Pericyclic Reactions. 1. Definition.
A pericyclic reaction is one in which bonds are made or broken in a concerted cyclic transition state.
A concerted reaction is one which involves no intermediates during the course of the reaction.
A stepwise and therefore non-concerted and non-pericyclic reaction is shown on the right.
.
Pericyclic reactions have certain characteristic properties, although as usual it is not difficult to find
exceptions to all these rules.
- There is a relatively small solvent effect on the rate of reaction (unless the reactants themselves happen to be
charged, ie carbonium or carbanions). Pericyclic reactions can occur in the gas phase with no
solvent. Quite recently, the use of water to accelerate pericyclic reactions (by perhaps a factor of 10 to 100)
has been much investigated, but the acceleration is largely due to the formation of hydrogen bonds specific to
the transition state.
- There is no nucleophilic or electrophilic component. This means that in the arrow pushing sense,
there is no beginning and no ending for the arrows, and the arrow pushing can occur in either a
clockwise or anti-clockwise direction.
- Normally, no catalyst is need to promote the reactions. However, many transition metal complexes
can catalyse pericyclic reactions by virtue of d-orbital participation. Lewis acids also catalyse
many forms of pericyclic reactions, either directly, or by changing the mechanism of the the reaction so that it becomes a stepwise process
(ie the right hand
diagram above) and
hence no longer a true pericyclic reaction. It is also possible to accelerate the reactions by the use of
pressure for those reactions involving a substantial decrease in volume, and catalysts also exist which provide
suitable "cavities" for promoting pericyclic reactions such as cycloadditions.
- Pericyclic reactions normally show a very high stereospecificity.
- Pericyclic reactions can be frequently promoted by light (denoted hv in the text) as well as heat
(denoted D; in the text). Normally, the stereochemistry under the two sets of conditions is different,
and it was thought opposite. Current thinking about the photochemical route is more complex!
- Pericyclic reactions are unusual in that very few enzymes which catalyse such reactions are known.
Artificial catalytic antibodies ('abzymes') can be created which can perform this feat,
"Diels-Alderase" does this for cycloadditions
and Chorismate mutase
for the Claisen rearrangement.