A-cleavage, of carbonyl

Two of the three general types of secondary reactions resulting from photochemical a-cleavage of carbonyls, namely molecular rearrangement and hydrogen transfer to yield aldehydes or ketenes, have been discussed. The third type of reaction observed, decarbonylation, will be discussed in this section. The discussion will begin with the decarbonylation of small ring carbonyls. By way of example of this type of reaction, diphenylcyclopropenone decarbonylates upon photolysis to yield diphenylacetylene(57) ... 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

All experimental observations with respect to these reactions, including the dependence on neither orbital character nor spin multiplicity of the excited state (which is in striking contrast to a-cleavage of carbonyl compounds) and the influence of substituents of the aryl moiety on the reaction rate, could be rationalized by correlation diagrams that were obtained on the basis of semiempirical MO calculations [105]. /1-Cleavage of phenoxyketones as well as photo-Fries and photo-Claisen rearrangements were characterized to be no photodissociations. 

Type A photorearrangement, 323, 329, 331 Type I cleavage of carbonyls, 135-171 application of extended Hiickel theory to, 179-181... 

A. Cleavage of Radical Anions of Diiron Carbonyl Dimers... 

This explains, for example, the tendency of some 1,2-diols to suffer oxidative carbon-carbon bond breakage under the action of PDC. Thus, although many 1,2-diols can be uneventfully oxidized to a-hydroxyketones with PDC,176 very often a cleavage of a carbon-carbon bond occurs, resulting in two carbonyl functionalities.177 Vicinal tertiary diols, sometimes, are smoothly oxidized to diketones by PDC.178... 

Adam W, Arnold MA, Saha-Moller CR (2001) Photooxidative damage of guanine in DG and DNA by the radicals derived from the a cleavage of the electronically excited carbonyl products generated in the thermolysis of alkoxymethyl-substituted dioxetanes and the photolysis of alkoxyac-etones. J Org Chem 66 597-604... 

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