Reaction Norrish type

Predict the products of a-cleavage (Norrish type 1) reactions of given carbonyl compounds. 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

In solution, the Norrish type 1 reaction of ketones results in the non-selective free-radical combination reactions to give products AA, AB and BB in the ratio of 1 2 1, whereas photolysis of ketones in zeolites produces ... 

The first reaction in Scheme 18.1 is a Norrish Type 2 reaction. It is the only reaction where the products contain no radical species at all and hence is a onetime, only reaction with no possibility of any further direct reaction propagation. The evidence for this reaction was obtained by Day and Wiles [25] who reported that the production of carboxylic acids was too high in relation to CO2 gas evolution to be accounted for by Norrish Type 1 reactions which they had previously discussed (see below). 

Norrish type 1 reaction, named after one of the pioneers of mechanistic investigations in organic photochemistry. 

The end-chain aromatic ketone (1690 cm-1) may react photochemically by a Norrish type 1 reaction, leading to the formation of benzaldehyde (1704 cm-1) and benzoic acid (1698-1732cm-1). No detailed mechanism can be proposed for the formation of benzoic anhydride (1725-1785 cm-1) since several plausible routes exist. 

The unsaturated carbonyl reactants can also themselves be derived from photolysis of bicyclic or tricyclic ketones via a Norrish type 1 reaction. For example, irradiation of the ketone 17 gave the cyclobutenyl aldehyde 18, which on further irradiation was converted to the oxetane 19 in good yield66. 

During to the UV exposure of polyolefin the most important chromophores species are the carbonyl groups formed in presence of air [18]. These species contribute to the initiation of photodegradation reactions, produce excite states of singlet and triplet and initiate Norrish type 1 reactions (1), which lead to the scission of the main polymeric chain [19]... 

To avoid synthetic problems, we prepared aldehyde 47, a substrate in which isomerization of the C-C double bond to the endocyclic position is less likely due to the extended conjugation of the alkene moiety. Compound 47 was obtained as a pure substance and also presents a substitution pattern that, according to the above postulates, should favor the ODPM rearrangement. However, irradiation of 47, using m-methoxyacetophenone as sensitizer, led to formation of the diene 48 (38%), as a result of photodecar-bonylation (Scheme 9). No ODPM product was formed in this process.Irradiation of the corresponding methyl ketone 49, under the same conditions as used for 47, afforded the product of 1,3-acyl migration 50 in 24% yield (Scheme 9). Again, no ODPM product was formed in this instance. The formation of 48 and 50 is reminiscent of Norrish Type 1 processes. However, in these cases, homolytic bond fission does not occur in the carbonyl nit excited state, as is the case in normal Norrish Type 1 reactions. ... 

As far as we are aware, these observations show for the first time that the well-known Norrish Type 1 reactions of P,y-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. The reason for this unusual reactivity may be that the Tj (itit ) excited states of 47 and 49 have sufficient energy to promote the homolytic bond fission between the carbonyl and the a-carbon due to the stabihty of the resulting pentadienyl radical 51. As a consequence, the photodecar-bonylation path competes favorably with the ODPM rearrangement. 

Solvation Effect on Norrish Type Reactions in Pinonic Acid... 

Norrish type reactions. Type I reaction involves a-cleavage giving rise to an acyl and an alkyl radical. It is generally observed in aliphatic ketones in the vapour state and at high temperatures. The acyl radical is essentially decarbonylated at high temperatures. 

Note that Norrish-type reactions are not only of importance in relation to various polymers containing ketonic impurities, but they also play a dominant role in the photolysis of all polymers containing carbonyl groups as constituent moieties, such as polyacrylates, polymethacrylates, poly (vinyl acetate), polyesters, and polyamides. 

As previously discussed in the introductory chapter, the Norrish type reactions account for photodecomposition of polymers with carbonyl moieties, such as polyacrylates, polyesters and polyketones, and through abstraction of the hydrogen atom [21, 22]. After forming of free radicals through Norrish I reactions, photodegradation further proceeds via autoxidation. 

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. 

A Norrish type I reaction for side-chain carbonyl functionahty is equation 7, and a Norrish type II reaction for side-chain carbonyl functionahty is equation 8. 

Norrish type I chemistry is claimed to be responsible for about 15% of the chain scission of ethylene—carbon monoxide polymers at room temperature, whereas at 120°C it promotes 59% of the degradation. Norrish I reactions are independent of temperature and oxygen concentration at temperatures above the T of the polymer (50). 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

Since the quantum yield of the Norrish type I reaction is generally low, it has been assumed that the initial homolytic cleavage is a reversible process. Evidence came from an investigation by Barltrop et al. which has shown that erythro-2,3-dimethylcyclohexanone 12 isomerizes to t/zreo-2,3-dimethylcyclohexanone 13 upon irradiation ... 

As a side reaction, the Norrish type I reaction is often observed. The stability of the radical species formed by a-cleavage determines the Norrish type 1/Norrish type II ratio. For example aliphatic methyl ketones 10 react by a Norrish type II-mechanism, while aliphatic tcrt-butyl ketones 11 react preferentially by a Norrish type I-mechanism. 

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. 

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... 

Reactions are known where both Norrish Type I and Norrish Type II reactions compete, and the substituents on and nature of the substrate will determine which leads to the major product." ... 

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

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