Pericyclic reaction
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In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted-reactions fashion. Pericyclic reactions are usually rearrangement reactions. The major classes of pericyclic reactions are:
- Electrocyclic reactions
- Cycloadditions
- Sigmatropic reactions
- Group transfer reactions
- Cheletropic reactions
In general, these are considered to be equilibrium processes, although it is possible to push the reaction in one direction by designing a reaction by which the product is at a significantly lower energy level; this is due to a unimolecular interpretation of Le Chatelier's principle. Pericyclic reactions often have related stepwise radical processes associated with them. Some pericyclic reactions, such as the [2+2] cycloaddition, are 'controversial' because their mechanism is not definitively known to be concerted (or may depend on the reactive system). Pericyclic reactions also often have metal-catalyzed analogs, although usually these are also not technically pericyclic, since they proceed via metal-stabilized intermediates, and therefore are not concerted.
The 'largest' pericyclic reaction was the [14 pi] ring closure on a corrin performed by Albert Eschenmoser.
Due to the principle of microscopic reversibility, there is a parallel set of "retro" pericyclic reactions, which perform the reverse reaction.
[edit] Pericyclic reactions in biochemistry
Pericyclic reactions also occur in several biological processes.
- Claisen rearrangement of chorismate to prephenate in almost all prototrophic organisms.
- [1,5]-sigmatropic shift in the transformation of precorrin-8x to 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 .
[edit] See also
[edit] References
- ↑ 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
