The IUPAC Gold Book defines pericyclic reactions as:
A chemical reaction in which concerted reorganization of bonding takes place throughout a cyclic array of continuously bonded atoms. It may be viewed as a reaction proceeding through a fully conjugated cyclic transition state. The number of atoms in the cyclic array is usually six, but other numbers are also possible. The term embraces a variety of processes, including cycloadditions, cheletropic reactions, electrocyclic reactions, sigmatropic rearrangements, etc. (provided they are concerted). (DOI: 10.1351/goldbook.P04491)
In general, the two criteria for a reaction to be considered as pericyclic are that it must be a concerted process with a cyclic transition state.
Four classes of pericyclic reactions are generally accepted:
cycloadditions, where two sigma bonds between two different components are formed in order to create a new ring. Examples include the Diels–Alder reaction as well as cheletropic additions;
electrocyclic reactions, where one sigma bond is formed across two ends of a conjugated system at the expense of a pi bond. An example is the Nazarov cyclisation;
sigmatropic rearrangements, where a sigma bond is moved from one position to another. These include well-known name reactions such as the Claisen and Cope rearrangements;
group transfer reactions, which are processes involving two components but do not form a ring. An example is the ene reaction.
The course of a pericyclic reaction is subject to strict orbital symmetry requirements. These are most famously discussed in terms of the Woodward–Hoffmann rules, formulated in the 1960s. They may also be understood in terms of frontier molecular orbital interactions, as described by Fukui. For their work, Hoffmann and Fukui won the 1981 Nobel Prize in Chemistry (Woodward had passed away two years earlier).
Due to their capability to form multiple bonds at once and generate molecular complexity, pericyclic reactions are often used in synthesis. Examples include (further discussion of all these may be found in Nicolaou and Sorensen's Classics in Total Synthesis I):
- the formation of ring B of estrone in Vollhardt's 1977 synthesis via a 4π-electrocyclic ring opening and a Diels–Alder reaction [pictured below];
- the rapid generation of the backbone in Heathcock's syntheses of the daphniphyllium alkaloids via a Diels–Alder reaction;
- Nicolaou's 1982 biomimetic syntheses of the endiandric acids utilising an impressive cascade of multiple electrocyclisations.
Further reading
- Most organic chemistry textbooks of undergraduate level and above will have a section on pericyclic reactions and Woodward–Hoffmann analysis.
- Fleming, I. Pericyclic Reactions, 2nd ed.; Oxford University Press: Oxford, U.K., 2015. (Oxford Chemistry Primer #67; 1st ed. in 1999)
- In a 2004 essay, E. J. Corey claimed that he had first suggested to Woodward how orbital symmetry could affect the stereoselectivity of electrocyclic reactions, thus laying the foundation for the Woodward–Hoffmann rules (J. Org. Chem. 2004, 69 (9), 2917–2919. DOI: 10.1021/jo049925d.) In response to this, Hoffmann gave his personal account of how the rules had come about (Angew. Chem. Int. Ed. 2004, 43 (48), 6586–6590. DOI: 10.1002/anie.200461440.).