Intermediate, biradical pericyclic

In 1938, there was still much discussion concerning the mechanism. Two possibilities were recognized, the first a one-step pericyclic process involving a cyclic transition state (138) and the second a two-step process involving an intermediate biradical (140) i.e.. 

Conversely, cases are known where % cycloadditions take place by the ERj mechanism in preference to an alternative allowed Diels-Alder process. A good example is the reaction of butadiene with trifluoroethylene to give the adducts (236)-(239). The fact that (236) and (239) are formed in equal amounts shows that the reaction is not a pericyclic cycloaddition but must take place via the biradical (240). In this the F2C—CFH bond is single, so rotation is possible, the configuration of the original reactants being lost. Even the 13% of normal Diels-Alder product may be, and probably is, formed by cyclization of the same intermediate biradical, either (240) or the precursor (241) of (238). 

The choice between these two possibilities (—G or —Gr) is still uncertain. Note that if the reaction is concerted, then it must take place by the reverse of a face-to-face n cycloaddition. If the reaction took place by the reverse of an allowed cis-rrans n cycloaddition (see p. 350), it could not be chemiluminescent because in an aromatic ( allowed ) pericyclic reaction, the ground-state and excited-state surfaces remain far apart throughout. Similar chemiluminescent reactions are also observed in the case of bicyclic oxetanes, a good example being the chemiluminescent oxidation " of lophine (115) by alkaline hydrogen peroxide. Here again it is uncertain whether the final step is a concerted — G -type process or a — G -type process involving an intermediate biradical. 

In connection with Eq. (22), yet another important factor differentiates our approach from usual quantum chemical analyses of reaction mechanisms. This difference concerns the fact that while a quantum chemical approach is in principle independent of any external information (all participating species appear automatically as various critical points on the PE hypersurface), in our model that is more closely related to classical chemical ideas some auxiliary information about the structure of the participating molecular species is required. This usually represents no problem with the reactants and the products since their structure is normally known, but certain complications may appear in the case of intermediates. This complication is not, however, too serious since in many cases the structure of the intermediate can be reasonably estimated either from some experimental or theoretical data or on the basis of chemical intuition. Thus, for example, in the case of pericyclic reactions that are of primary concern for us here, the intermediates are generally believed to correspond to biradical or biradicaloid species with the eventual contributions of zwitterionic structures in polar cases. 

It is conceivable that the difference in the Doering and Cooke results can be attributed to the isopropyl group in a-thujene which would make the non-allylic radical center tertiary in the proposed intermediate. In the Cooke and Andrews work the corresponding radical center would be secondary. Possibly the difference is enough to make the biradical longer lived and better able to achieve geometrical equilibrium in the experiments of Doering and coworkers. Whether the stereochemistry observed by Cooke and Andrews is the result of involvement of one or more pericyclic processes or whether it reflects dynamic phenomena in the biradical (see next section) is an open question. 

The photochemical production of vinylcyclopropane derivatives from compounds having two 7t-moieties bonded to an sp3-hybridized carbon648 is termed the din-methane rearrangement, also known as the Zimmerman reaction.649 A very broad spectrum of di-7t-systems can lead to photoproducts that are usually not obtainable by alternative routes.632,633 The reaction may be classified formally as a [l,2]-shift but, according to the proposed stepwise biradical mechanism,650 651 1,3- and 1,4-biradical (BR) intermediates and also the second 7t-bond may be involved652 (Scheme 6.29). A concerted (pericyclic) pathway for the di-7t-methane reaction from the excited singlet state is, however, not excluded. Typically, the singlet state reaction occurs upon direct... 

Biradicals have been proposed as intermediates mainly in pericyclic reactions. Generally, they have been obtained by the cleavage of cyclic azo compounds. The biradicals will be formed in a singlet or triplet state depending on whether the decomposition took place from the singlet or triplet state of the azo compound (Scheme 4.65). 

First, the most important step in the analysis of the above scheme requires us to characterize the structure of the intermediate since it is only when its structure is known with sufficient certainty that the predictions based on the value of the overlap determinants can be reliable. In general, the question of the structure of the intermediate can, of course, be quite complicated, but in the case of pericyclic reactions, which are of concern here, the situation is slightly more simple. This is due to the fact that the set of structures which could play the role of the eventual intermediates is restricted only to species of a biradical and/or zwitterionic nature [60,61], so that the proposal of the structure of the eventual intermediate need not be so complicated. Thus, e.g., in the case of 2j + 2g ethene dimerization, the corresponding intermediate can be naturally identifi with the tetramethylene biradical. In such a case, the whole two step reaction scheme can be desribed as follows ... 

In a similar way it would be possible to analyze the mechanism of any other pericyclic reaction, provided the structure of the intermediate is known with sufficient reliability. This requirement can be probably satisfied for the reactions of unsubstituted systems where owing to low polarity of the system the intermediate can be reasonably approximated by the biradical structures. A slightly more complex situation can occur, however, in the case of substituted skeletons, where the... 

The major value of free radical group increments is in the prediction of stabilities of proposed radical and biradical intermediates in various thermal and photochemical reactions. For example, we might want to determine whether an observed thermal rearrangement occurs homolytically, via biradical intermediates, or by a concerted, pericyclic process. One valuable piece of information is AH° of the proposed biradical intermediate. If it is too high for the biradical to lie on the reaction path, then the biradical route can be rejected. We will see examples of this type of analysis in Chapter 15. 

Ultimately, these experimental studies cannot completely rule out biradical intermediates in the Cope rearrangement, especially for the heavily substituted systems. In the end, only theory can make a definitive statement about the structure of a transition state. The interplay between theory and experiment in the Cope rearrangement and other pericyclic processes has at times been contentious. In addition, differing theoretical models have often made diametrically opposed predictions (see the Going Deeper highlight on page 900). This further fueled the debate over the true nature of pericyclic transition states. 

As we saw in the previous section, reactions that are formally of pericyclic type may in fact take place by an alternative two-step ERj mechanism. One can, moreover, envisage a continuous transition between these two extremes. Thus the Diels-Alder reaction between butadiene and ethylene might take place in one step via a symmetric pericyclic transition state (253) or it might take place in two steps via a biradical (254). In the latter case, formation of the biradical should involve passage over an intermediate transition state... 

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