Pericyclic reactions are reactions in which “all first order changes in bonding relationships takes place in concert on a closed curve” (Woodward & Hoffmann). More simply, the term “pericyclic” covers all concerted reactions involving a cyclic flow of electrons through a single transition state. Pericyclic reactions can be predicted and controlled to a great degree, which makes them very useful in synthesis.
There are broadly four classes of pericyclic reaction:
Sigmatropic – These are unimolecular isomerisations, and involve the movement of a σ-bond from one position to another. An illustration would be the first step of the Claisen Rearrangement: Note the nomenclature of this reaction, being described as a [i,j] shift. For example, this following is a [1,7] shift:
Electrocyclic – These are unimolecular. They are characterised by ring opening or closing with a σbond forming at one end. Ring closing is more common, since this is formation of a σ-bond at the expense of a π-bond, but ring strain can lead to opening. Examples are:
Cycloaddition – This is the largest class of pericyclic reaction. It is characterised by two fragments coming together to form two new σ-bonds in a ring. Some examples are Diels-Alder and Ozonolysis reactions, which are described below.
Chelotropic reactions are a specific type of cycloaddition, where the two bonds are made or broken at the same atom. The classic example of this is carbene addition to a double bond.
Group Transfer – There are only a few of these reactions, the most common of which is the ene reaction (see further down). They resemble [1,5] sigmatropic shifts, since a σ-bond moves, and they also resemble Diels-Alder, but replacing a π-bond with a σ-bond.
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