Stepwise electron transfer

Kinetics of the reaction of p-nitrochlorobenzene with the sodium enolate of ethyl cyanoacetate are consistent with this mechanism. Also, radical scavengers have no effect on the reaction, contrary to what would be expected for a chain mechanism in which aryl radicals would need to encounter the enolate in a propagation step. The reactant, /i-nitrophenyl chloride, however, is one which might also react by the addition-elimination mechanism, and the postulated mechanism is essentially the stepwise electron-transfer version of this mechanism. The issue then becomes the question of whether the postulated radical pair is a distinct intermediate. 

As depicted in Scheme 1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. How the dynamics of concerted electron transfer/bond breaking reactions (heretofore called dissociative electron transfers) may be modeled, and particularly what the contribution is of bond breaking to the activation barrier, is the first question we will discuss (Section 2). In this area, the most numerous studies have concerned thermal heterogeneous (electrochemical) and homogeneous reactions. 

Although the most numerous investigations of dissociative electron transfer have concerned thermal reactions, photoinduced dissociative electron transfer has also attracted a great deal of recent theoretical and experimental attention. As discussed in Section 6, one of the key questions in the field is whether photoinduced dissociative electron transfers are necessarily endowed with a unity quantum yield as one would predict on purely intuitive grounds. Quantum yield expressions for the concerted and stepwise cases are established and experimental examples are discussed. 

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then thermodynamics will favor one or the other mechanism according to equation (18). AG )eav is also the standard free energy of cleavage of the ion radical. 

Note, however, that the concerted/stepwise dichotomy discussed here concerns cases in which the intermediate is unstable toward cleavage that occurs by means of intramolecular dissociative electron transfer. As discussed in the foregoing sections, for primary radicals that cleave homolytically, criteria based on the life-time of the intermediate may be pertinent. 

There is thus an apparent continuity between the kinetics of an electron transfer leading to a stable product and a dissociative electron transfer. The reason for this continuity is the use of a Morse curve to model the stretching of a bond in a stable product in the first case and the use of a Morse curve also to model a weak charge-dipole interaction in the second case. We will come back later to the distinction between stepwise and concerted mechanisms in the framework of this continuity of kinetic behavior. 

How can these photochemical and electrochemical data be reconciled With the benzylic molecules under discussion, electron transfer may involve the n or the cr orbital, giving rise to stepwise and concerted mechanisms, respectively. This is a typical case where the mechanism is a function of the driving force of the reaction, as evoked earlier. Since the photochemical reactions are strongly down-hill whereas the electrochemical reaction is slightly up-hill at low scan rate, the mechanism may change from stepwise in the first case to concerted in the second. However, regardless of the validity of this interpretation, it is important to address a more fundamental question, namely, whether it is true, from first principles, that a purely dissociative photoinduced electron transfer is necessarily endowed with a unity quantum yield and, more generally, to establish what are the expressions of the quantum yields for concerted and stepwise reactions. 

Two situations may be distinguished according to whether the electron transfer step of the stepwise mechanism lies in the normal region or in the inverted... 

It is also worth emphasizing that recent theoretical work on photoinduced stepwise and concerted electron transfer/bond-breaking reactions opens the route to a more systematic combination than before of the electrochemical and photochemical approaches to the same problems. 

It is a pleasure to acknowledge the essential contribution of Dr. C. P. Andrieux to most of the work reported above as well as that of Dr. D. Lexa in the field of porphyrins, Professor Moiroux and Dr. A. Anne to cation radical reactivity, Dr. M. Robert to photoinduced dissociative electron transfer and to the stepwise/concerted competition and Drs. P. Hapiot and Medebielle to recent work on thermal SRn1 reactions. Many students from our group have also contributed effectively to the work, namely, C. Costentin, G. Delgado, V. Grass, A. Le Gorande, C. Tardy and D. L. Wang. Fruitful and pleasant... 

Researchers studying the stepwise kinetics of nitrogenase electron transfer using stopped-flow kinetic techniques have presented other scenarios. One hypothesis presents kinetic evidence that dissociation of Fe-protein from MoFe-protein is not necessary for re-reduction of Fe-protein by flavodoxins.13 These authors state that the possibility of ADP-ATP exchange while Fe-protein and MoFe-protein are complexed with each other cannot be excluded and that dissociation of the complex during catalysis may not be obligatory when flavodoxin is the Fe-protein reductant. This leads to the hypothesis that MgATP binds to the preformed Fe-protein/... 

Cu,Zn superoxide dismutase. Essentially, these observations support a stepwise one-electron model again. Interestingly, the oxidation state of copper does not change during the catalytic reaction, i.e. the sole kinetic role of the histidine coordinated metal center is to alter the electronic structures of the substrate and 02 in order to facilitate the electron transfer process between them. 

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... 

Several other systems have been shown to involve substantial structural changes upon electron transfer, possibly giving rise to potential inversion. They can be divided into two categories. In the first, which includes the reduction of fra/r.v-2,3-dinitro-2-butcnc that we just discussed, electron transfer and structural change are concerted. In the second, they occur in a stepwise manner. Examples of such reactions are given in Section 2.5.7, where the question of the concerted versus stepwise character of electron transfer and structural change in the case of electron transfer-triggered isomerization is also addressed. 

The initial steps of the Kolbe reaction, the oldest organic electrochemical reaction, constitute a good illustration of the loss of an acid moiety upon oxidative electron transfer (Scheme 2.24). The issue of the stepwise versus concerted character of the electron transfer/bond-breaking process in this reaction is discussed in Chapter 3. 

As depicted in Scheme 3.1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. RX is a commonly used designation for an alkyl halide. Many experimental studies of dissociative electron transfers have indeed taken as examples the reductive cleavage of alkyl halides. However, many other compounds have been investigated in the framework of reaction Scheme 3.1 in the organic and inorganic field, for reductions as well as for oxidations. 

In the case of stepwise processes, the cleavage of the primary radical intermediate (often an ion radical) may be viewed in a number of cases as an intramolecular dissociative electron transfer. An extension of the dissociative electron transfer theory gives access to the dynamics of the cleavage of a primary radical into a secondary radical and a charged or neutral leaving group. The theory applies to the reverse reaction (i.e., the coupling of a radical with a nucleophile), which is the key step of the vast family of... 

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