Stepwise and concerted

AV > 1C I A, two-dimensional behavior, which corresponds to an intermediate case between the stepwise and concerted regimes, persists up to zero temperature. 

The demonstration that formation of the nucleophile adduct R-Nu results in the same proportional decrease in the yields of the alkene and solvent adducts, so that the ratio of the yields of these reaction products is independent of [Nu-]. If the solvolysis and elimination reactions proceed by competing stepwise and concerted pathways, respectively, then the yield of R-OSolv will decrease with increasing trapping of the carbocation intermediate by added nucleophile, while the yield of alkene from elimination will remain constant, so that the ratio [R-OSolv]/[Alkene] will decrease as [Nu ] is increased. 

Only low yields of the azide ion adduct are obtained from the reaction of simple tertiary derivatives in the presence of azide ion 2145 46 and it is not possible to rigorously determine the kinetic order of the reaction of azide ion, owing to uncertainties in the magnitude of specific salt effects on the rate constants for the solvolysis and elimination reactions. Therefore, these experiments do not distinguish between stepwise and concerted mechanisms for substitution reactions at tertiary carbon. 

Implications in the dichotomy between stepwise and concerted mechanisms 164... 

It follows that a proper description of the stepwise and concerted reaction pathways requires a three-dimensional representation as illustrated by Figs 8a and 8b. 

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. 

IMPLICATIONS IN THE DICHOTOMY BETWEEN STEPWISE AND CONCERTED MECHANISMS... 

Fig. 14 Potential energy profile for stepwise and concerted mechanisms with (solid lines) and without (dotted lines) an attractive interaction between the caged fragments in the product cluster. The case of the reduction of a neutral substrate is represented. It can be transposed for reductions of a positively charged substrate or for oxidations of neutral or negatively charged substrates.

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. 

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. 

Fig. 10.11 The stepwise and concerted mechanisms for the Diels-Alder reaction between butadiene and ethylene. The reactants (lower left) proceed to the product, cyclohexene (lower right) either through a two step, two transition state mechanism involving the formation of a diradical intermediate (top center), or more directly through the symmetric synchronous transition state (bottom center) (Storer, J. W., Raimondi, L., and Houk, K. N., J. Am. Chem. Soc. 116, 9675 (1994))...

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.)...

Another question of interest is that of the transition between stepwise and concerted pathways. A schematic representation of the problem is given in Fig. 6 this is schematic in the sense that the system is represented as depending on a single reaction coordinate, viz., the distance between the atoms forming the bond to be broken during the reaction. This is obviously the main reaction coordinate as far as the concerted pathway (48) and the decomposition of RX- (47) are concerned. It is also an important reaction... 

In Other classes of organic halides, for example perfluoroalkyl and vinyl halides, the distinction between stepwise and concerted electron-transfer-bond-breaking upon reduction by outer sphere heterogeneous and/or homogeneous electron donors is less unambiguous than in the case of aryl and alkyl halides. As discussed in Section 3, they also present the interest of being active substrates in Sg l reactions. 

The reductive cleavage of iodobenzene and 3-methyliodobenzene was studied by cyclic voltammetry in both DMF and acetonitrile at 21 and 56 °C at different scan rates and has shown that there is a transition between stepwise and concerted mechanisms at lower scan rates. 1-Iodonaphthalene undergoes a stepwise reductive cleavage with mixed kinetic control by electron transfer and follow-up bond breaking, whatever the scan rate. ... 

Nucleophilic Substitution at Benzyl Derivatives. The sharp break from a stepwise to a concerted mechanism that is observed for nucleophilic substitution of azide ion at X-l-Y (Figs. 2.2 and 2.5) is blurred for nucleophilic substitution at the primary 4-methoxybenzyl derivatives (4-MeO,H)-3-Y. For example, the secondary substrate (4-MeO)-l-Cl reacts exclusively by a stepwise mechanism through the liberated carbocation intermediate (4-MeO)-T, which shows a moderately large selectivity toward azide ion ( az/ s = 100 in 50 50 (v/v) water/ trifluoroethanol). The removal of an a-Me group from (4-MeO)-l-Cl to give (4-MeO,H)-3-Cl increases the barrier to ionization of the substrate in the stepwise reaction relative to that for the concerted bimolecular substitution of azide ion. The result is that both of these mechanisms are observed concurrently for nucleophilic substitution of azide ion at (4-MeO,H)-3-Cl in water/acetone solvents. These concurrent stepwise and concerted nucleophilic substitution reactions of azide ion with (4-MeO,H)-3-Cl show that there is no sharp borderline between mechanisms for substitution at primary benzylic carbon, but instead a region of overlap where both mechanisms are observed. 

Generally, only a single stepwise or concerted pathway for aliphatic nucleophihc substitution is detected by experiment because of the very different activation barriers for formation of the respective reaction transition states for these reactions. The description of the borderline between stepwise and concerted nucleophilic substitution reactions presented in this chapter has been obtained through a search for those rare substrates that show comparable barriers to these two reactions and through the characterization of the barrier for nucleophile addition to the putative carbocation intermediate of the stepwise reaction in the region of this change in mechanism. 

The description of the borderline between stepwise and concerted nucleophilic substitution remains murky in cases where there is no significant stabilization of the transition state for the concerted reaction through the coupling of bond cleavage and formation. The reason is that there are no simple experimental protocols to detect the point at which the energy well for the carbocation intermediate of the stepwise reaction in the upper right hand corner of Figure 2.3 is transformed into... 

T. L. Amyes and J. P. Richard, Concurrent Stepwise and Concerted Substitution Reactions of 4-Methoxybenzyl Derivatives and the Lifetime of the 4-Methoxybenzyl Carbocation, J. 

The utilization of electrochemistry for electron transfer in organic reactions dates back to Kolbe (equation 2), and continues to be a subject of great The respective roles of stepwise and concerted dissociative... 

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... 

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