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 |
p_{s} + q_{a} or p_{a} + q_{s} |
p_{s} + q_{s} or p_{a} + q_{a} |
4n +2 |
p_{s} +q_{s} or p_{a} + q_{a} |
p_{s} + q_{a} or p_{a} + q_{s} |
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