THE THEORY OF PERICYCLIC REACTIONS

An appreciation of the role of symmetry in the construction of orbital models for bonding is essential to the understanding of the theory of pericyclic reactions. The introduction to symmetry presented in Chapter 10 will serve as background for the remainder of this chapter. We summarize its main conclusions here before proceeding.12... 

For a more detailed treatment of the theory of pericyclic reactions the reader is referred to modern texts on reaction mechanism.25... 

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. 

R. Ponec Similarity Models in the Theory of Pericyclic Reactions... 

Once the theory of pericyclic reactions was developed, it was recognized that the conversion of Dewar benzene to benzene is an electrocyclic reaction. This conversion involves two pairs of electrons one pair of pi electrons and one pair of sigma electrons of the Dewar benzene. (The third pair of electrons is located in exactly the same place in both the reactant and the product and so is not involved in the reaction.) An electrocyclic reaction involving two pairs of electrons must occur by a conrotatory motion if it is to be thermally allowed. However, the conrotatory opening of Dewar benzene is geometrically impossible, because it would result in a benzene with a trans double bond, a compound with too much angle strain to exist. 

The [4 + 2] cycloaddition was discovered long before the theory of pericyclic reactions was developed. It is more commonly known as the Diels-Alder reaction, named after O. Diels and K. Alder, who shared the 1950 Nobel Prize in chemistry for developing this reaction. The Diels-Alder reaction occupies a very important place among the tools of the synthetic organic chemist because it provides a method for the construction of six-membered rings from acyclic precursors with excellent control of stereochemistry. 

Since Criegee s pioneering work in the late 1930s, the accepted mechanism for hydroxylation has been a concerted cycloaddition of alkene to the 0=0s=0 system.58 The development of the theory of pericyclic reactions in the 1960s and 1970s helped cement this proposal firmly in the minds of many workers.59 However, subsequent work has found few (if any) documented organometallic examples that followed the Woodward-Hoff-mann rules. 

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