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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 reaction is named after Ronald George Wreyford Norrish.

Type I

The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates. 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 bond from either state, two radical fragments are obtained.[2] The size and nature of these fragments depends upon the stability of the generated radicals; for instance, the cleavage of 2-butanone largely yields ethyl radicals in favor of less stable methyl radicals.[3]

Norrish type I reaction
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, with racemisation at the α-carbon.
  • Two organic residues can recombine with formation of a new carbon-carbon bond, with the loss of carbon monoxide.[2] The rate and yield of this product depends upon the bond-dissociation energy of the ketone's α substituents. Typically the more α substituted a ketone is, the more likely the reaction will yield products in this way.[4][5]
  • The abstraction of an α-proton from the carbonyl fragment may form a ketene and an alkane.
  • The abstraction of a β-proton from the alkyl fragment may form an aldehyde and an alkene.
Norrish type I reaction
Norrish type I reaction

The synthetic utility of this reaction type is limited, for instance it often is a side reaction in the Paternò–Büchi reaction. One organic synthesis based on this reaction is that of bicyclohexylidene.[6]

Type II

A Norrish type II reaction is the photochemical intramolecular abstraction of a γ-hydrogen (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.[7] Norrish first reported the reaction in 1937.[8]

Norrish type II reaction
Norrish type II reaction

Secondary reactions that occur are intramolecular recombination of the two radicals to a substituted cyclobutane in the Norrish–Yang reaction,[9] or fragmentation to an enol and an alkene.

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.[10] 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).

In one study [11] the photolysis of an Acyloin derivative in water in presence of hydrogen tetrachloroaurate (HAuCl4) generated nanogold particles with 10 nanometer diameter. The species believed to responsible for reducing Au3+ to Au0 [12] is the Norrish generated ketyl radical.

Norrish application nanogold synthesis
Norrish application nanogold synthesis

No fewer than three Norrish-type reactions feature in the classic 1982 total synthesis of dodecahedrane.

An example of a synthetically useful Norrish type II reaction can be found early in the total synthesis of the biologically active cardenolide ouabagenin by Baran and coworkers.[13] The optimized conditions minimize side reactions, such as the competing Norrish type I pathway, and furnish the desired intermediate in good yield on a multi-gram scale.

Type II Norrish reaction in Phil Baran's total synthesis of the biologically active cardenolide ouabagenin.
Type II Norrish reaction in Phil Baran's total synthesis of the biologically active cardenolide ouabagenin.

See also

References

  1. ^ 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
  2. ^ a b "IUPAC Gold Book - Norrish Type I photoreaction". IUPAC. 24 February 2014. doi:10.1351/goldbook.N04219. Retrieved 31 March 2014.
  3. ^ Blacet, F. E.; N. Pitts Jr., James (1950). "Methyl Ethyl Ketone Photochemical Processes". Journal of the American Chemical Society. 72 (6): 2810–2811. doi:10.1021/ja01162a544.
  4. ^ Yang, Nien-Chu; D. Feit, Eugene; Hui, Man Him; Turro, Nicholas J.; Dalton, Christopher (1970). "Photochemistry of di-tert-butyl ketone and structural effects on the rate and efficiency of intersystem crossing of aliphatic ketones". Journal of the American Chemical Society. 92 (23): 6974–6976. doi:10.1021/ja00726a046.
  5. ^ Abuin, E.B.; Encina, M.V.; Lissi, E.A. (1972). "The photolysis of 3-pentanone". Journal of Photochemistry. 1 (5): 387–396. doi:10.1016/0047-2670(72)80036-4.
  6. ^ 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.
  7. ^ "IUPAC Gold Book - Norrish Type II photoreaction". IUPAC. 24 February 2014. doi:10.1351/goldbook.N04218. Retrieved 31 March 2014.
  8. ^ Norrish, R. G. W.; Bamford, C. H. (31 July 1937). "Photo-decomposition of Aldehydes and Ketones". Nature. 140: 195–6. doi:10.1038/140195b0.
  9. ^ "IUPAC Gold Book - Norrish–Yang reaction". IUPAC. 24 February 2014. doi:10.1351/goldbook.NT07427. Retrieved 31 March 2014.
  10. ^ 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
  11. ^ 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
  12. ^ Technically Au3+ is reduced to Au2+ which then forms Au+ and Au3+ by disproportionation followed by final reduction of Au1+ to Auo
  13. ^ Renata, H.; Zhou, Q.; Baran, P. S. (3 January 2013). "Strategic Redox Relay Enables A Scalable Synthesis of Ouabagenin, A Bioactive Cardenolide". Science. 339 (6115): 59–63. doi:10.1126/science.1230631.