- A pericyclic reaction is characterized as a change in bonding relationships that takes place as a continuous, concerted reorganization of electrons.
- The term "concerted" specifies that there is one single transition state and therefore no intermediates are involved in the process. To maintain continuous electron flow, pericyclic reactions occur through cyclic transition states.
- More precisely: The cyclic transition state must correspond to an arrangement of the participating orbitals which has to maintain a bonding interaction between the reaction components throughout the course of the reaction
The Five Major Classes of Pericyclic Reactions
1. ELECTROCYCLIC RING CLOSURE/RING OPENING:
- An electrocyclic ring closure is the creation of a new sigma bond at the expense of the terminal π orbitals of a conjugated π system.
- There is a corresponding reorganization of the conjugated pi system. We classify the reaction according to the number of electrons involved.
2. CYCLOADDITION REACTIONS/CYCLOREVERSION REACTIONS:
- Involves the union of two smaller, independent π systems.
- Sigma bonds are created at the expense of π bonds.
- Referred to as [m + n] additions when a system of m conjugated atoms combines with a system of n conjugated atoms.
- A cycloreversion is simply the reverse of a cycloaddition.
3. CHELETROPIC REACTIONS:
- A special group of cycloaddition/cycloreversion reactions.
- Two bonds are formed or broken at a single atom.
- The nomenclature for cheletropic reactions is the same as for cycloadditions.
4. SIGMATROPIC REARRANGEMENTS:
- Involves migration of a sigma bond from one position in a conjugated system to another position in the system, accompanied by reorganization of the connecting π bonds.
- The number of π and sigma bonds remains constant.
- The rearrangement is an [m, n] shift when the sigma bond migrates across m atoms of one system and n atoms of the second system.
5. GROUP TRANSFER REACTIONS:
- In a group transfer reaction one or more group of atoms get transferred to a second reaction partner.