The English used in this article or section may not be easy for everybody to understand. (May 2012)
In organic chemistry, a pericyclic reaction is a type of chemical reaction between organic compounds. In the case of pericyclic reactions, the transition state of the molecule is a ring (has a cyclic geometry), and the reaction goes forward in a concerted way. Pericyclic reactions are usually rearrangement reactions. The most important groups of pericyclic reactions are:
- Electrocyclic reactions
- Sigmatropic reactions
- Group transfer reactions
- Cheletropic reactions
- Dyotropic reactions
In general, pericyclic reactions are equilibrium processes. However, it is possible to push the reaction in one direction if the product is at a significantly lower energy level. This is applying Le Chatelier's principle to a reaction involving a single molecule.
Many pericyclic reactions have similar stepwise radical processes connected with them. Chemists disagree whether some reactions are pericyclic reactions. For example, it is not definitively known whether the [2+2] cycloaddition mechanism is concerted (or may depend on the reactive system). Many pericyclic reactions have similar reactions that are metal-catalyzed. But these metal-catalyzed reactions are also not really pericyclic. The metal catalysts stabilize the reaction intermediates. So the reaction is not concerted, but rather metal-stabilized.
Due to the principle of microscopic reversibility, there is a parallel set of "retro" pericyclic reactions, which perform the reverse reaction.
Pericyclic reactions in biochemistryEdit
Pericyclic reactions also happen in several biological processes:
- Claisen rearrangement of chorismate to prephenate in almost all prototrophic organisms.
- [1,5]-sigmatropic shift when precorrin-8x is being changed into hydrogenobyrinic acid
- non-enzymatic, photochemical electrocyclic ring opening and a (1,7) sigmatropic hydride shift in vitamin D synthesis.
- a conversion of Isochorismate into salicylate and Pyruvate in a catalyzed, true pericyclic reaction.
- A New Type of Corrin Synthesis. Yasuji Yamada, D. Miljkovic, P. Wehrli, B. Golding, P. Loliger, R. Keese, K. Miiller, and A. Eschenmoser. Angew. Chem. Int. Edit. 1969, 8(5),343-348. [permanent dead link]
- Isochorismate Pyruvate Lyase: A Pericyclic Reaction Mechanism? Michael S. DeClue, Kim K. Baldridge, Dominik E. Künzler, Peter Kast, and Donald Hilvert J. Am. Chem. Soc.; 2005; 127(43) pp 15002 - 15003; (Communication) DOI: 10.1021/ja055871t Abstract[permanent dead link]
- In this experiment isochorismate is deuterated in one specific position and subjected to the lyase. Two important observations rule out other reaction mechanisms, ionic or base promoted. From the kinetic isotope effect (value 2.34) it can be inferred that carbon to hydrogen bond breaking occurs in the transition state of the rate determining step. NMR spectroscopy shows that the deuterium atom alone is transferred to the pyruvate molecule.