Stepwise and concerted processes

When a bond is broken upon electron transfer, a first question arises are electron transfer and bond breaking stepwise (46, 47) or concerted (48). 

The outcome of the competition is represented in Fig. 5 in terms of the location of the half-wave potential of the RX reduction wave (i.e. the current-potential curve), relative to the standard potential of the RX/ RX- couple, E° (Andrieux et al., 1978). As concerns the competition, three main regions of interest appear in the diagram. On the left-hand side, the follow-up reaction is so slow (as compared to diffusion) that the overall process is kinetically controlled by the parameter A, i.e. by electron transfer and diffusion. Then, going upward, the kinetic control passes from electron transfer to diffusion. In the upper section d in the lower section 

On the extreme right-hand side of the diagram, the follow-up reaction has become so fast that it prevents the back electron transfer. Kinetic control is then by the forward electron transfer and the half-wave potential is then, once more, given by (53). It becomes more and more positive of the standard potential as the electron-transfer step (46) becomes faster and faster. Situations are thus met in which the overall process is kinetically controlled by an endergonic electron transfer due to the presence of a fast follow-up reaction. For such fast electron transfers, the reaction would have been controlled by diffusion in the absence of the follow-up reaction (upper left-hand part of Fig. 5). 

In between these two extreme situations, the overall process may be kinetically controlled by the follow-up reaction. This will occur if the electron transfer is intrinsically fast. Then the expression for , 2 (54), 

When the follow-up reaction becomes so fast that the thickness of the reaction layer comes close to molecular dimensions, the above analysis breaks down because the diffusion of RX- ceases to obey Pick s law. An extreme situation in this connection is when the reaction is so fast that RX- has no time to diffuse away from the electrode and collapses instead at the surface. The follow-up reaction should then be viewed as a surface reaction and the half-wave potential is given (Saveant, 1980b, 1983) by (55), where 

---

The distinction between stepwise and concerted processes is clear in principle even though establishing whether an overall transformation is one or the other may be problematical [ 5 ]. Once it has been established that two aspects of an overall transformation are concerted, the... 

Modelling of outer sphere electron transfer 5 Application to organic redox systems 15 Modelling of dissociative electron transfer 21 Stepwise and concerted processes 23 Aryl halides 37 Alkyl halides 54 Other examples 63... 

On the other hand, the reactions of esters with amines generate the aminolysis products. A theoretical study " on ester aminolysis reaction mechanisms in aqueous solution shows that the formation of a tetrahedral zwitterionic intermediate (Scheme 9.3) plays a key role in the aminolysis process. The rate-determining step is the formation or breakdown of such an intermediate, depending on the pH of the medium. Stepwise and concerted processes have been studied by using computation methods. Static and dynamic solvent effects have been analyzed by using a dielectric continuum model in the first case and molecular dynamics simulations together with the QM/MM method in the second case. The results show that a zwitterionic structure is always formed in the reaction path although its lifetime appears to be quite dependent on solvent dynamics. 

It follows that the value of the electrochemical transfer coefficient may allow the distinction between stepwise and concerted electron-transfer-bond-breaking reactions when a chemical bond of normal strength is involved (Andrieux and Saveant, 1986b Andrieux et al., 1990b). If the reduction wave possesses the characteristics of a process controlled by slow electron transfer rather than controlled by a follow-up reaction, and if a is significantly larger than 0.5, then one can conclude that the reaction proceeds in a stepwise manner. The same is true when the wave exhibits the characteristics of a process controlled by a follow-up reaction, electron transfer remaining at equilibrium. 

Fig. 6 Stepwise and concerted electron transfer and bond breaking. Schematic representation of the potential energy surface, (a) Stepwise process, a > 0.5. (b) Concerted process, a < 0.5. (Adapted from Andrieux et ai, 1985.)...

The importance of the counter-ion has been recognized. The counter-ion influences the spin multiplicities and, potentially, the relative rates of the stereospecific concerted and stepwise non-concerted processes. The calculations indicate that the active manganese-oxo species in the Jacobsen epoxidation may either be a high-spin quintet or triplet, depending on the ligand. 

The question of stepwise vs. concerted processes in the catalysis of symmetry-forbidden reactions becomes more important when the postulated forbidden-to-allowed process itself is questioned. Certainly, stepwise processes in catalysis are common, and have been known to intervene in catal5dic reactions for some time. So to establish that an overall chemical transformation A B proceeds through distinct steps e.g., A - X - - B) is not, in itself, chemically significant. To suggest, however, that a chemical transformation A - B can proceed smoothly on the coordination sphere of a transition metal, and essentially nowhere else, introduces the possibihty of novel chemistry, not encountered in or outside of catalysis. The mere observation that a symmetry-forbidden reaction A - B proceeds in the presence of a metal, does not, in itself, mean that the symmetry-restricted transformation A B proceeded in a concerted manner on the coordination sphere of that metal. Here, the question of stepwise vs. concerted takes on new significance, for it bears on the possible existence of a special kind of chemical transformation. 

Stereochemical and theoretical aspects of hetero Diels-Alder reactions have been intensively studied and recently reviewed. A clear definition for the endo- and exo-orientation regarding intermolecular all-carbon and hetero Diels-Alder reactions was recently suggested by Tietze in a review covering the literature of the last decade. The reaction mechanisms of several hetero Diels-Alder reactions have been investigated and calculated and concerted processes as well as stepwise reaction pathways have been proposed depending on the compounds employed and the reaction conditions. 

In addition, with the implementation of metals and the use of a variety of enophiles (RCHO, O2, PTAD, RNO, Se02, R COR", benzyne, etc.), there is evidence for many of these reactions to be stepwise and oftentimes very difficult to distinguish concerted reactions from stepwise reactions. The broader definition of a radical, stepwise or concerted process of the Alder-Ene reaction will be utilized here. 

Density functional theory calculations have been used to estimate the mechanistic course of hydration and hydrolysis reactions of a-oxocarboxylic acid derivatives (esters and amides), with solvent effects allowed for. For the uncatalysed reactions, stepwise and concerted routes are comparable in energy, whereas catalysis via water assistance significantly favours the stepwise process. 

Only one exception to the clean production of two monomer molecules from the pyrolysis of dimer has been noted. When a-hydroxydi-Zvxyljlene (9) is subjected to the Gorham process, no polymer is formed, and the 16-carbon aldehyde (10) is the principal product in its stead, isolated in greater than 90% yield. This transformation indicates that, at least in this case, the cleavage of dimer proceeds in stepwise fashion rather than by a concerted process in which both methylene—methylene bonds are broken at the same time. This is consistent with the predictions of Woodward and Hoffmann from orbital symmetry considerations for such [6 + 6] cycloreversion reactions in the ground state (5). 

Concerted mechanisms are possible for acyl, phosphoryl, and sulfonyl transfer. By thinking about the stepwise limits for each possible concerted process, one can, even with quite crude calculations, make some judgment about the feasibility of the concerted process. This area of chemistry is as yet unsettled and will see some changes in what is now generally accepted. 

Despite the study of cycloadditions to alkylidenecyclopropanes being far from fully exploited, the results accumulated in the literature are enough for a comprehensive review which will present the state of the art. The present review will deal with all the processes of cycloadditions, without discriminating between the nature, concerted or stepwise, of the processes. It will cover those reactions which involve exclusively the exocyclic double bond of methylenecye-lopropane and alkylidenecyclopropanes in the formation of three-, four-, five-and six-membered rings. 

---