Norrish II reaction

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. 

In Section VIII we apply the same model in greater detail to Norrish II reactions of ketones. By doing so, a set of similar mechanistic criteria can be viewed in many constraining environments, allowing a more systematic picture of the model to be drawn. 

To place in perspective the Norrish II reactions of ketones in constrained... 

There has been a tendency to call both the E and C pathways as Type II or Norrish II reactions since it is well established that they emanate from common intermediates. Wagner has suggested that the C pathway be called more appropriately the Yang reaction [255]. Whether this is followed will depend upon the tolerance of photochemists for the proliferation of name reactions. We group the two processes together while recognizing the fundamental work of Yang. 

The major features of the Norrish II reactions which are germane to this chapter are included in Scheme 41. Note that each structure in Scheme 41 represents a family of conformers which are related by similarities in both structure and reactivity. A ketone molecule in collision-free space can exist in a variety of conformations produced by rotation around single C—C bonds. In a linear alkanone, the conformation of lowest energy is all-trans although local minima can exist where there is one or more gauche arrangements. 

The influence of these various effects may be manifested in measurable parameters of the reaction like the overall quantum yields (On) and the photoproduct ratios for fragmentation to cyclization (E/C) and for trans to cis cyclobutanol formation (t/c) as shown in Scheme 41. The values of these quantities and their variations as the media are changed can provide comparative information concerning the relative importance of solvent anisotropy on Norrish II reactions, also. Specifically, they reveal characteristics of the activity of the walls and the size, shape, and rigidity of the reaction cavities occupied by electronically excited ketones and their BR intermediates. 

Another factor which should influence only minimally Norrish II reactions in fluid isotropic media is the size and shape of the photoproducts relative to each other and to the reactant ketone. However, in media that provide reaction cavities with stiff walls, this factor may be of paramount importance. 

As shown in Figure 42 for the Norrish II reactions of a simple ketone, 2-nonanone, not only do the shapes of the products differ from those of the reactant, but so do their molecular volumes [265]. Interestingly, the volume of the fragmentation products, 1-hexene and 2-hydroxypropene (which ketonizes to acetone), are closer in volume to 2-nonanone than is either of the cyclization products. They are also capable of occupying more efficiently the shape allocated by a stiff solvent matrix to a molecule of 2-nonanone in its extended conformation the cross-sectional diameter of either of the cyclobutanols is much larger than that of extended 2-nonanone or the fragmentation products when spaced end-on. Both of these considerations should favor fragmentation processes if isomorphous substitution for the precursor ketone in the reaction cavity is an important requirement for efficient conversion to photoproducts. 

VIII. NORRISH II REACTIONS IN ORGANIZED MEDIA A. Neat Crystalline Phases... 

Scheffer, Trotter, and co-workers have provided elegant demonstrations of the distance and angular dependence between a carbonyl oxygen atom and a gamma hydrogen atom on the ease of initial abstraction [276], Their singlecrystal X-ray structural information allows the various courses of Norrish II reactions of neat solids to be understood in the context of topochemical control [11,13]. For instance, they have noted that the cyclic diones 77 (n = 7,8,10, and 12) follow different Norrish II pathways depending upon the conformations of the individual molecules in their crystals (Eq. 9) [277]. 

An even more dramatic example of the potential lack of selectivity afforded to the Norrish II reactions of ketones by supposedly very ordered systems than that described in the 76 systems is provided by neat samples of the mesomorphic alkanophenones (81) [278]. These molecules are capable of existing in nematic and smectic B mesophases (see Figure 16) as shown in Scheme 42. The instability of the monotropic smectic B phase of 81a and smectic B phase of 81b did not allow their photoreactions to be examined these smectic phases became solids soon after the initiation of irradiation. 

However, the monotropic nematic phase of 81a was sufficiently stable to allow its Norrish II reactions to be examined. Similarly, Norrish II product ratios from irradiation of the smectic B phases of 81c and 81d were easily measured. Although the packing of molecules in the solid phases of the 81 homologues is unknown, inferential evidence supports their being layered also. 

Several interesting examples of solid inclusion complexes with ketone guests which undergo the Norrish II reactions have been examined. They illustrate the breadth of reaction cavity types and resultant selectivities that can be expected in such systems. 

A similar study of Norrish II reactions has been conducted on complexes of aryl ketones in Dianin s compound 1 [295], a nonpolar host whose channels are effectively truncated at each 11A of length by a 2.8-A constriction from 6 hydrogen-bonding hydroxyl groups (see Figure 3) [296]. Table 13 summarizes the results from complexes with ketones expected to undergo primarily the Norrish II reactions [297]. As befits the rather large (and mostly) nonpolar reaction cavities, the E/C and t/c ratios in Table 13 provide evidence for relatively little control by the channels of Dianin s compound over the fate BRs. Even in the most selective case from 5-methyl-... 

Although all the products can be rationalized on the basis of 7-hydrogen abstraction followed by cyclization or rearrangement-cyclization of 1,4-biradical intermediates, the mechanism has been shown to involve analogous zwitterionic intermediates [299b, 301], Although they are not strictly Norrish II reactions, transformations of 98 will be considered so for the purposes of discussion since the photoproducts and the mechanisms of their formation are very similar to those expected of Norrish II processes. 

In another set of surfactant systems comprised of 50% potassium stearate (KS) and water, 50% potassium palmitate (KP) and water, and 50% 1 1 KS/l-octadecanol (KSO) and water, the Norrish II reactions of the homologous series of 2- and sym-alkanones (2-105 and s-105, respectively, with n as the total number of carbon atoms in the alkanone) have been investigated [309]. 

This assertion is borne out by the small deviations from fluid-isotropiclike values of the photoproduct ratios found in the Norrish II reactions of the two alkyl perfluoroalkyl ketones 76 (with m, n = 7,8 and 9,10) in the (macroscopically) highly ordered smectic phases of F10H10 [273,274]. As... 

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). 

The photocyclization reaction is more efficient with p-anomers, and the new C—C bond created at the anomeric center is obtained by preferential axial anomeric hydrogen abstraction followed by carbocyclization. Norrish II reactions generally result from hydrogen abstraction at the y-position according to Scheme 31 to give cyclobutanols or degradation products. For 68, (n = 1) photoabstraction at the 5-position of the anomeric hydrogen leads to cyclopentanols 69 (Scheme 37) [67]. 

The previously described Norrish II reaction is used in the photolysis of pyruvate esters of carbohydrates and nucleosides [90-94]. The presence of different protecting... 

In the case of polyamides and polyesters the most important photolytic reactions are the Norrish I and II reactions (see Scheme 1). The Norrish I reaction leads to chain cleavage and radicals that might initiate oxidation, the Norrish II reaction only leads to chain cleavage. The main question for these polymers is What is the relative importance of photolysis and photo-oxidation ... 

A second obvious problem with the ordinary definition of adiabatic reactions is the vagueness of the term product. If the product is what is actually isolated from a reaction flask at the end, few reactions are adiabatic. (Cf. Example 6.7.) If the product Is the first thermally equilibrated species that could in principle be isolated at sufficiently low temperature, many more can be considered adiabatic. A triplet Norrish II reaction is diabatic if an enol and an olefin are considered as products. It would have to be considered adiabatic, however, if the triplet 1,4-biradical, which might easily be observed, were considered the primary photochemical product. (See Section 7.3.2.)... 

On irradiation with ultraviolet light, the activated ketone groups present can take part in two different types of free radical, bond-breaking reactions. In organic photochemistry, these two reactions are referred to as Norrish I and Norrish II Reactions, and their mechanisms are shown below for the degradation of copolymers of ethylene and carbon monoxide [46, 47] ... 

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