The Woodward-Hoffmann Rules

The conclusions (from correlation diagrams) about pericyclic reactions can be generalized to a set of selection rules for pericylic reactions -

the Woodward-Hoffmann rules:

These rules can be expressed in a number of ways. A summary for pericyclic cycloaddition reactions is:

p + q	Thermally allowed	Photochemically allowed
4n	ps + qa or pa + qs	ps + qs or pa + qa
4n +2	ps +qs or pa + qa	ps + qa or pa + qs

You don’t need to learn these for this course!

· p and q are the number of electrons in the two p systems which are undergoing the cycloaddition reaction.

· s indicates suprafacial attack with respect to one of the p components, and

· a indicates antarafacial with respect to one of the p components.

From the table outlining the rules, when p and q add up to give a number which can be written as (4n +2), with n as an integer (0, 1, 2…), the thermal cycloaddition reaction is allowed when it is suprafacial with respect to both components (or antarafacial with respect to both components).

An example of the Woodward-Hoffmann Rules in action: the Diels-Alder Reaction