|
|
- Two extended Pi systems may cyclize in a Pericyclic manner
to gain two sigma bond and loose two Pi bond.
- This is an intermolecular net double addition reaction.
- The reactions are classified by the number of Pi electrons (not the
atoms) contributed by each extended Pi systems, 2 + 2, 2 + 4, etc.
- Reactions involving 4n Pi electrons must proceed thermally via a supra (syn)
antara (anti) addition (sideways) to both Pi systems and therefore are
limited to reactions with reduced sterics such as reactions with ketenes or
the non-polar environment (no salt) Wittig reaction or Photochemically via a
supra-supra addition to both Pi systems.
- Reactions involving 4n + 2 Pi electrons must proceed thermally via a
supra (syn) supra (syn) addition to both Pi systems such as the Diels-Alder
reaction, ozonolysis, etc.
- For the Diels-Alder reaction, the regiochemistry is controlled by
electrostatic attraction and therefore the initial major product is a
kinetic, not a thermodynamic product.
- For the Diels-Alder reaction, the stereochemistry for forming bicyclic
products follows the endo rule, which can be simplified to trans on the bond
forms cis on the ring and vice versa.
| Pi electrons\Conditions |
Heat (not light) |
Light |
| 4n, n = 1, 2, ... (e.g. 2 + 2) |
supra-antara |
supra-supra |
| 4n+2, n = 0, 1, ... (4 +2) |
supra-supra |
supra-antara |
|
|
Cycloaddition Reactions