Barton esters photochemical fragmentation

Other pathways of radical generation involve thermal or photochemical fragmentation of perfluoroacyl peroxides [14] or photochemical fragmentation of perfluoro-alkylsulfonyl bromides [15]. The in situ formation of Barton esters from perfluoroacyl chlorides and thiopyridone-N-oxide has also been used as a convenient source of radicals [16] (Scheme 2.99). 

Oxidation Reactions. Hypoiodite intermediates may be generated from the reaction of simple alcohols with NIS. When conducted under photochemical irradiation, the products of Barton-type or fragmentation reactions of alkoxyl radical intermediates may be obtained. Aldehydes are oxidized to methyl esters via hemiacetal intermediates by reaction with NIS in methanol at rt. However, such conditions are not effective for the oxidation of simple alcohols. The combination of NIS and Tetrabutylammonium Iodide in dichloromethane has been developed for the oxidation of a variety of alcohols to the corresponding carbonyl corrpounds (eq 8). This reagent system is most widely used for the oxidation of lactols to lactones, in which near-quantitative yields are generally obtained under mild conditions (eq 9). ... 

Tin Hydride-Mediated Reaction Allyltin Method Photochemical Fragmentation of Barton Esters Direct Photolysis of Cobaloximes Direct Photolysis of Organotellurium Compounds Miscellaneous... 

Free radicals generated under photochemical conditions from thiohydroxamates add readily to the NN double bond of diaziridines and in particular to the 3-bromo or 3-trifluoromethyl-3-phenyldiazirine to produce diaziridinyl radicals (Scheme 25) these intermediates undergo dimerization and fragmentation to yield the corresponding imines, which can be hydrolyzed to the corresponding amines. Barton esters can also be used for the direct preparation of nitroso compounds. From a synthetic standpoint, the yields are moderate and the method is limited to primary and secondary carboxylic acids, which form dimers as an end product. Tertiary nitroso compounds do not dimerize and further react with the radical present under the reaction conditions. 

There are a variety of photochemical reactions involving free nitrous oxide (NO) as persistent radical. Often there is an initial fragmentation, as presented in Scheme 6 for TV, 7V-dimethyl-/V-nitro-samine. One example is the Barton reaction of nitrite esters (Scheme 15). It allows the functionalization of methyl groups in steroids and utilizes an intermediate 1,5-hydrogen atom migration, which converts the initially formed oxygen-centered radical to a carbon-centered species.69... 

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