Competion ratios Concerted” mechanism

The observation that the transition state volumes in many Diels-Alder reactions are product-like, has been regarded as an indication of a concerted mechanism. In order to test this hypothesis and to gain further insight into the often more complex mechanism of Diels-Alder reactions, the effect of pressure on competing [4 + 2] and [2 + 2] or [4 + 4] cycloadditions has been investigated. In competitive reactions the difference between the activation volumes, and hence the transition state volumes, is derived directly from the pressure dependence of the product ratio, [4 + 2]/[2 + 2]p = [4 + 2]/[2 + 2]p=i exp —< AF (p — 1)/RT. All [2 + 2] or [4 + 4] cycloadditions listed in Tables 3 and 4 doubtlessly occur in two steps via diradical intermediates and can therefore be used as internal standards of activation volumes expected for stepwise processes. Thus, a relatively simple measurement of the pressure dependence of the product ratio can give important information about the mechanism of Diels-Alder reactions. 

Evidence in favor of the stepwise mechanism is provided by the products from the irradiation of 6-hepten-2-one, Eq. (61) (Yang et al., 1963). The products (VII) and (VIII) are produced in 4 to 1 ratio. Although this result establishes the occurrence of the stepwise mechanism, it does not exclude the concerted mechanism as a competing route. A similar conclusion was reached from a study in which optically active (Orban e< al., 1963) starting materials were irradiated [Eqs. (62) and (63)]. 

In marked contrast to that of cyclobutanes, the cycloreversion of cyclobutanones to ethene and ketene88 is most probably a concerted process.89-94 For example, the fact that the pyrolysis of 2-propylcyclobutanone (13) at 350 °C gives ethene and pent-l-ene in the ratio of 3.8 1 is not easily explained by a diradical mechanism.90 This transformation presumably involves two competing cycloreversion reactions. As expected for a concerted process, the conversion through the less sterically congested transition structure is favored. For this reason, ethene is generated as the major alkene.90... 

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