Stepwise vs. Concerted Mechanisms

How the two mechanisms can be distinguished from each other experimentally is the issue addressed in the following section. The next two sections are devoted to deciphering the factors that drive the reaction toward one or the other mechanism. A first series of factors are related to the molecular characteristics of the starting material. Once these 

FIGURE 3.9. Potential energy profiles for the concerted and stepwise mechanisms for a thermal reductive process. E is the electrode potential for an electrochemical reaction and the standard potential of the electron donor for an homogeneous reaction. For an oxidative process, change all the — into + and donor into acceptor. 

ELECTRON TRANSFER, BOND BREAKING, AND BOND FORMATION 

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As an analogous example, the behavior of sulfonium salts can be mentioned. At a mercury electrode, both trialkyl (Colichman Love 1953) and triaryl (Matsuo 1958) sulfonium salts can be reduced, with the formation of a sulfur-centered radical. This radical is adsorbed at the mercury surface and then eliminates the carboradical. The latter, just after its abstraction, captures one more electron and transforms into carboanion. This is the final stage of reduction. The mercury surface cooperates in both successive one-electron steps (Luettringhaus Machatzke 1964) (Scheme 2-20). This scheme is important for the problem of hidden adsorption, but it cannot to be generalized in the sense of a stepwise vs. concerted mechanism of dissociative electron transfer. As shown, the reduction of some... 

Scheme 6.2). In the previous mechanism, B was formed in concert with electron transfer, whereas the present mechanism follows the stepwise pathway. (The stepwise vs. concerted dichotomy is discussed in detail in Chapter 3 for the cased where the B —> B step is a bond-breaking reaction.) Although equation (6.51) still applies, equation (6.58) is replaced by... 

The theoretical principles of cycloaddition reactions are well understood and there have been many computational studies (see Pericyclic Reactions The Diels-Alder Reaction). Often the hetero-cycloaddition reaction shows similar characteristics to the carbocyclic analog, but a number of special features have been noted. In heterocyclic chemistry the cycloaddition reactions are often dipolar computational studies show that a concerted mechanism is followed in the gas phase. However, a number of studies have noted that these dipolar cycloaddition reactions become stepwise when solvent effects are included (via the reaction field method), with a consequent loss of stereospecificity." Other characteristics of hetero-cycloaddition reactions which have been studied include the endo/exo selectivity" and the regiose-lectivity (for example, [2-1-2] vs. [2-1-4])." High levels of electron correlation are generally required in order to establish these selectivities. 

These aspects introduce different mechanistic patterns expected for the 1,3-DC reactions, as compared with DA cycloadditions (concerted vs stepwise with some zwitterionic character). This result may again be traced to the electrophilicity difference at the ground states of the reacting pairs.39 These results suggest that the description of the reactivity and the reaction mechanism involved in the 1,3-DC processes can be systematized as in the case of the DA cycloadditions. Such a model should be able to determine the charge transfer pattern and to decide which of the partners is acting as nucleophile/electrophile in a polar process, or even to anticipate a concerted pathway in those cases where the electrophilicity/nucleophilicity difference is small. 

Concerted vs. stepwise transfers. We now address the question of whether the hydride and proton transfers are concerted, or whether the hydride transfer precedes the proton in the pyruvate to lactate reaction direction. Our TPS study showed that both mechanisms are possible. In Fig. 25 we show the distribution of the time lag between the hydride and proton transfer for all reactive trajectories. We note that both concerted and sequential transfers are possible, and that 74% of the trajectories have a time lag greater than 10 fs, indicating that the majority of reactive trajectories have sequential transfer steps. 

Fig. 4 Concerted (top) vs stepwise (bottom) mechanism in the olefin epoxidation with Mo diperoxo complexes. R=NMe2 or alkyl...

Bauld has carried out a large number of mechanistic studies of radical cation mediated Diels—Alder and cyclobutanation reactions, as discussed in detail in two recent reviews. Much of the above discussion concerning the concerted vs. stepwise nature of the dimerization reactions also applies to the addition of an alkene radical cation to a different alkene. Although the addition of alkene radical cations to dienes can lead to both cyclobutane and Diels-Alder products, the latter usually predominate for dienes with at least modest s-cis conformer populations. It is clear that in some cases the Diels-Alder adducts arise via rearrangement of initial divinylcyclobutane products. However, cyclobutanation and Diels-Alder adduct formation have been demonstrated to occur by independent pathways in other systems. There is also considerable experimental and theoretical data in support of a concerted but nonsynchronous mechanism for these reactions. 

Several explanations are proposed to explain the switch in stereoselectivity from exo-61a to exo-61h using different Lewis acids, such as different orientation of the vinyl ether (s-cis vs. s-trans), or change in the mechanism (concerted vs. stepwise) [93d]. Apparently, bulky aluminum Lewis acids, such as ATPh induce nitroalkenes to react with chiral dienophiles via the s-trans vinyl ether conformation (Figure 16.3), whereas SnCU induces nitroalkenes to react... 

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