A Norrish reaction, named after Ronald George Wreyford Norrish, is a photochemical reaction taking place with ketones and aldehydes. Such reactions are subdivided into Norrish type I reactions and Norrish type II reactions.[1] While of limited synthetic utility these reactions are important in the photo-oxidation of polymers such as polyolefins,[2] polyesters, certain polycarbonates and polyketones.
The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates (α-scission). 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.[3] 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.[4]
Several secondary reaction modes are open to these fragments depending on the exact molecular structure.
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.[7]
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.[8] Norrish first reported the reaction in 1937.[9]
Secondary reactions that occur are fragmentation (β-scission) to form an alkene and an enol (which will rapidly tautomerise to a carbonyl), or intramolecular recombination of the two radicals to a substituted cyclobutane (the Norrish–Yang reaction).[10]
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.[11] 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[12] 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[13] is the Norrish generated ketyl radical.
Leo Paquette's 1982 synthesis of dodecahedrane involves three separate Norrish-type reactions in its approximately 29-step sequence.
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 Phil Baran and coworkers.[14] 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.
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