Selective Two-electron Reduction

The selective two-electron reduction of C60 to C60H2 has been achieved by photo-induced electron transfer in benzonitrile-TFA solution with 10-methyl-9,10-dihydroa-cridine (AcrHy.65 The proposed mechanism begins with electron transfer from AcrIU to C60 to afford the radical ion pair (C60 AcrHj ). The strongly acidic AcrHj species protonates Q,(l to afford the C6qH radical which is rapidly converted into the dihydrofiillerene (C60H2) by electron transfer from AcrH in the presence of TFA. The... 

Fukuzumi S, Suenobu T, Kawamura S, Shida A, Mikami K (1997) Selective two-electron reduction of C60 by 10-methyl-9,10-dihydroacridine via photoinduced electron transfer. Chem Commun 291-292... 

Scheme 18 Selective two-electron reduction of Cffl to 1,2-C6oH2.

The reduction of Ceo and Cvo has normally been achieved by the use of strong re-ductants such as BH3, which yield not only CeoHv and CV0H2 but also polyhydride mixtures [104, 230-237]. The use of the triplet excited state of Ceo has made it possible to attain the selective two-electron reduction of Ceo to l,2-CeoH2 by 10-methyl-9,10-dihydroacridine (AcrH2) which is a mild hydride donor (Eq. 14) [225, 238],... 

Under different conditions (in aqueous electrolyte) the selectivity of the cleavage reaction may be perturbed by the occurrence51-53 of a dimerization process. Thus, while the major process remains the two-electron reductive pathway, 20% of a dimer (y diketone) may be isolated from the cathodic reduction of PhC0CH2S02CH3. The absence of crosscoupling products when pairs of / -ketosulphones with different reduction potentials are reduced in a mixture may indicate that the dimerization is mainly a simple radical-radical coupling53 and not a nucleophilic substitution. 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

Electrochemical and Electrocatalytic Reduction. The one-electron reduction of C02 yields the radical anion C02-, which reacts with an H source to give the formate ion. The reaction, however, is not selective because various other reactions may take place. An alternative and more promising approach is the two-electron reduction of C02 in the presence of a proton source to afford formic acid. The latter process requires a considerably lower potential (—0.61 V) than does the one-electron reduction (—1.9 V) consequently, the electrolysis in the presence of catalysts may be performed at lower voltages. The control of selectivity, however, is still a problem, since other two-electron reductions, most importantly reduction to form CO and H2, may also occur.101127 The reduction of C02 to CO, in fact, is the subject of numerous studies. Electrochemical and electrocatalytic reductions of C02 in aqueous solutions have been studied and reviewed.11,128-130... 

The presence of an acid (CF3COOH) is also reported to be essential for the two-electron reduction of C60 by AcrH2 via photoinduced electron transfer from AcrH2 to the triplet excited state of C60 in benzonitrile (PhCN) to yield 1,2-C60H2 selectively [101] ... 

Scheme 5.17).40 A three-step reduction-deprotection protocol liberated the aratz -vicinal diamine 21. A six-membered cyclic transition-state structure was proposed to account for the anti selectivity after a two-electron reduction of the nitrone. The chiral A-tert-butylsulfinyl group directs the attack of the carban-ion to the Sz-face of the C=N double bond of the imine. 

Grobe and Hegge demonstrated that aluminum powder in fV-methylpyrrolidinone (NMP) can also be used for the reduction of bromotrifluoromethane in the presence ofchlorotrimethyl-silane to produce trimethyl(trifluoromethyl)silane, a method more convenient than the modified Ruppert procedure. The C Br bond of bromochlorodifluoromethane is selectively reduced by aluminum powder in. V-methylpyrrolidinone in the presence of ehlorotrimethylsilane giving (chlorodifluoromethyl)trimethylsilane in 80% isolated yield. The process appears to involve a two-electron reduction of the C Br bond of bromochlorodifluoromethane followed by trapping of the resulting CFjCU" anion by chlorotrimethylsilane. Aluminum powder, corresponding to a net consumption of two electrons per mole of bromochlorodifluoromethane. is needed for the reduction. 

Depending upon the nature of the /3-diketonato ligand, they may exhibit a single two-electron reduction or two separate one-electron reductions. In any case, the electron transfer processes are coupled to more or less fast release of the diketonato ligand, which means that copper(II) is the only stable oxidation state in the system. Table 17 summarizes the formal electrode potentials for the Cu(II) —Cu(I) — Cu(0) reductions of selected copper(II) )3-diketonates. 

A sophisticated use of DPSC has been that of the selection of step potentials for the detection of various intermediates. One such example was provided by Amatore et al, showing that the formation Cr2(CO)io on a short timescale upon reduction of Cr(CO)6 was by an ErCiC2 mechanism involving the dimerization of two Cr(CO)5 molecules [7]. This mechanism differs from the one observed on a longer timescale, where Cr(CO)5 , formed in a two-electron reduction of Cr(CO)6, can react with Cr(CO)6 to yield Cr2(CO)io and CO. 

Table 7. Two-electron reduction potential for selected bi- and trinuclear metal-carbonyl complexes (V relative to the SCE obtained from reactivity studies).

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