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Did you know how Norish type II reactions create mysterious cyclic compounds?
In the field of chemistry, the Norrish reaction, named after British chemist Ronald George Wreyford Norrish, is a photochemical reaction involving ketones and aldehydes. Generally speaking, these reactions can be divided into two categories: Norish type I reactions and type II reactions. Although these reactions have limited synthetic utility, they play an important role in the photooxidation of polymers, particularly in materials such as polyolefins, polyesters, and some polycarbonates and polyketones.
Overview of Type I reactions
Norish type I reactions are the photochemical cleavage of aldehydes and ketones, a phenomenon known as alpha-cleavage. When a carbon group absorbs a photon, the carbon group enters a photochemical singlet state and eventually undergoes internal cross-conversion to possibly produce a triplet state. When the α-carbon bond is broken, two free radical fragments are generated, the properties and stability of which will depend on the inherent ability to generate the free radical.
For example, when 2-butanone is cleaved, the stable ethyl radical is produced primarily rather than the less stable methyl radical.
These fragments can rejoin back to the original carbon group, possibly undergoing gradual changes in the process. The abstraction of hydrogen atoms may form alkenes or aldehydes, a process that has limited synthetic utility because such reactions often occur as side reactions of other reactions.
The secret of type II reactions
The Norrish Type II reaction is characterized by the photochemical internal extraction of gamma-hydrogen, a hydrogen atom located three carbon positions between the carbon group and the carboxyl group, which results in the formation of a 1,4-diradical as the main light products. This reaction was first reported by Norrish in 1937.
The generated diradicals may undergo β-cleavage to produce alkenes and rapidly interconverting ketones, or may produce substituted cyclobutanes through dimerization, which is called the Norish-Young reaction.
Applications of Norish reaction
The study of the Norish reaction has also received attention in environmental chemistry, especially the study of the photolysis of aldehydes, such as heptanal, which is ubiquitous in the earth's atmosphere. Under close to atmospheric conditions, the photolysis process of heptaldehyde will produce 62% of 1-pentene and acetaldehyde, as well as cyclic alcohols such as cyclobutanol and cyclopentanol. The source of these compounds comes from type II channel.
In addition, some experiments have also shown that photolysis of a ketone derivative in water can also produce gold nanoparticles with a diameter of 10 nanometers. This reaction involves the free radicals generated by Norrish.
Latest research and synthetic applications
In the latest synthetic methods, for example, Leo Paquette's polycyclic synthesis in 1982 demonstrated the practical application of Norish-type reactions, emphasizing the indispensable importance of this reaction in organic synthesis. In addition, Phil Baran and his research team successfully optimized the conditions for utilizing type II reactions to minimize competing reactions during the synthesis of bioactive compounds such as cardioglycosides, thereby obtaining ideal intermediates on the doklam scale.
Conclusion
In summary, Norish type II reactions are more than just a photochemical reaction mechanism, functioning in different ways in synthetic and environmental sciences. As research continues, how will we use these reactions to unlock more chemical synthesis possibilities?
