Monoelectron transfer

This cycle involves, first, a monoelectronic transfer from the nickel (0) complex to the aryl halide affording a Ni(I) complex and then an oxidative addition affording a 16 electron-nickel (II) which undergoes a nucleophilic substitution of Nu-, then a monoelectronic transfer occurs once again with a second aryl halide, and, last, a reductive elimination of the arylated nucleophile regenerates the active Ni(I) species. 

In this case the decisive redox process, which would be specifically inhibited by the hydroquinone, would be the formation of the copper(I) complex corresponding to a monoelectronic transfer from copper(O) to the aryl halide. 

It is important to note that the substitution of trifluoromethyl and perfluoroalkyl halides goes through a specific process. The displacement of the halogen atom never occurs via the usual Sn 1 or Sn2 processes rather it occurs either via a halophilic attack and a monoelectronic transfer (SnrI) or via an a-elimination of a fluoride ion and... 

The anodic dissolution of zinc [233, 264] was investigated in solution H3BO3 + NH4CI + Na2S04 at pH 4.4 using electrochemical impedance spectroscopy and EQCM. In the same solution, zinc anodic dissolution of different galvanized steel sheets of zinc [265] was studied. The postulated [41, 266] mechanism of Zn oxidation in acidic solution corresponds to two consecutive monoelectronic transfers. 

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

Phase transfer was used to catalyze nucleophilic aromatic substitutions by Makosza et al. in 1974.201 202 Zoltewicz203 has given a good early review of various methods and has compared other techniques that make use of polar solvents, transition metals, and monoelectron transfers. 

The formation of a metal anion from metal halide and lithium can be rationalized by a double and successive monoelectronic transfer, with (or without when weak M14—M14 bond is involved) formation of a transient digermane (Scheme 8). 

The formation of exciplex is used to explain the reaction behavior of simple alkene. However, evidence of monoelectron-transfer processes are reported for electron-rich alkenes [19]. 

As we said before, in our opinion H is able to react by way of monoelectronic transfers and very often apparent simultaneous transfers of two electrons must rather be very fast consecutive mono-electronic transfers. 

The mechanism of the amines or alcohols arylation catalyzed by nickel(II) complexes has not been elucidated until now (refs. 7, 17), even though the arylation of nucleophiles catalyzed by nickel(0) complexes is better understood. In this last case it is generally admitted that the reaction proceeds by an oxidative addition step, followed by a nucleophilic substitution, and then a reductive elimination of the arylation product (Scheme 4). According to the work of Kochi (ref. 18), the oxidative addition of the haloarene on a nickel(O) complex takes place through a monoelectronic transfer from the metal to the aryl halide with simultaneous formation of a nickel(I) intermediate, the actual catalyst of the reaction (ref. 6). 

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

The transient character of unstable species is intrinsically because of at least one fast reaction which they undergo as soon as they are formed (for example coalescence reaction in the case of atoms and clusters). This reaction therefore induces competition with any redox reaction which could be regarded as determining the redox potential of a transient entity. In particular, the competition does not enable the establishment of a reversible equilibrium of electron transfer with another suitable system. Thus, the redox potential of short-hved species must be evaluated from kinetic methods - the pulse technique enables us to observe whether or not electron transfer involving the transient species and a series of donor/acceptor couples, used as monitors, is elfective, and thus to establish by a bracketing method the value of the imknown redox potential. Only elementary monoelectronic transfers are considered. Thus, note that one of the forms of the reference couple, reduced or oxidized, can also be a transient radical. 

Other studies (Zhuang et al., 1999 Milardovic et al., 2006 Rodriguez Cid De Leon et al., 2011 Ahmed et al., 2012) have also reported the electrochemical behavior of this radical by cyclic voltammetry, showing that the redox process of DPPH is a monoelectron transfer and is fast, reversible, and controlled by diffusion. 

Because of the greater potential dependence of the direct charge transfer than of the monoelectron transfer, the first one must always predominate at high overvoltage even if at the equilibrium potential the monoelectron transfer is energetically favored, so that it cannot be considered as barred. Moreover, if the intermediates occurring in multistep mechanisms are relatively unstable, direct charge transfer may become the fastest of the parallel paths even at equilibrium. 

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