tert-Butyl Hydroperoxide and Tetrabutylammonium Iodide-Promoted Free Radical Cyclization of -Imino-N-arylamides and -Azido-N-arylamides

The oxidizing system of tert-butyl hydroperoxide (TBHP) and tetrabutylammonium iodide (TBAI) is capable of generating -(arylaminocarbonyl)iminyl radicals from ethyl 2-(N-arylcarbamoyl)-2-iminoacetates. These iminyl radicals preferably undergo intramolecular ipso attack on the benzene ring to give azaspirocyclohexadienyl radicals, which are readily captured by molecular oxygen under an oxygen atmosphere to yield azaspirocyclohexadienones. In the absence of oxygen, the reaction affords quinoxalin-2-one products. This oxidizing system is also effective to convert -aryl--azido-N-arylamides to the corresponding iminyl radicals under basic conditions (sodium tert-butoxide, t-BuONa), and the subsequent cyclization of these iminyl radicals results in the formation of azaspirocyclohexadienone products in high yields under an oxygen atmosphere. Plausible mechanisms are proposed to rationalize the experimental results, and factors influencing the reactions are discussed.