conditions, the overall rate constant of the photochemical reductive dissolution of lepidocrocite in the presence of oxalate is pH-dependent. Thus, the pH dependence of the rate reflects more than the pH dependence of oxalate adsorption at the lepidocrocite surface. Various pH effects may account for this observed pH dependence of ka. One possibility is catalysis of detachment of the reduced surface iron centers by protonation of their neighboring hydroxo and oxo groups. The following question then arises How does the observed rate constant, ka, depend on surface protonation The general rate expression of the proton-catalyzed dissolution of oxide... 

Our results do not allow us to decide which of these pH effects is predominant. The experimental observation, however, that in the pH range between 3 and 5 the overall rate constant depends on the concentration of surface protons to the power 1.6 may be an indication that proton catalysis of the detachment of surface Fe(II) is an important factor. In this case, the detachment of surface Fe(II) would be the rate-determining step of the overall process. The experiments presented here serve as an illustrative example, pointing out that reductive dissolution of oxide minerals may be catalyzed by protons, and hence that the rates of proton-catalyzed and of reductive dissolution may not be merely additive. However, more experimental evidence is needed to evaluate the validity of applying the rate expression of the proton-catalyzed dissolution to the overall rate constant of reductive dissolution. 

Although methanol undergoes addition to the C-C double bond of an (alk-l-enyl)cyclopropane in a proton-catalyzed reaction, " generally methanol addition is carried out in a two-step reaction involving methoxymercuration followed by sodium borohydride reduction. The pure product from the first step can be isolated, but this isolation is usually not performed ... 

Fig. 3. Generic reaction sequence for the FASs. ACP, acyl carrier protein AT, acetyltransferase MT, malonyl transferase KS, P-ketoacyl synthase KR, P-ketoacyl reductase DH, dehydrase ER, enoyl reductase TE, thioesterase FT, palmitoyl transferase. In the animal FAS the acetyl and malonyl loading reactions are catalyzed by the same acyl transferase and the chain-termination reaction is catalyzed by a thioesterase. In the fungal FAS, the malonyl loading and palmitoyl unloading reactions are catalyzed by the same acyl transferase. Stereochemical analyses in the laboratories of Comforth and Hammes established that in both animal and fungal FASs the KS-catalyzed condensation reaction proceeds with inversion of configuration at the malonyl C2 position, followed by KR-catalyzed reduction of the 3-keto moiety to the 3R alcohol by transfer of the pro-4S hydride from NADPH, and DH-catalyzed dehydration to a trans-enoyl moiety by the syn elimination of the 2S hydrogen and the 3/f hydroxyl as water. However, the stereochemistry of the final reduction reaction catalyzed by ER domain proceeds with different stereochemistry. The animal FAS transfers the pro-4R hydride of NADPH to the pro-3/f position with simultaneous addition of a solvent proton to the pro-2S position, whereas the fungal FAS takes the pro-4S hydride of NADPH into the pro-3S position and the solvent proton is incorporated at the pro-25 position.

Three possible mechanisms for carrying out this transformation have been proposed [55]. One mechanism involves protonation of Np in the dinitrogen Ugand to yield Mo(N2H) followed by addition of an electron. The second involves reduction of Mo(N2) to Mo(N2) followed by protonation of Np in Mo(N2). The third involves a proton-catalyzed reduction followed by a protonation. These proposals wiU be discussed in that order. 

Single-potential step chronoamperometry The reduction of protons catalyzed by hydrogenase in the presence of a mediator (viologen) was used as a model of EC at mechanism not only in CV experiments [87], but also for chronoamperometric studies [88]. During the cathodic potential step, the current was sampled in 10 ms intervals. In this way I-t curves were obtained for blank solutions, for solutions with mediator and for solutions with mediator and with increasing amounts of hydrogenase, respectively. The current-time curves are schematically drawn in Fig. 22. Important is the characteristic shape for the catalytic reaction the trend to achieve the steady-state in the presence of both the mediator and the enzyme is evident. 

Fig. 3.6 Schrock s postulated proton-catalyzed reductive protonation.

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Complex IV consists of 13 peptides, two heme A groups (cytochrome a and a3> and two or three Cu atoms (Table 2). It spans the inner membrane and protrudes into the intermembrane space. Complex IV catalyzes the reduction of dioxygen by oxidized cytochrome c, and four protons derived from the matrix are consumed in the reaction. 

Scheme 58 Proposed mechanism of the reduction of protons to H2 catalyzed by 22...

In the same year, Evans and coworkers reported the electrochemical reduction of protons to H2 catalyzed by the sulfur-bridged dinuclear iron complex 25 as a hydrogenase mimic in which acetic acid was used as a proton source [201]. The proposed mechanism for this reaction is shown in Scheme 60. The reduction of 25 readily affords 25 via a one electron reduction product 25. Protonation... 

The cationic complex [CpFe(CO)2(THF)]BF4 (23) can also catalyze the proton reduction from trichloroacetic acid by formation of Fe-hydride species and may be considered as a bioinspired model of hydrogenases Fe-H Complexes in Catalysis ) [44]. This catalyst shows a low overvoltage (350 mV) for H2 evolution, but it is inactivated by dimerization to [CpFe(CO)2l2-... 

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