Concerted and Stepwise Reactions

A great many reactions in solution can profitably be treated by an extension of the earlier ideas to include more than one progress variable. We will introduce the idea with the general reaction 

In the pure concerted reaction there is no need to invoke the cationic or anionic intermediates in describing the transition state, but it now becomes evident that some deviation from this idealized route may be possible, and then we need a way to comment upon and to measure the extent to which the cationic or anionic character is mixed in in the transition state. This is now widely accomplished with the aid of energy surfaces of the type shown schematically in Fig. 5-19. Depending on the nature of the surface, the reaction path may follow a route far from the diagonal representing the pure concerted reaction, and the primary goal is to identify the location of the transition state on this surface. 

Energy surfaces of this type appear to have been described first by Hughes et al. in 1936. Their current popularity and utility are a consequence of developments by Albery, More O Ferrall, and Jencks, and they are sometimes called More O Ferrall plots or More O Ferrall-Jencks plots. We will refer to them as RIP diagrams, this designation arising from the symbolism in Fig. 5-19. 

To make this more specific. Table 5-3 gives examples of several reaction types that fit the RIP pattern. Consider nucleophilic substitution on saturated carbon. The concerted mechanism is the one-step bimolecular 5, 2 process  

A stepwise mechanism is also possible this is the two-step unimolecular SnI reaction  

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Concerted and stepwise reactions. Transition between the two mechanisms 129... 

How molecular structure controls the mechanism 133 Transition between mechanisms upon changing the driving force 134 Does concerted mechanism mean that the intermediate does not exist 140 Reaction coordinates in concerted and stepwise reactions 140... 

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. 

The terms cycloaddition and cyclization generally refer to concerted and stepwise reactions, respectively. Nevertheless, in this section they will be used indiscriminately since, in most of the processes, the mechanisms are uncertain. Formation of non-cyclic products proceeding through a cydization/cycloreversion pathway is also included in this section. 

A plateau on the PE hypersurface is not the only feature that can serve to muddy the distinction between concerted and stepwise reactions. Another occurs when an intermediate, even one in a quite deep PE well, is located off the direct path from reactant to product. An example comes from the nominal [3,3] sigmatropic rearrangement of 1,2,6-heptatriene. If heated alone, in the gas phase, the reactant rearranges cleanly to the [3,3] product, 3-methylene-1,5-hexadiene. However, Roth et al. showed that an intermediate could be trapped when the reaction was conducted in high-pressure oxygen ( ). They postulated that the intermediate was the... 

Every discussion of the Diels-Alder reaction for 1,3-butadiene includes the observation that cycloaddition should occur only from the s-cis conformer to produce czs-cyclohexene. This conformational selectivity, however, further implies that cycloaddition of s-trans- 1,3-butadiene should lead to trans-cyclohexene, but the s-trans region of this potential surface has remained unexplored. A study of this problem shows that the concerted and stepwise reaction paths exist for both diene conformers, connecting them to the respective cyclohexene isomers.661 It is also demonstrated that the usual paradigm for the Diels-Alder reactions is incomplete a thorough understanding of this archetypal reaction requires consideration of the full range of processes shown in Scheme 6.10, not just those involving the s-cis conformer. 

In reactions of mechanistic borderline, the reaction pathway may not follow the minimum energy path, but the reaction proceeds via unstable species on the PES. In other cases, the reacting system remains on the IRC but does not become trapped in the potential energy minimum. In some cases, intermediates are formed in reactions that should be concerted, whereas in other reactions a concerted TS gives an intermediate. Thus, the question of concerted versus stepwise appears too simple and the definition of concerted and stepwise reactions becomes unclear. In some reactions, the post-TS dynamics do not follow IRCs, and path bifurcation gives two types of products through a common TS. 

Reaction Coordinate Diagrams 209 The Rate-Determining Step 213 Composition of the Transition State 216 Position and Height of the Energy Barrier 220 Concerted and Stepwise Reactions 230... 

More interesting than this first qualitative result is, however, the comparison of the relative ease of concerted and stepwise reaction mechanisms. Here it is possible to see that whereas in the case of allowed conrotatory reaction the concerted... 

As can be seen from the Table, the values of g p do indeed remedy the insufficiency of the first order index r and correctly predict the reaction preferred for the concerted process by the Woodward-Hofi nann rules. Much more interesting than this reproduction of Woodward-Hofl nann rules is, however, the comparison of relative ease of concerted and stepwise reaction mechanisms. Thus, e.g., in the case of 2+2 ethene dimerization where the original criterion failed, the new enhanced criterion based on the second order similarity indices clearly suggests that the preferred mechanism of forbidden s + s dimerization is the stepwise one. This is in a complete agreement with both experimental and theoretical data for this process [125,126]. 

Although we obtained good results in the case above, for systems such as these involving possible biradical transition structures (which is often the case in the comparison of concerted and stepwise reaction mechanisms) then higher levels of electron correlation are sometimes required, and possibly even the use of MCSCF methods. 

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