Norrish reaction
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The '''Norrish reaction''' in [[organic chemistry]] describes the [[photochemical reaction]]s taking place with [[ketone]]s and [[aldehydes]]. This type of reaction is subdivided in '''Norrish type I reactions''' and '''Norrish type II reactions''' <ref>''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 </ref>. The reaction is named after [[Ronald George Wreyford Norrish]].
==Norrish I==
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 [[Photoexcitation|excited]] to a photochemical [[Diradical|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.
[[Image:NorrishtypeIreaction.png|center|600px|Scheme 1. Norrish type I reaction]]
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-[[hydrogen|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 <ref>''Bicyclohexylidene'' Nicholas J. Turro, Peter A. Leermakers, and George F. Vesley [[Organic Syntheses]], Coll. Vol. 5, p.297 ('''1973'''); Vol. 47, p.34 ('''1967''') [http://www.orgsynth.org/orgsyn/prep.asp?prep=cv5p0297 Online article].</ref>.
==Norrish II==
A '''Norrish type II reaction''' is the photochemical [[intramolecular]] abstraction of a [[gamma|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'').
[[Image:NorrishTypeII.png|center|400px|Scheme 2. Norrish type II reaction]]
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]].
==Scope==
The Norrish reaction has been studied in relation to [[environmental chemistry]] with respect to the photolysis of the [[aldehyde]] [[heptanal]], a prominent compound in Earth's atmosphere <ref>''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}}</ref>. 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 ([[cyclobutanol]]s and [[cyclopentanol]]s) 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).
In one study <ref>''Facile Photochemical Synthesis of Unprotected Aqueous Gold Nanoparticles'' Katherine L. McGilvray, Matthew R. Decan, Dashan Wang, and Juan C. Scaiano [[J. Am. Chem. Soc.]]; '''2006'''; 128(50) pp 15980 - 15981; (Communication) {{DOI|10.1021/ja066522h}}</ref> the photolysis of an [[Acyloin]] derivative in water in presence of [[chloroauric acid|hydrogen tetrachloroaurate]] (HAuCl<sub>4</sub>) generated [[nanogold]] particles with 10 [[nanometer]] diameter. The species believed to responsible for reducing Au<sup>3+</sup> to Au<sup>0</sup> <ref>Technically Au<sup>3+</sup> is reduced to Au<sup>2+</sup> which then forms Au<sup>+</sup> and Au<sup>3+</sup> by [[disproportionation]] followed by final reduction of Au<sup>1+</sup> to Au<sup>o</sup></ref> is the Norrish generated [[ketyl]] radical.
[[Image:NorrishApplication.png|400px|center|Norrish application nanogold synthesis]]
No less than three Norrish-type reactions feature in the classic 1982 total synthesis of [[dodecahedrane]]
==External links==
* IUPAC [[Gold Book]] [http://www.iupac.org/goldbook/N04219.pdf Norrish type I definition]
* IUPAC [[Gold Book]] [http://www.iupac.org/goldbook/N04218.pdf Norrish type II definition]
==References==
<div class="references-small"><references/>
[[Category:Free radical reactions]]
[[Category:Photochemistry]]
[[it:Reazioni di Norrish]]
[[Category:Name reactions]]