Norrish reaction
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The Norrish reaction in organic chemistry describes the photochemical reactions taking place with ketones and aldehydes. This type of reaction is subdivided in Norrish type I reactions and Norrish type II reactions [1].
The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates (scheme 1). The carbonyl group accepts a photon and is excited to a photochemical singlet state. Through intersystem crossing the triplet state can be obtained. On cleavage of the α-carbon carbon bond from either state, two radical fragments are obtained.
Several secondary reaction modes are open to these fragments depending on the exact molecular structure.
- The fragments can simply recombine to the original carbonyl compound (path A).
- By extrusion of carbon monoxide in path B two organic residues can recombine with formation of a new carbon carbon bond
- When the carbon fragment has an α-proton available it gets abstracted forming a ketene and a saturated hydrocarbon in path C
- When the alkyl fragment contains a β-proton it gets abstracted with formation of an aldehyde and an alkene.
The synthetic utility of this reaction type is limited. It often is a side reaction for instance in the Paterno-Büchi reaction. One organic synthesis based on this reaction is that of bicyclohexylidene [2].
A Norrish type II reaction is the photochemical intramolecular abstraction of a γ-hydrogen (which is a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct (IUPAC definition) (scheme 2).
Secondary reaction are either intramolecular recombination of the two radicals to a cyclobutane compound (path A) or fragmentation to an enol and an alkene.
The reaction was first reported about in 1937 by Ronald George Wreyford Norrish.
[edit] Scope
The Norrish reaction has been studied in relation to environmental chemistry with respect to the photolysis of the aldehyde n-heptanal, a prominent compound in Earth's atmosphere [3]. Photolysis of heptanal in conditions resembling atmospheric conditions results in the formation of 1-pentene and acetaldehyde in 62% chemical yield together with cyclic alcohols (cyclobutanols and cyclopentanols) both from a Norrish type II channel and around 10% yield of hexanal from a Norrish type I channel (the initially formed n-hexyl radical attacked by oxygen).
[edit] External links
[edit] References
- ^ Named Organic Reactions, 2nd Edition, Thomas Laue and Andreas Plagens, John Wiley & Sons: Chichester, England, New York, 2005. 320 pp. ISBN 0-470-01041-X
- ^ Bicyclohexylidene Nicholas J. Turro, Peter A. Leermakers, and George F. Vesley Organic Syntheses, Coll. Vol. 5, p.297 (1973); Vol. 47, p.34 (1967) Online article.
- ^ Photolysis of Heptanal Suzanne E. Paulson, De-Ling Liu, Grazyna E. Orzechowska, Luis M. Campos, and K. N. Houk J. Org. Chem.; 2006; 71(17) pp 6403 - 6408; (Article) DOI:10.1021/jo060596u
