Norrish II process

Reaction Cavities of Alkanophenones in Neat Solid and Liquid-Crystalline Phases. As mentioned previously, solid-state studies on the Norrish II processes of alkyl aryl ketones are unambiguous with respect to the triplet multiplicity of the reactive excited state. On the other hand, a bulky aryl auxochrome can create complications during the transformation of the excited triplet states to photoproducts in neat anisotropic phases. 

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. 

The E-anomeric hydrogen of compound 74 is too far away to permit abstraction and the Norrish II process results from the y-hydrogen abstraction of the aglycone chain. In this way, O-vinylglycosides 75 can be formed [74] (Scheme 39). 

This restriction does not apply to the Norrish II process which does not involve radical products. 

It is significant therefore that the Norrish II process predominates until the onset of oxidative crystallisation which also requires mobility of the polymer segments to permit relaxation reallignment of the polymer chains. 

The majority of the work on the fundamental processes of photodegradatlon of acrylic polymers has been carried out by Grassle and co-workers (5-7). For radiation with wavelength X > 300.0 nm photodegradatlon is initiated by absorption of a photon by the carbonyl group of the ester side chains. Subsequent fragmentation depends on local stereochemistry and temperature. The principal modes of fragmentation are described by the Norrish I and Norrish II processes ( ). 

Hence the Norrish II process leads to formation of an alkene [3(a)] or chain scission [3(b)], The fate of the pol3nneric macro-radical depends on the sequence distribution of the copolymer. Consider the macroradlcal ... 

CO and abstraction or recombination products of alkyl fragments derived from the ester side chain fragments The Norrish II process is unimportant for short chain esters, (e.g. methylmethacrylate), for longer chain esters it should be considered. 

Scheme 31. Application of the Norrish II process to the oxidation of carbohydrates...

The intramolecular abstraction of a hydrogen atom from the y-carbon of a pho-toexcited carbonyl group is called the Norrish II process (Scheme 31). This reaction has been applied to the oxidation of the anomeric carbon by irradiation of 2-oxo propyl glycoside 187 [118] or glycosyl pyruvic esters 188 [119], Binkley [119] similarly prepared the kojic acid diacetate 189 by photochemically induced oxidation of the glucosyl pyruvic ester 190. 

Oxime esters of aliphatic acids (15) have been used as two-stage photobase generators for lithography in microelectronies with improved image quality. The involved photoreaetion is a Norrish II process with concomitant aromatisation. ... 

Even with no unpaired electrons present, even-electron ions can undergo reactions thought to be characteristic of a radical site, even at sites remote from the charge (Gross 1988). Note that similar reactions occur in organic chemistry y-H/)9-cleavage rearrangements of neutral molecules take place in both closed-shell (retro-ene reaction) and open-shell (Norrish II process) systems, while these and p-H decompositions occur thermally in esters, xanthates, sulfoxides, and carbamates. For the EE" ions of Equation 8.35 (Kraft and Spiteller 1967),... 

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