Norrish type II fragmentation

The irradiation of cyclohexyl benzoylformate (62) yields the hydroxyphenyl ketene (63) by a Norrish Type II fragmentation process. If the ketene is generated in the presence of imines (64), successful thermal trapping results in the formation of p-lactams (65). 

Dibenzoylbutane undergoes Norrish Type II fragmentation to yield acetophenone and l-phenylbut-3-en-l-one.65 66 61... 

Norrish Type II fragmentation reaction is the excitation of the carbonyl group of cycloketones by irradiation followed by intermolecular hydrogen abstraction usually from... 

Pincock and co-workers have examined the Norrish type II fragmentation of various aromatic esters. In contrast to alkyl aryl ketones, esters with lowest k,k triplets undergo the reaction with a low quantum efficiency, which is induced by intramolecular electron transfer. 

Intramolecular Electron Transfer and Norrish Type II Fragmentation... 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Although the Paterno-Buchi reaction is of high synthetic potential, its use in organic synthesis is still not far developed. In recent years some promising applications in the synthesis of natural products have been reported. The scarce application in synthesis may be due to the non-selective formation of isomeric products that can be difficult to separate—e.g. 6 and 7—as well as to the formation of products by competitive side-reactions such as Norrish type-I- and type-II fragmentations. 

If there are hydrogen atoms in the y-position relative to the acyl group, irradiation of an imidazolide leads to a 1,2-shift of the acyl group (step one) followed by a Norrish type II or type I fragmentation (step two) [41,[51... 

Frequently B will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction) as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly(phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photodegradation, reaction 2. 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

One of the most common photochemical reaction pathways of carbonyl compounds is the formation of a diradicaloid excited state which is able to abstract a hydrogen atom at the y (or, more rarely, e) position, followed by either fragmentation or recombination. This process, which is known as the Norrish type II reaction, has a parallel in the photochemistry of nitro groups the intramolecular hydrogen abstraction of excited ortho-nitrotoluene is actually one of the very early synthetic photochemical transformations [9]. It has been exploited in a family of photolabile protecting groups, most prominent among which are derivatives of ortho-nitrobcnzyl alcohol, as introduced in 1966 by Barltrop et al. (Scheme 13.1) [10, 11],... 

A laser flash study of the photoreactions of hexan-2-one and 5-methylhexan-2-one has provided evidence for the existence of the triplet 1,4-biradicals produced by the y-hydrogen abstraction typical of Norrish Type II reactivity. The photochemical behaviour of the alkanone, nonan-5-one, in urea inclusion compounds has been studied. In solution, irradiation of nonan-5-one yields hexan-2-one, propylene, and two cyclobutanols. In the clathrate, the fragmentation products were essentially the same but only one cyclobutanol was observed. The cyclization fragmentation ratio was established as 0.67, compared with 0.32 in methanol. The authors suggest that the CIS-cyclobutanol has less stringent rotational requirements and that it is this isomer (43) which is formed in the clathrate. 

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. 

Pincock and his co-workers have studied the photochemical fragmentation reactions of the esters (31). This system has an in-built electron accepting sensitiser. When (31a-c) are irradiated in methanol the principal reaction is fission to yield the styrene (32) and p-cyanobenzoic acid. The other products formed from the reactions are the styrene addition products (33)-(35). The authors propose that the Norrish T) e II process in this instance involves a proton transfer and this occurs within the zwitterionic biradical formed as the primary intermediate on electron transfer. Further proof of the authenticity of this mechanism was obtained by irradiation of the deuteriated derivatives (31 d, e). The results of a study of the photochemical decomposition of benzyl phenylacetate, as a suspension in water over Ti02, have been reported. Bond fission is the result of irradiation of (36) in cyclohexane/ethyl acetate. A Norrish Type II hydrogen abstraction occurs with the elimination of the enone moiety. This affords a path to the CD ring system (37) of vitamin D. 

The chromone (52) undergoes photochemical addition of ethene. The primary product from this cycloaddition, presumed to be (53), is photo-chemically reactive and is converted into (54) and (55). The former of these is a key intermediate in a synthetic strategy to tricothecene analogues. Both (54) and (55) arise via the Norrish Type II reactivity of (53). Thus hydrogen abstraction from the methoxy substituent by the excited carbonyl group results in a 1,4-biradical that either ring closes to (54) or fragments with the loss of methanal to yield the enol of (55). 

---