Acid Catalysis in Electron Transfer

Acid Catalysis in Electron Transfer flavin analogue (FI)... 

According to Eq. 6, the value becomes larger with decrease in Aa, i.e., the larger pAa value. In general, the pAa value becomes larger in an aprotic solvent such as MeCN as compared with the value in water, since the solvation energy for proton in an aprotic solvent is much less than that in water [85]. Thus, acid catalysis in electron transfer reactions is expected to be much more efficient in acetonitrile... 

Bronsted acid catalysis in electron transfer reactions can also be applied to catalysis of metal ions in electron transfer reactions, since not only protons but also metal... 

Bronsted acid catalysis in electron transfer described in Section 1.3.1 has also been effective for redox reactions via the electron transfer step. As shown in the case of metal ion-catalyzed hydride transfer reactions (see above), hydride transfer reactions from an NADH analogue to /7-benzoquinones also proceed via Bronsted acid-catalyzed electron transfer [255, 256]. Since NADH and ordinary NADH model compounds are subjected to the acid-catalyzed hydration [98, 257, 258], an acid-stable NADH model compound, 10-methyl-9,10-dihydroacridine (AcrH2), was used as a hydride donor to / -benzoquinone (Eq. 24) ... 

As shown above, the acid catalysis on electron transfer increases the overall efficiency of the photochemical reactions via photoinduced electron transfer without affecting the products. However, there are some cases when addition of an acid to a particular photoinduced electron transfer process results in... 

Ribonucleotide reductase differs from the other 5 -deoxyadenosyl-cobalamin requiring enzymes in a number of respects. Hydrogen is transferred from coenzyme to the C2-position of the ribose moiety without inversion of configuration. Also since lipoic acid functions in hydrogen transfer, exchange with solvent protons takes place. Furthermore, exchange between free and bound 5 -deoxyadenosylcobalamin occurs rapidly during catalysis. Evidence for a Co(I)-corrin as an intermediate for this reduction is presented in our section on electron spin resonance. 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

The catalytic role of Mg + in Scheme 12 is ascribed to the 1 1 and 1 2 complex formation of Q and Mg +, which results in an increase in kobs with an increase in [Mg +], exhibiting first- and second-order dependences on [Mg ], respectively (Figure 7). This contrasts with the Lewis acid catalysis in conventional concerted Diels-Alder reactions, in which the catalyst is believed to activate dienophiles (not the radical anions) by coordination to the acidic metal center [208-212]. However, the exact catalytic mechanism of numerous Lewis acid-catalyzed Diels-Alder reactions [208-212] has yet to be clarified, including a possible contribution of Lewis-acid catalyzed electron transfer step. 

As shown above, acid catalysis in an electron transfer process accelerates the rate of the overall redox reaction via the electron transfer without affecting the final products. However, there is a case when acid catalysis in an electron transfer process results in a drastic change of the products in the overall redox reactions (see below). In general, the radical cations of organic compounds (RH +) are much stronger carbon acids in solution as compared with the parent compounds (RH) [271], and... 

Lewis Acid Catalysis in C C bond Formation via Electron Transfer... 

Schmittel, M., Kiau, S. Thermal and electron-transfer induced reactions of enediynes and enyne-allenes. Part 9. Electron-transfer versus acid catalysis in enediyne cyclizations. Liebigs Ann. Chem. 1997,1391-1399. 

The same mechanistic dichotomy for HAT reactions, one-step (concerted) HAT versus sequential (stepwise) electron and proton transfer (Scheme 2.1), is applied to hydride transfer reactions, one-step (concerted) hydride transfer versus sequential (stepwise) ET followed by proton-electron (or hydrogen) transfer.13,40 64 68 Such one-step versus multistep pathways have been discussed extensively in hydride transfer reactions of dihydronicotinamide coenzyme (NADH) and analogues, particularly including the effect of metal cations and acids, 69-79 because of the essential role of acid catalysis in the enzymatic reduction of carbonyl compounds by NADH.80 In contrast to the one-step hydride transfer pathway that proceeds without an intermediate, the ET pathway would produce radical cation hydride donors as the reaction intermediates, which have rarely been observed. The ET pathway may become possible if the ET process is thermodynamically feasible. 

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