Successive electron transfers

The proposed mechanism for the conversion of the furanone 118 to the spiro-cyclic lactones 119 and 120 involves electron transfer to the a -unsaturated methyl ester electrophore to generate an anion radical 118 which cyclizes on the /3-carbon of the furanone. The resulting radical anion 121 acquires a proton, giving rise to the neutral radical 122, which undergoes successive electron transfer and protonation to afford the lactones 119 and 120 (Scheme 38) (91T383). 

The cleavage mechanism can be clarified by cyclic voltammetries as shown in Figure 5. In aprotic solution (curves a) steps (l) and (2) correspond to the successive electron transfers leading finally to the dianion. On the other hand, in protic solution (curve c), step (2) has disappeared while step (l) has grown and then obviously corresponds to an ECE process. Anyhow, and whatever the medium, step (3) is identified as that in which the produced olefin (here 1,1-diphenylethylene) is reduced in all cases. 

Thus a single two-electron wave is observed and only one product, the alcohol, can be isolated. Finally, at high pH neither the ketone nor the radical anion are protonated by this basic medium and it is not until the dianion, formed by successive electron transfers, that protonation occurs. 

Dissolving-Metal Reduction of Aromatic Compounds and Alkynes. Dissolving-metal systems constitute the most general method for partial reduction of aromatic rings. The reaction is called the Birch reduction,214 and the usual reducing medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve as a proton source. The reaction occurs by two successive electron transfer/proto-nation steps. 

Reduction of acetylenes can be done with sodium in ammonia,220 lithium in low molecular weight amines,221 or sodium in HMPA containing /-butanol as a proton source,222 all of which lead to the A-alkene. The reaction is assumed to involve successive electron transfer and protonation steps. 

The first question is whether the redox systems can be subjected to successive electron-transfer reactions in extended redox sequences. What one needs to know thereby are the number of charges that can be transferred and what is the Coulombic repulsion arising between the charged subunits. The experimental methods that have to be applied are obvious. Cyclic... 

The conclusion from the above examples is that under appropriate experimental conditions these systems can be subjected to successive electron-transfer reactions forming highly charged derivatives with intact molecular frameworks. 

The three terms on the left-hand side of equation (2.10) correspond to each of the three rate-limiting factors successively electron transfer, followup reaction, and diffusion (the parameters Af and 2 measure the competition between each of the first two factors with the third). 

At the end of this first stage of the chemical catalysis process, the metalloporphyrin is left under the form of a metal(III) bromide. The reaction that closes the catalytic loop is thus the reduction of this species into the metal(I) complex by means of two successive electron transfers from the electrode. This is a fast process since the electrode potential is adjusted so as to reduce rapidly the metal(II) complex. 

The two successive electron transfer reactions are assumed to obey the Butler-Volmer law with the values of standard potentials, transfer coefficient, and standard rate constants indicated in Scheme 6.1. It is also assumed, matching the examples dealt with in Sections 2.5.2 and 2.6.1, that the reduction product, D, of the intermediate C, is converted rapidly into other products at such a rate that the reduction of B is irreversible. With the same dimensionless variables and parameters as in Section 6.2.4, the following system of partial derivative equations, and initial and boundary conditions, is obtained ... 

The catalytic preparation of esters and amides under mild and waste free reaction conditions using readily available starting materials is a desirable goal. The first redox process of this type using heterocyclic carbenes was reported by Castells and co-workers in 1977 in which aldehydes were oxidized to esters in one-pot in the presence of nitrobenzene [104], Furfural 169 is converted into methyl 2-furoate 170 in 79% yield Eq. 15. Nitrobenzene is the presumed stoichiometric oxidant for the oxidation of the nucleophilic alkene XXX to the acyl azolium XXXI by successive electron transfer events. The authors observe nitrosobenzene as a stoichiometric byproduct. This type of reactivity is also observed when cyanide is used as the catalyst. Miyashita has expanded the scope of this transformation using imida-zolylidene carbenes [105-107]. 

In this case, the formation of a surface oxide (Oads) occurs electrochemically with two successive electron transfers. Therefore, if step (7.28) is rate determining, the mechanism is EE with a predicted Tafel slope of 40 mV at low OHads coverage. 

The reach of cyclic voltammetry is vast. It has been applied to the investigation of simple electron-transfer reactions those with two successive electron transfers (so-called EE reactions) and with multiple electron transfers (EEE) involving electron transfer to and from compounds, say, with several benzene rings. The technique has been applied to complex sequences in which an electron transfer is followed by a chemical reaction step, and then by another electron transfer (ECE reactions), etc. The complexity of some of the reaction sequences investigated by cyclic voltammetry lends itself well to calculations that need computers the classic work of Feldburg in this direction (digital simulation) has been already mentioned (Section 7.5.19.2). 

A rather important role appears to be played by electron tunneling reactions in photosynthesis. As has been noted earlier, a number of successive electron transfer reactions at the primary stage of photosynthesis provide a photocatalytic separation of charges... 

That MV2+ is doubly positively charged is also of consequence, for whilst this results in a net repulsion from positively charged sensitizers, it does mean that in the event of successful electron transfer, both products are positively charged and are thus repelled from each other, leading to net product formation. 

The mechanism of polyamine hydrogenation (Fig. 6.8) is believed to involve successive electron transfer (from polyamine to fullerene) - proton transfer (from polyamine radical cation to fullerene radical anion) steps (Briggs et al. 2005 Kintigh et al. 2007). At or near room temperature, aliphatic amines and polyamines are known to hydroaminate [60]fullerene (Miller 2006), likely also involving preliminary electron transfer - proton transfer steps followed by free radical coupling of C and N based radicals (Fig. 6.8). At elevated temperatures in polyamine solution, however, this latter free radical coupling step becomes uncompetitive with... 

In references [10, 12], the particular case was studied of the successive electron transfers of a molecule with n identical and non-interacting centers by following an... 

Interestingly, some of these processes are mimicked by reactions with nucleophiles and it is clear that one-electron transfer from the nucleophile is involved. Indeed, a remarkable process [32], Scheme 10,begins with perfluorodecalin (26) and must proceed via intermediate polyfluorocycloalkene derivatives, e.g. (27), in which successive electron transfers occur, and the final product is a naphthalene derivative (28). So far, this is the only case in which a saturated per-fluorocarbon has been reported to react in this way, to give meaningful products. 

An interesting example, and the one where these ideas were first applied,40 is the Birch reduction of aromatic compounds by sodium in liquid ammonia containing alcohol. These reactions seem to take place by two successive electron transfers, each followed by capture of a proton, i.e. 

Inspecting the emission features of the conjugates, we see the already known characteristics of a successful electron-transfer reaction. Regardless of the... 

The so-called acyloin condensation consists of the reduction of esters—and the reduction of diesters in particular—with sodium in xylene. The reaction mechanism of this condensation is shown in rows 2-4 of Figure 14.51. Only the first of these intermediates, radical anion C, occurs as an intermediate in the Bouveault-Blanc reduction as well. In xylene, of course, the radical anion C cannot be protonated. As a consequence, it persists until the second ester also has taken up an electron while forming the bis(radical anion) F. The two radical centers of F combine in the next step to give the sodium glycolate G. Compound G, the dianion of a bis(hemiacetal), is converted into the 1,2-diketone J by elimination of two equivalents of sodium alkoxide. This diketone is converted by two successive electron transfer reactions into the enediolate I, which is stable in xylene until it is converted into the enediol H during acidic aqueous workup. This enediol tautomerizes subsequently to furnish the a-hydroxyketone—or... 

Fig. 20 Pictorial illustration of the hypothetical mechanism of the action of oxalate on the reduction of [188Re04]. Oxalate ions react first with the teraoxo anion forming an intermediate Re(VII) complex and causing the concomitant expansion of the coordination sphere of the metal from tetrahedral to octahedral. Successively, electron transfer takes place from Sn2+ ions to the octahedral metal center...

The reason for the E selectivity lies in the mechanism of the elimination. The first step is believed to be two successive electron transfers from the reducing agent (sodium metal) to the sulfone. Firstly, a radical anion is formed, with one extra unpaired electron, and then a dianion, with two extra electrons and therefore a double negative charge. The dianion fragments to a transient carbanion that expels acetate or benzoate to give the double bond. 

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