Protonation reduction step

The second experimental system explored the reduction mechanism of another azo-dye, known as Sudan III (I-(4-pheuyIazophenyIazo)-2-naphthoI) [91]. Sudan III contains two azo groups rendering two successive two-electron, two-proton reduction steps at the mercury surface. Figure 2.68 shows a typical SW voltammetric response of Sudan III recorded in a borate buffer at pH 10.00. The first reduction step is chemically reversible, while the second one is irreversible. More importantly, the second reduction step proceeds at potential about 230 mV more negative than the first one, thus causing a well-separated voltammetric peak. The overall mechanism... 

Fig. 2. The first, most important protonation reduction step.

Trans stereochemistry of the alkene product is established during the second reduction step when the less hindered trans vinylic anion is formed from the vinylic radical. Vinylic radicals undergo rapid cis-trans equilibration, but vinylic anions equilibrate much less rapidly. Thus, the more stable trans vinylic anion is formed rather than the less stable cis anion and is then protonated without equilibration. 

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. 

The mechanism of the electroreductive cyclization reaction has been studied in some detail [22], The initial thought was that it occurred via the cyclization of the radical anion derived, for example, from 25 in the first reduction step. A moment s reflection, however, reveals that there are many more mechanistically viable pathways, especially when one realizes that the transformation involves five steps - two electron transfers (symbolized below by e and d , the latter corresponding to a homogeneous process), two protonations ( p ), and cyclization ( c ). In principle, these could occur in any order, and with any one of the steps being rate-determining. 

It is quite difficult to reduce benzene or pyridine, because these are aromatic stmctures. However, partial reduction of the pyridine ring is possible by using complex metal hydrides on pyridinium salts. Hydride transfer from lithium aluminium hydride gives the 1,2-dihydro derivative, as predictable from the above comments. Sodium borohydride under aqueous conditions achieves a double reduction, giving the 1,2,5,6-tetrahydro derivative, because protonation through the unsaturated system is possible. The final reduction step requires catalytic hydrogenation (see Section 9.4.3). The reduction of pyridinium salts is of considerable biological importance (see Box 11.2). 

Reduction of phenyl vinyl sulphones in dimethylformamide containing phenol as proton donor causes loss of phenylsulphinate ion. The reaction probably involves a series of electron and proton addition steps [74]. In absence of a proton source, phenyl vinyl sulphone radical-anion undergoes a dimerization reaction discussed on p. 57. Reactions of alkyl substituted vinyl sulphones are complicated by alkene migration in the presence of electrogenerated bases. Dimers are formed and further reduction leads to loss of phenylsulphinate ion [81] (Scheme 5.3). 

Reduction of alkynes with sodium in ammonia,147 lithium in low-molecular-weight amines,148 or sodium in hexamethylphosphoric triamide containing /-butanol as a proton source149 leads to the corresponding is-alkene. The reaction is assumed to involve successive electron-transfer and proton-transfer steps. 

Exchange of the existing 5- and 6-protons is faster than the reduction step, so the deuteration of uracil actually produces the tetradeutero derivative 376 <2001JLR7>. Thymine behaves similarly, producing a trideutero derivative <2001JLR7>. Analogous results were seen with the tritiation of uracil, where the tetratritio derivative was obtained <2002MI295>. 

CH3CN, dimethylsulfoxide, dimethylfor-mamide (DMF) and pyridine, of course, is reversible at the timescale of cyclic voltammetry the first unambiguous studies appeared in 1965, the radical being identified by electron spin resonance (ES R) [34, 35]. The reversibility has been demonstrated by cyclic voltammetry in pyridine even in a basic medium, the second reduction step occurring at a much more negative potential is irreversible [36]. In the presence of proton donors, and, of course, in protic solvents, it is known that O is unstable and that the reduction of O2 proceeds via a two-electron step [10, 27, 37]. The superoxide ion is moderately basic... 

In the water photolysis system, the potential of Q should be lower than —0.41 V and that of C2 higher than 0.82 V. For the proton reduction, two electron process (Eq. (4), E0 = —0.41 V) is much more favorable than the stepwise reaction in which the first step (Eq. (5))... 

Ellipsoidal cryptands can also be synthesized by direct alkylation procedures <77AG(E)720,80CB1487), obviating the need for a diborane or lithium aluminum hydride reduction step. In the case of [l.l.l]cryptand (15a) yields of the final amine alkylation step are enhanced by the amine proton itself acting as a template (81CC777). 

The formation of [Fen(CN)5H20]3- as a product sets the basis for the catalytic processing of nitrite reduction by hydrazine under appropriate pH conditions. As shown in Fig. 13, nitrite binds to the aqua ion and rapidly converts to NO+. After the attack by N2H4, the adduct reorganization, associated with proton migration steps, favor... 

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