Norrish Type processes

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 ... 

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

When applied to ketones, this is called Norrish Type / cleavage or often just Type I cleavage. In a secondary process, the acyl radical R —CO can then lose CO to give R radicals. Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the 0—0 bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R. The latter is an important source of radicals R , since the other product is the very stable N2. 

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

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. 

First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrish-type I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after a-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the investigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical. 

If the intersystem crossing process is efficient at this excitation, then the Norrish type II rearrangement must occur from the triplet state. This is further substantiated by a reduction in loss of tenacity with increasing concentration of triplet state quencher. The reduction in loss of tenacity may be equated with interruptions of the chain scission process(es). 

Scheme 5.22. Domino Wolff/Cope rearrangement/Norrish type I fragmentation/recombination process.

Das zur Diskussion stehende 7t, n-angeregte Keton geht eine a-Spal-tung (Norrish Type I Process n>) zum entsprechenden 1,5-, 1,6- oder 1,7-Alkyl/Acyl-Biradikal ein, das... 

The process of intramolecular abstraction of an hydrogen atom from the y carbon atom by the excited carbonyl group is commonly referred to as Norrish type II reaction. The resulting diradical can close to a cyclo-... 

The end result of these studies showed very clearly that two major processes were important, i.e. photolysis and photo-oxidation. Photolysis reactions were posited to be the result of the well-known Norrish Type 1 and Norrish Type 2 cleavage reactions. As we shall see, the Type 1 cleavage followed by several subsequent reactions can account for many of the observed degradation products. 

The Norrish type I process is not important for the photolysis of diaryl ketones. A reason suggested... 

Now aryl free radicals are extremely unstable, they are not stabilised by resonance. Since the stability of a product can act as a driving force for the reaction to proceed along that path then we can say that the more unstable the product is, the less likely that path will be followed. Hence Norrish type I process is energetically unfavourable for diaryl ketones. 

Norrish type-I reaction, has been studied over the years in extreme detail, with every imaginable physical and theoretical method at hand. Data gathered through studying such reactions on the femtosecond time scale, together with new theoretical work prompted by the dynamics observed, have provided a detailed picture of the processes involved and a fresh perspective on nonconcerted ot-cleavage events. 

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. 

A recent development in the synthesis of 2-benzazepine-l,5-diones is the photoaddition of alkenes to A-alkylphthalimides. A-Alkylphthalimides capable of undergoing a Norrish Type II process follow the route outlined in Scheme 35 (path a) to yield hydrobenz-azepinediones (264) by ring opening of the azacyclobutanol (262) (78ACR407). 

---