Coupling Reactions Involving Electron-transfer Processes

Multicomponent Coupling Reactions Involving Electron-transfer Processes 

Multi-Component Reactions by Radical/Anion Species Hybridization 

An efficient addition reaction of THF to aldimines is possible when the dimethylzinc/air system is employed [58], The reaction can be conducted in an 

The reaction of t-BuBr with CO in the presence of SmCp2 leads to the formation of acylsamarium species presumably via a radical formation/carbonylation/ reduction sequence, When heptanal is added to the reaction mixture, a ketol is 

A related three-component reaction involving dienes employs a cobalt catalyst [67]. Unlike the above case, the third component here is a carbanion species, such 

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Multicomponent Coupling Reactions Involving Electron-transfer Processes 187... 

A systematic description of all possible combinations of homogeneous chemical processes coupled to electron transfer at an electrode surface is impossible because an infinite range of theoretically possible reaction schemes can be constructed. Unfortunately, a consistent form of nomenclature for defining the possible web of reaction pathways has not yet been invented. However, the lUPAC nomenclature [89] is of assistance with respect to simple reaction schemes. In this article, the commonly employed descriptors for electron transfer (E) and chemical (C) sequences of reaction steps, e.g. ECEC, will be used for a sequence of reactions involving electron transfer-chemical process-electron transfer-chemical process. Reaction schemes involving branching of a reaction pathway will be considered later. 

Polyaniline is the conducting polymer most commonly used as an electrocatalyst and immobilizer for biomolecules [258-260]. However, for biosensor applications, a nearly neutral pH environment is required, since most biocatalysts (enzymes) operate only in neutral or slightly acidic or alkaline solutions. Therefore, it has been difficult or impossible to couple enzyme catalyzed electron transfer processes involving solution species with electron transport or electrochemical redox reactions of mostly polyaniline and its derivatives. Polyaniline is conducting and electroactive only in its protonated (proton doped) form i.e., at low pH valnes. At pH values above 3 or 4, polyaniline is insulating and electrochemically inactive. Self-doped polyaniline exhibits redox activity and electronic conductivity over an extended pH range, which greatly expands its applicability toward biosensors [209, 210, 261]. Therefore, the use of self-doped polyaniline and its derivatives could, in principle. 

Three important concurrent reactions involving electron transfer may occur in ATRP (Figure 8.22 E is the redox potential of the couple shown in parentheses) (i) disproportionation of the ATRP (usually Cu -based) activator, (ii) oxidation or reduction of organic radicals to carbocations or carbanions, respectively, and (iii) formation of organometallic species via a reaction between radicals and the (usually lower oxidation state) complexes catalyzing the ATRP process. 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

The latter reaction must involve a large number of molecular steps and may be a much slower process. The mechanisms of a few inorganic electron transfer processes have been summarized by Taube (1968). The presence of very slow reactions when several redox couples are possible means that the Eh value measured with an instrument may not be related in a simple way to the concentrations of species present, and different redox couples may not be in equilibrium with one another. Lindberg and Runnells (1984) have presented data on the extent of disequilibrium... 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

The enantioselective oxidative coupling of 2-naphthol itself was achieved by the aerobic oxidative reaction catalyzed by the photoactivated chiral ruthenium(II)-salen complex 73. 2 it reported that the (/ ,/ )-chloronitrosyl(salen)ruthenium complex [(/ ,/ )-(NO)Ru(II)salen complex] effectively catalyzed the aerobic oxidation of racemic secondary alcohols in a kinetic resolution manner under visible-light irradiation. The reaction mechanism is not fully understood although the electron transfer process should be involved. The solution of 2-naphthol was stirred in air under irradiation by a halogen lamp at 25°C for 24 h to afford BINOL 66 as the sole product. The screening of various chiral diamines and binaphthyl chirality revealed that the binaphthyl unit influences the enantioselection in this coupling reaction. The combination of (/f,f )-cyclohexanediamine and the (R)-binaphthyl unit was found to construct the most matched hgand to obtain the optically active BINOL 66 in 65% ee. 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

The unique electrocatalytic role of benzoic acid-protected silver nanoclusters in the Wolff rearrangement of a-diazo ketones has been disclosed. The presence of a Agn°/Agn+ redox couple facilitates a non-classical electron-transfer process, involving chemical reactions interposed between two electron-transfer steps occurring in opposite directions.32... 

Bimolecular excited state electron transfer reactions have been investigated extensively during the last decade (1-3). Electron transfer is favored thermodynamically when the excitation energy E of an initially excited molecule A exceeds the potential difference of the redox couples involved in the electron transfer process. 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

The photochemical reaction of photosynthesis involves the removal of an electron from an excited state of the special chlorophyll that acts as an excitation trap. The movement of the electron from this trap chi to an acceptor begins a series of electron transfers that can ultimately lead to the reduction of NADP+. The oxidized trap chi, which has lost an electron, can accept another electron from some donor, as in the steps leading to O2 evolution. Coupled to the electron transfer reactions in chloroplasts is the formation of ATP, a process known as photophosphorylation. In this section we will consider some of the components of chloroplasts involved in accepting and donating electrons a discussion of the energetics of such processes will follow in Chapter 6 (Section 6.3). 

The qualitative reaction profile given in Fig. 11 shows how a light-induced single-electron transfer process can be coupled to suitable follow-up steps to facilitate the formation of permanent two-electron photoredox products. Larger bond and shape reorganizations of excited state molecules, which typically involve the population of CT states or the formation of Jahn-Teller distorted species, are very helpful to achieve such... 

The carbonyl-carbon kinetic isotope effect (KIE) and the substituent effects for the reaction of lithium pinacolone enolate (112) with benzaldehyde (equation 31) were analyzed by Yamataka, Mishima and coworkers ° and the results were compared with those for other lithium reagents such as MeLi, PhLi and AllLi. Ab initio (HF/6-31-I-G ) calculations were carried out to estimate the equilibrium isotope effect (EIE) on the addition to benzaldehyde. In general, a carbonyl addition reaction (equation 32) proceeds by way of either a direct one-step polar nucleophilic attack (PL) or a two-step process involving electron transfer (ET) and a radical ion intermediate. The carbonyl-carbon KIE was of primary nature for the PL or the radical coupling (RC) rate-determining ET mechanism, while it was considered to be less important for the ET rate-determining mechanism. The reaction of 112 with benzaldehyde gave a small positive KIE = 1.019),... 

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