Acid-catalyzed electron transfer

Such a second-order dependence of kel on [HC104] has also been reported for the acid-catalyzed electron transfer from cis-diethylcobalt(III) complex, c/s-[Co(bpy)2]+, to p-benzoquinone derivatives in H20-EtOH (5 1 v/v) under high acidic conditions such that two t otons are involved for the one-electron reduction of p-benzoquinone derivatives [45] ... 

The ke[ values of photoinduced electron transfer reactions from [Ru(bpy)3]2 + to various nitrobenzene derivatives in the presence of 2.0 mol dm-3 HC104 are listed in Table 1, where the substituent effect is rather small irrespective of electron-withdrawing or donating substituents. A similar insensitivity to the substituent effect is also observed in the acid-catalyzed photoinduced electron transfer from [Ru(bpy)3]2+ to acetophenone derivatives [46,47]. The stronger the electron acceptor ability is, the weaker is the protonation ability, and vice versa. Thus, the reactivity of substrates in the acid-catalyzed electron transfer may be determined by two reverse effects, i.e., the proton and electron acceptor abilities, and they are largely canceled out. Such an insensitive substituent effect shows a sharp contrast with the substituent effect on the acid-catalyzed hydride transfer reactions from Et3SiH to carbonyl compounds, in which the reactivity of substrates is determined mainly by the protonation ability rather than the electron acceptor ability. 

Even simple metalloporphyrins such as [(TPP)M]+ where M = Co, Fe and Mn can enable reduction of O2 to H2O by ferrocene derivatives (Fc) used as an electron donor in the presence of HCIO4 in acetonitrile [214, 215]. However, the rate-determining step is the two-electron reduction of O2 to H2O2 (Scheme 15) [214, 125]. Nonetheless, the catalytic effect of [(TPP)M]+ for the reduction of O2 by Fc is remarkable, because the oxidation of ferrocene by O2 hardly occurred in the presence of HCIO4 without [(TPP)M]+ [216]. The rate-determining step for the [(TPP)M]+-catalyzed two-electron reduction of O2 to H2O2 has been shown to be the initial electron transfer from Fc to [(TPP)M]+ as shown in Scheme 15 [215]. The reduced metalloporphyrin, (TPP)M, is rapidly oxidized by acid-catalyzed electron-transfer reduction of O2 by Fc, regenerating [(TPP)M]+ via formation of the putative hy-droperoxo complex (Scheme 15) [215]. 

The rate constants (ket) of electron transfer from Fc to [(TPP)M] agree well with those evaluated in light of the Marcus equations [91] for outer-sphere electron transfer (Eq. 4) [215]. Such agreement clearly demonstrates that electron transfer from Fc to [(TPP)M]+ in Scheme 15 proceeds via an outer-sphere pathway. In contrast to this, the ket value of the acid-catalyzed electron transfer from (TPP)Co to O2 is 10 -fold larger than that expected from an outer-sphere electron transfer [215]. Such huge enhancement of the observed rate relative to that calculated for outer-sphere electron transfer indicates the strong inner-sphere nature of acid-catalyzed electron transfer from (TPP)Co to O2 this should result in formation of the hydroperoxo complex, [(TPP)Co02H]+ (Scheme 15, M = Co). Other metalloporphyrins (M = Fe and Mn) can also act as efficient catalysts of the reduction of... 

O2 by Fc in the presence of HCIO4 in acetonitrile. The inner-sphere nature of the acid-catalyzed electron transfer from (TPP)M to O2 is essential to the catalytic cycle. Otherwise [(TPP)M]+ would act solely as an electron mediator and no acceleration of the overall electron transfer from Fc to O2 would be achieved. 

Acid catalysis is also effective for the electron transfer reduction of flavins, which are also important coenzymes in the biological redox reactions [80-82], Flavin analogues (FI la and lb) are known to be protonated at the N-1 position in a strongly acidic aqueous solution as shown in Scheme 5 (pa s 0) [83]. In an aprotic solvent such as acetonitrile (MeCN), the protonation of FI occurs much more readily than in H2O [84]. The one-electron reduced radical FIH can also be protonated to give F1H2 + in acetonitrile (Scheme 5) [84]. In such a case, an acid-catalyzed electron transfer from 6W-[R2Co(bpy)2] to F1H+ in MeCN occurs to yield F1H2 (Eq. 7) [84] ... 

Acid-catalyzed electron transfer plays an important role in reduction of not only carbonyl compounds but also other substrates such as O2 [90, 91], N02 [92], nitrobenzene derivatives [93, 94], nitrosobenzene derivatives [93, 94] and sulfoxides [95, 96], The ket value for the photoinduced electron transfer from [Ru(bpy)3] + to nitrobenzene increases parabolically with increase in [HCIO4] [94]. This indicates that PhN02 is doubly protonated in the photoinduced electron transfer reaction to give PhN02H2 + (Eq. 9) ... 

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. 

Hydride transfer via Brnnsted acid-catalyzed electron transfer... 

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) ... 

Figure 14. Plot of logfcobs for the acid-catalyzed reduction of aldehydes and ketones by AcrH2 in the presence of HCIO4 (2.7 x 10 M) in MeCN at 333 K vs. logfcet for the acid-catalyzed electron transfer from [Ru(bpy),il + to the same series of substrates in the presence of HCIO4 (2.0 M) at 298 K. Numbers refer to the aldehydes and ketones (1, acetaldehyde 2, propionaldehyde 3, butyraldehyde ...

An acid-catalyzed electron transfer reaction can be combined with other reactions to construct an overall catalytic reaction, where catalysis in an overall reaction is... 

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