Photochemical reactions Norrish

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. 

In some photochemical reactions both Si and T take part in the reaction. For example, both the Si and Tt states of aliphatic ketones, such as hexan-2-one, take part in Norrish type 2 reactions ... 

The photochemical reaction that has been most thoroughly investigated from the ionic chiral auxiliary point of view is the well-known Norrish/Yang type II reaction. One example, taken from the work of Patrick, Scheffer, and Scott [36], deals with derivatives of 7-methyl-7-benzoylnorbornane-p-car-boxylic acid (37a, Scheme 8). This compound was treated with a variety of optically pure amines to afford the corresponding 1 1 salts (37b), and in an... 

A second example from Salem s work which illustrates the role of symmetry is the photochemical hydrogen abstraction by ketones, termed the Norrish type II reaction (Norrish, 1937). A typical case is illustrated in (69). 

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. 

A conceptual model which is the centerpiece of this chapter is developed in Section III. This is preceded (Section II) by a brief introduction to various organized media. The validity and generality of the model is examined by two approaches. In the first (Sections IV-VI), selected photochemical reactions belonging to various classes and chromophores are presented as supporting examples. In the second (Sections VII and VIII), a critical reevaluation of the results reported on Norrish II reactions in a number of organized media is made on the basis of the model. However, although we examined the literature examples on the basis of our model, we often have deviated from the initial explanations offered by the authors. 

Intramolecular photochemical reactions need the presence of light sensitive groups such as carbonyls to induce transformations by oc-cleavage (Norrish I reaction) or by y-H-abstraction (Norrish II reaction). 

Photochemical procedures have also been reported. For instance, some phenacyl glycosides undergo a Norrish type II photochemical reaction to give lactones on photolysis with pyrex-filtered UV light.24... 

The Norrish-Yang reaction [63] is a widely used photochemical reaction, which consists of an intramolecular hydrogen abstraction by a photoexcited ketone, followed by carbon-carbon bond formation in the (l,n)-biradical intermediates (Scheme 9.38). 

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],... 

As discussed in Sec. 3.3.1.4, besides ketones also imides can be used as reactants in the Norrish-Yang reaction. For clarity, photochemical reactions of imides, which are initiated by a hydrogen abstraction both... 

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 the context of chiral induction in photochemical reactions, the Norrish-Yang cyclization, which has been extensively studied in isotropic and anisotropic media, has been examined. Both cis and trans cyclobutanols exist as pairs of enantiomers (Scheme 36). These reactions are good candidates for photoreactions inside zeo-... 

The topochemical principles have also been applied to the interpretation of mframolecular photochemical reactions of polymorphic materials depending on polymorphic form. 1,14-cyclohexacosanedione 6-XLII can be prepared as conformational dimorphs that undergo Norrish type II photochemistry which can be correlated with the molecular conformations in the two modifications, one leading to a cw-butanol, while the other leads to a trans product (Gudmunsdottir et al. 1996). Similarly, these principles can be invoked to understand the relative photochemical stability of two polymorphs of the active ingredient in a compound developed for the treatment of psoriasis (Lewis 2000). 

In the thermolysis of cyclohexanone virtually all of the products arise from the (8-cleaved structure 5.23) Jwo effects are important here. In the ion the a-cleavage structure is specifically stabilized (see below) and the jff-cleavage structure 9, would be specifically destabilized by the heteroatom which is at an active site in the odd alternant n system associated with the carbonyl. The primary photolysis products of cyclohexanone 24) are much more closely correlated to its mass spectrum than the thermolysis products are. This is because n- n excitation can be relaxed by an a-cleavage (Norrish type I) analogous to 7. The analogy for the photochemical reactions is, however, far from perfect because of the stabilizing effect of the oxygen atom on the even alternant even electron ions, e.g. 10. The photolysis of... 

Wagnerhas reviewed the influence of conformational flexibility on the outcome of photochemical reactions, especially hydrogen abstraction reactions of the Norrish Type II kind and related processes. Kinoshita and Naito have published a short review dealing with 1,4-biradicaIs in the Norrish Type II reaction. A study of Norrish Type II degradation in polymers has been reported. ... 

Ritchie and Norrish introduced photometry into the study of the photochemical reaction of H2 and CI2 by measuring with 3650 A light the time dependence... 

Khan, Norrish, and Porter decomposed acetaldehyde by means of flash photolysis. The rate of light absorption was about 10 photons/ cm. sec. Temperature rises were controlled to be between 3 and 190°C. The reactant concentration was about 10 molecules/cm., and the rate constant was about 10 . Thus the product kA is about 10 . The intersection of log kA = —I and log J = 21 is deep inside the non-chain region of Table I. Thus essentially all products are expected to arise from radical-radical reactions, or from primary photochemical reaction. The observed ratio of methane to ethane was slightly greater than two, indicating the primary process... 

Stiver and Yates have studied the photochemical reactions of some hydroxy-keto steroids (28, 29). Irradiation of the isomeric compounds (28a, 29a) showed that the products obtained, (30) and (31) respectively, had retained the configuration of the carbon to which the hydroxy group is attached. The use of deuteriated derivatives (28b, 29b) has identified the hydrogen abstraction processes involved in the conversion of these ketones into the lactones (30b) and (31b) respectively. The authors " propose that there are two major factors which control the stereospecificity of the reactions. These are the shape of the hydroxy-bearing C-atom and the hydrogen transfer within the biradical formed on Norrish Type I fission. The stability of the biradical intermediate clearly plays an important part in determining the outcome of the reactions. 

In addition to the normal photochemical reactions of saturated ketones, p,y-unsaturated carbonyl compounds undergo carbonyl migration via a [1,3] shift. Compound 139 in Scheme 45 represents a typical example. Compounds with an alkyl substituent in the P position such as 140 may also undergo a Norrish type II reaction (Kiefer and Carlson, I%7), while for ketones with electron-rich double bonds such as 141, oxetane formation is also observed (Schexnayder and Engel, 1975). 

A detailed study of the photochemical reactions of the ketones (38) and (39) in the solid phase has been reported. " Both of these systems readily undergo Norrish Type II hydrogen abstraction in solution and it was this fact that attracted the authors to the systems. One of the facets of the work focussed... 

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