Stepwise mechanism criterion

When a proposed intermediate is so unstable that it cannot exist, i.e. it would have a lifetime less than that of a bond vibration ( 10 13 s), the reaction must proceed by a concerted mechanism (see reference [11], p. 5). The proton-transfer steps and other covalent bondforming and bond-breaking processes are concerted but with varying degrees of coupling between their motions. However, it is still not clear whether a concerted mechanism can occur when the intermediates, which would be formed in an alternative stepwise mechanism, have significant lifetimes. This is an important question for reactions catalysed by enzymes because the nature of the intermediate itself will control whether the enzyme- and non-enzyme-catalysed mechanisms are forced to be similar if the sole criterion for a concerted mechanism is the stability of the intermediate. 

Where the value of Prr is larger than the product rp, r,p, the concerted mechanism can be expected to be preferred, whereas the greater value of the product allows one to expect rather the preference of the stepwise mechanism. As an example of the practical use of the above criterion let us discuss again the electrocyclic transformation of butadiene to cyclobutene. The individual alternative reaction mechanisms are described, by the scheme given in previous chapter. The evaluation of the relative ease of individual reaction paths requires to calculate first the values... 

A new theoretical approach to thermal cycloadditions has appeared, and the [2 + 2] reaction is one which is discussed in detail. It is proposed that the distinction between stereospedfic and non-stereospedlic is not a good criterion for formulating a [2 + 2] cycloaddition as concerted or otherwise. The net result of competing effects (e.g. steric and electronic) may cause differently stereospecific, concerted reactions to compete effectively. The apparent non-stereospeciflcity could therefore be a result of concurrent, different, stereospecific cycloadditions rather than of a single non-stereospecific, stepwise mechanism. 

Inevitably there is some overlap between the present chapter and that on Cycloaddition reactions of enamines because many such reactions result in the formation of heterocycles. The criterion for inclusion in this chapter is isolation of, or evidence for, discrete acyclic intermediates. Often the mechanism of the reaction is not known the mere suspicion that the cycloaddition proceeds in a stepwise manner was thought sufficient justification for mentioning it here. 

The deuterium isotope effect on the rate constant of the double-proton transfer in the solid state can be used as a criterion to judge whether the double-proton transfer proceeds in a simultaneous or in a stepwise manner. The difference between ratios of rate constants /chh/ hd and /chd/ dd depends on the mechanism of the double-proton transfer. If a double-proton transfer occurs in a stepwise manner, the slower step in the /chd process, that is, a deuteron transfer, becomes rate-determining. Consequently the ratio/chh/ hd should be large. On the other hand, since deuteron transfers are ratedetermining in both /chd and /cdd processes, the ratio /chd/ dd is expected to be small. By contrast, if a proton transfer and a deuteron transfer occur simultaneously, the ratios of /chh/ dd and /chd/ dd should be of the same order. The observed ratios for benzoic acid dimers in crystals were found to be /chh/ hd = 23, / hd/ dd = 10 at 15 K (Stbckli et ai, 1990). Accordingly, the double-proton transfer is considered to occur concertedly in benzoic acid dimers [16]. 

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]. 

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