Radicals Generated by Homolytic Cleavage Processes Thermolysis and Photolysis

2 RADICALS GENERATED BY HOMOLYTIC CLEAVAGE PROCESSES THERMOLYSIS AND PHOTOLYSIS 

An interesting example for radicals being generated by thermolysis is exemplified in Equation 9.6 [22]. The carbon-centered radical 34 generated from 33 via thermal C—O bond homolysis reacts by intramolecular HAS to yield the oxindole 35. It should be noted that, within these reactions, 2,2,6,6-tetramethylpiperidine-l-oxyl radical (TEMPO) acts as an oxidant in the rearomatization step  

In a similar approach, quinolines 40 were prepared starting from alkoxyamine 36 and isonitriles 37 induced by TEMPO using an inter-intramolecular sequence [23] (Eq. 9.7). The precursor 36 via thermal C O homolysis and addition with aryl isonitrile 37 affords the corresponding imidoyl radical 38. Radical 38 can further react in a 5-ero-type cyclization to give the primary radical 39, which can undergo HAS and oxidation to provide the corresponding quinoline 40  

The initiating radicals (Inif) can also be generated from peroxides, azo compounds (Init-Init), etc., via thermolysis or photolysis (Eq. 9.8). A hydrogen abstraction reaction between radical Inif and a given H-donor (R-H) affords Init-H and a new radical (R ) that can initiate the radical chain reaction (Eq. 9.9)  

This procedure was employed for the synthesis of 6-substituted phenanthridine 43 from isonitriles 41 (Eq. 9.10). In this example, thermolysis of dibenzoyl peroxide (BPO) was used to generate two benzoyloxy radicals (PhCOO ), which abstracted the a-H of dioxane (Z = 0) [24] or from cyclohexane (Z=CH2) [25]. The radical 42 is produced and then intramolecular radical cyclization took place from it to finally form the 6- substituted phenanthridine 43 in good yields  

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