One-electron reducing agent

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

Another useful reagent for reduction of a-acetoxyketones and similar compounds is samarium diiodide.233 Sml2 is a strong one-electron reducing agent, and it is believed that the reductive elimination occurs after a net two-electron reduction of the carbonyl group. 

Samarium diiodide is another powerful one-electron reducing agent that can effect carbon-carbon bond formation under appropriate conditions.257 Aromatic aldehydes and aliphatic aldehydes and ketones undergo pinacol-type coupling with Sml2 or SmBr2. 

An Alternative Mechanism. Considering the facility of the electron transfer reactions to which a great deal of this symposium has been devoted, we have to worry whether our "proton transfer" reactions may not really be the result of electron transfer in the reverse direction followed by hydrogen transfer. As Bergman (26) has recently reported that another hydride anion may act as a one-electron reducing agent, and as we have evidence implicating 0s(C0) H as an intermediate in a number of... 

Table 2 Common one-electron reducing agents and their (approximate ) reduction potentials ( V vs. SCE)...

The ion Cu" is extremely labile. Rate constants for the formation of maleate or fumarate complexes are =10 M s Ref. 281. It can be prepared in an acid perchlorate solution by reaction of Cu with a one-electron reducing agent such as Cr, or Eu Ref. 282. Although there is a marked tendency for disproportionation, solutions of Cu are metastable for hours in the absence of oxygen, particularly when concentrations of Cu(I) are low and the acidity is high. Espenson has capitalized on this to study the rates of reduction by Cu of some oxidants, particularly those of Co(III), Table 5.7 (see Prob. 6(c) Chap. 5). 

Electron attachment to the LUMO of a neutral organic acceptor produces a radical anion [61]. This process can be initiated either chemically using a one-electron reducing agent [62, 63], electrochemically by cathodic reduction [64, 65] or photochemically in the presence of an electron donor in its excited state [12, 66]. 

The one-electron reduction of diquaternary salts of 2,2 -bipyridine has attracted much attention. When an aqueous solution of diquat dibromide (75) is treated with a one-electron reducing agent, such as zinc dust, the solution acquires an intense green color. This is due to the formation of the stable radical cation 87. The one-electron transfer is completely reversed by air. In theory the radical cation 87 can take up another electron to form the neutral species 88. The stability of the radical cation 87 is due to the ability of the odd electron to be located at any of the nuclear positions because of the near coplanarity of the two pyridine rings. The potential 0 of the first one-electron transfer occurs at — 0.35 V in aqueous solution... 

Very little has been reported regarding 2,4 -bipyridine. As expected, it is quaternized preferentially on the y-pyridyl ring " and with excess methyl iodide the diquaternary salt l,r-dimethyl-2,4 -bipyridinium diiodide is ob-tained. ° The latter salt, like diquat, is reduced to a radical cation by one-electron reducing agents. The potential E of the one-electron transfer in aqueous solution is —0.64 lV-alkyl-2,4 -bipyridinones have been... 

These solutions of electrons are not mere laboratory curiosities. In addition to being strong bases, they are also good one-electron reducing agents. For example, pure samples of alkali metal superoxides may be readily prepared in these solutions ... 

Recently, Sml2 has emerged as a potent one-electron reducing agent for carbonyl and... 

The data of Tables 9.3 and 9.4 indicate that 02 is a moderate one-electron reducing agent [cytochrome-c (Fem) is reduced in H2014,15 and iron(III) porphyrins in dimethylformamide] ... 

Sml2, which can be prepared conveniently from samarium powder and 1,2-diiodoethane in THF, finds application as a versatile one-electron reducing agent in organic synthesis. Two typical synthetic procedures mediated by Sml2 are the pinacol coupling of aldehydes and the Barbier reaction, as shown in the following schemes ... 

So far we have discussed the synthesis of useful P-chiral phosphorus intermediates that can be converted into a variety of phosphines by reaction with a nucleophile (and deprotection). However, Imamoto and co-workers found that one-electron reducing agents react with resolved menthyl phosphine oxides and boranes to give metal phosphides that retain their configuration at phosphorus (Scheme 26).49... 

It is a commonplace to say that there has been explosive growth in the use of lanthanides in organic chemistry. For many years, the use of cerium(iv) compounds as oxidants was widespread, but more recently a whole range of other compounds have made their appearance. Thus samarium(ii) compounds are now routinely used as one-electron reducing agents and the use of trifluoromethanesulfonate ( triflate ) salts of scandium and the lanthanides as water-soluble Lewis acid catalysts is widespread. Beta-diketonate complexes and alkoxides have also come into use there are even applications of mischmetal in organic synthesis. 

Two major mechanisms have to be taken into consideration for the alkylation of Co -corrins. The classical mechanism of a bimolecular nucleophilic substitution reaction at carbon (the Co -corrin acts as a nucleophile) leads to /3-aUcylated Co -corrins with high diastereoselectivity. Secondly, an electron transfer-induced radical process (where the Co -corrin acts as a one-electron reducing agent) may also lead to cobalt alkylation. The observed formation of incomplete a-aUcylated Co -corrins under kinetically controlled conditions has been proposed to occur via this path. The high nucleophilic reactivity of Co -corrins and their diastereoselective nucleophilic reaction on the ( upper ) /3-face are not increased by the nucleotide function on the ( lower ) a-face rather they appear to be an inherent reactivity of the corrin-bound tetracoordinate Co -center. Among the organometallic B12 derivatives prepared to date, neopentylcobalamin, benzylcobalamin, and... 

Cobaltocene is a one-electron reducing agent, which makes the overall reaction a non-complementary electron transfer. Thus, akinetic barrier is expected, althongh the redox potential indicates the reaction is highly favorable. ... 

The (CsMe5)3Ln complexes can also act as a one-electron reducing agent. Two representative examples are shown in Figure 8.4. A reasonable explanation is that (C5Me5)3Ln gives up one electron to form a [(C5Me5)2Ln]+ cation and a CsMes radical, followed by an electron transfer to the reactant and the dimerization of CsMes radicals. 

The reduction potentials ofLn(III)/Ln(II) range from -0.35 V (Eu) to -2.6 V(Nd). Therefore all of the divalent organolanthanide complexes, except for Eu(II), have a strong reducing ability, and can be used as one-electron reducing agents. 

It can be useful to replace an amino group with hydrogen, since this can offer a route to compounds difficult to prepare by other direct methods. The most reliable means of achieving this is by conversion to the diazonium salt and subsequent reaction with phosphinic acid (hypophospho-rous acid, H3PO2), catalysed by copper(I) salts. A free-radical mechanism is proposed, in which copper(I) ion acts as a one-electron reducing agent and initiates a chain reaction (Scheme 8.13). 

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