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.

Contents 1 Norrish I 2 Norrish II 3 Scope 4 External links 5 References

[edit] 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 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].

[edit] Norrish II

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 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 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).

In one study ^[4] 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 Au^{0 [5]} is the Norrish generated ketyl radical.

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

[edit] External links

• IUPAC Gold Book Norrish type I definition
• IUPAC Gold Book Norrish type II definition

[edit] References