Pericyclic reactions intermediates

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

Diels-Alder reactions are found to be little influenced by the introduction of radicals (cf. p. 300), or by changes in the polarity of the solvent they are thus unlikely to involve either radical or ion pair intermediates. They are found to proceed stereoselectively SYN with respect both to the diene and to the dienophile, and are believed to take place via a concerted pathway in which bond-formation and bond-breaking occur more or less simultaneously, though not necessarily to the same extent, in the transition state. This cyclic transition state is a planar, aromatic type, with consequent stabilisation because of the cyclic overlap that can occur between the six p orbitals of the constituent diene and dienophile. Such pericyclic reactions are considered further below (p. 341). 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

The intermediates 2 and 4 can then be traced back to simple precursors via retro-Diels-Alder condensations, which are very well known stereospecific pericyclic reactions. 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

Pericyclic reactions are concerted reactions that take place in a single step without any intermediates, and involve a cyclic redistribution of bonding electrons. The concerted nature of these reactions gives fine stereochemical control over the generation of the product. The best-known examples of this reaction are the Diels-Alder reaction (cyclo-addition) and sigmatropic rearrangement. 

Pericyclic reactions represented for many years a difficult mechanistic problem because the apparent absence of intermediates left few concrete features that could be subjected to experimental study. Application of some fundamental principles of orbital theory, initiated in 1965 by Woodward and Hoffmann1 and since developed extensively by them2 and by others,3 have provided new... 

Despite our increasing knowledge of the structures and reactions of organometallic intermediates, it is virtually impossible to plan complex ionic reactions by using known rate constants from simple model systems. Rate constants are potentially more useful when planning pericyclic reactions because they are easier to measure and because pericyclic reactions are much less susceptible to medium effects than ionic reactions. However, the need to evaluate the rates of competing pericyclic reactions is relatively rare (often there is only one reasonable possibility). 

Pericyclic reactions are the third distinct class. They have cyclic transition structures in which all bond-forming and bond-breaking takes place in concert, without the formation of an intermediate. The Diels-Alder reaction and the Alder ene reaction are venerable examples. The curly arrows can be drawn in either direction—clockwise, as here, but equally well anti- clockwise. They could even be drawn with fishhook arrows, and would still... 

Each of the following transformations is the result of two successive pericyclic reactions. Draw the structures of the intermediates A-E, and identify the class of pericyclic reaction to which each step belongs ... 

The characteristic feature of all pericyclic reactions is the concertedness of all the bond making and bond breaking, and hence the absence of any intermediates. Naturally, organic chemists have worked hard, and devised many ingenious experiments, to prove that this is true, concentrating especially on the Diels-Alder reaction. The following is an oversimplified description of some of the most telling experiments. 

In the third group, pericyclic reactions, there are no intermediate ions or radicals, but the reactant and reagent orientate themselves so as to form a cyclic transition state (usually, but not necessarily, six-membered in type) around which there is an electron flow leading to the formation of the new bonding arrangement [e.g. (iii)]. 

Although such an understanding of the reaction mechanism is in principle applied in the theory of pericyclic reactions, the above general picture is in this case slightly complicated by the specific (introduced in the course of historical development) classification of reaction mechanisms in terms of concertedness and/or nonconcertedness. Concerted reactions are intuitively understood as those reactions for which the scission of old bonds and the formation of the new ones is synchronised, whereas for nonconcerted reactions the above bond exchange processes are completely asynchronised. Moreover, since the above asynchronicity is also intuitively expected to induce the stepwise nature of the process, the nonconcertedness is frequently believed to require the presence of intermediates, whereas the concerted reactions are believed to proceed in one elementary step. 

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. 

In a similar way, it would be possible to analyse the mechanisms of any other pericyclic reaction, provided the structure of the intermediate is known with sufficient certainty. This requirement can probably be satisfied for the reactions... 

This opens up the possibility of a systematic investigation of pericyclic reactions not only for model cases of parent unsubstituted systems, but for inclusion if zwitterionic contributions also enable the analysis of the eventual mechanistic changes induced by the polar substitution. As an example, the push-pull substituted Diels-Alder system will be analysed, in which the diene component is substituted in position 1 by a donor, and dienophilic component in position 6 by an acceptor substitution. In order to avoid the problems with the relative wieght of individual limiting structures of the intermediate (Eq. 30), the coulombic integrals modelling the substitution in the HMO wave function were arbitrarily set to a = 3/ and a = — 3) so that there is sufficient polarity in the system to secure the approximation of the intermediate by pure zwitterionic structure Z,. 

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