Norrish type II cleavage

Typical chemical reactions of photoexcited aldehydes and ketones are cleavage reactions, usually designated as Norrish Type I [equation (54)], II [equation (55)] and III [equation (56)], hydrogen abstraction [equation (57)] and cycloadditions, such as the Paterno-Buchi reaction [equation (58)]. Of these, Norrish Type II cleavage and the related... 

This reaction, called Norrish Type II cleavage, involves intramolecular abstraction of the y hydrogen followed by cleavage of the resulting diradical (a... 

These cycloadducts, at their most elementary level, are excellent intermediates for the synthesis of 3-substituted furan derivatives. For example, Kawanisi and coworkers reported a synthesis of perillaketone 174 in which the critical step was a Paterno-BUchi photocycloaddition between furan and 4-methylpentanal in the presence of methanesul-fonic acid (Scheme 39)82. This reaction furnished two initial photoadducts, 172 and 173. The unexpected product 173 presumably arises from a Norrish Type II cleavage of 4-methylpentanal to give acetaldehyde, and subsequent cycloaddition with furan. The desired cycloadduct 172 was then converted uneventfully to 174 via acid-catalyzed aromatization and oxidation. 

It is interesting to note that although in principle ketone 48 can undergo secondary Norrish type II cleavage to give a mixture of methyl 4-acetyIbenzoate... 

Intramolecular hydrogen abstraction from a g carbon occurs when certain ketones are irradiated. For e.g., vapours of Hexan-2-one on photolysis gives an alkene and a ketone (via the enol form). It is referred to as Norrish type II cleavage. 

One possible explanation for the lack of correspondence between emitting and reacting states is reaction of the singlet. In the case of benzophenone, there is little question that the reaction involves only the triplet state, since triplet quenchers can effectively inhibit the reaction. This need not be the case with all carbonyl compounds. For example, it has been shown that both the n,ir singlet and triplet states can be involved in the Norrish type II cleavage of alkyl ketones (25-27). At high concentrations, piperylene quenches only that part of the 2-hexanone cleavage which arises from the triplet. The rate constants for... 

Since the photochemistry of many compounds that have been used as triplet sensitizers has been well studied, we will not attempt to cover these reactions in detail. Unless the investigator is unaware of them, common photochemical processes such as the Norrish Type II cleavage are not ordinarily a complication and as will be mentioned later, they can actually serve as mechanistic probes. A discussion of the mechanisms of triplet energy transfer1,3,9 is beyond the scope of this review as are other specific reactions which have been recently covered elsewhere. 

Tor synthetically useful examples of Norrish type II cleavage, see Ncckers Kellogg Prins Schouslra J. Ore. Chem. 1971, 36. 1838. 

Substituents in a-Position. To rationalize the ratio between Norrish type II cleavage and cyclization (see preceding chapter) the influence of a-substituents on cyclization were studied exclusively on straight-chain aryl ketones with preferential y-hydrogen abstraction. 

However, the Norrish Type II cleavage doesn t matter in carbonyl compounds with (y )-hydrogen abstraction. In this case, one has to bear in mind that electron-donating a-substituents can promote the Norrish Type I cleavage by stabilizing the resulting radical. For that reason, a-cleavage can compete with the desired cyclization. 

Photochemical reactions of alkenyl phenylglyoxylates have been additionally studied but PET induced photocyclization only occurred when alkenyl group was situated at a proper distance and in a suitable configuration [20]. Otherwise, dimerizations, Norrish Type II cleavage and... 

In Norrish type II cleavage, the O radical abstracts H from the y-carbon in a six-membered TS, and the 1,4-diradical then fragments to give an alkene and an enol, the latter of which tautomerizes to the ketone. Sometimes, the 1,4-diradical undergoes radical-radical combination to give a cyclobutane, instead. The Norrish type II cleavage is closely related to the McLafferty rearrangement that is often seen in the mass spectra of carbonyl compounds. 

There are numerous bridging possibilities. The extraordinary variety can be exemplified by the pyrolysis (700 °C) of 188 to give 190 (40 %) 10S), the anodic oxidation of 191 to give 192 (55 %)107), and the photochemical (mass spectrometrical) Norrish type-II cleavage ( McLafferty-rearrangemenf ) of carbonyl compounds with y-H-atoms (193) to give enols and alkenes 108). 

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