Norrish reaction

Norrish reactions are name reactions in the field of organic chemistry. Is photochemical cleavage of carbonyl compounds such as ketones or aldehydes. They are divided into two types, which are referred to as Norrish type I reaction and Norrish type II reaction. The reaction was first published in 1932 by Ronald George Wreyford Norrish, after whom it is named. For generating the radical UV radiation of a specific wavelength is required.

For the Norrish reactions there are few synthetic applications. Norrish Type I occurs as an undesirable side reaction in the Paterno - Büchi reaction as well as a side reaction of Norrish type II.

Norrish type I reaction

As it is in the Norrish reaction is a reaction with the free-radical reactive intermediates, which can react further with a low selectivity, multiple reaction pathways are possible.

The reaction starts with the absorption of a photon by the carbonyl to form a radical is generated in the singlet state. This is a triplet state can be formed by intersystem crossing. To what extent this step runs, depends on the reactivity of Singulettradikals and the speed of intersystem crossing. However, both conditions lead to radical cleavage of the carbon -carbon bond or carbon-hydrogen bond at the carbonyl group. At which the two residues, the carbonyl radical is bonded, is dependent on the stability of the alkyl radicals.

The radical pair formed can then react in different ways:

  • Recombination to the starting compound
  • Elimination of carbon monoxide and subsequent combination of the radicals
  • The alkyl radical represents a hydrogen radical is split off from the carbonyl radical, which is an alkane and a ketene arise.
  • The carbonyl radical splits off a hydrogen radical from the alkyl radical, which is an aldehyde and an alkene produced.

Norrish type II reaction

The Norrish type II reaction can proceed at compounds having a hydrogen atom at the γ - position. It also starts with the photochemical generation of a biradical. Over a six -membered transition state, however, can be abstracted in γ - position in this case, a hydrogen radical, with a biradikalischer alcohol is formed. This intermediate can then react in two ways:

  • Combination to form a derivative of cyclobutane
  • Fragmentation to an enol and an alkene
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