Oxidative Dimerization of Quinolic Nitroxides in the Presence of Trichloro- and Trifluoro-Acetic Acid. Crystal Structures of 6,6'-bis-(1-oxide-1,2,6,8a-tetrahydroquinoline)ylidene and of 2,3-Diphenylquinoline

Quinolinic nitroxides 1a-c react with trichloro- (TCA) and trifluoro- (TFA) acetic acid to give dimers 3a-c and quinolines 4a-c as the main products. Products 3a-c are explained as arising via the intermediate formation of the radical cation 5, which forms the final dimer through an oxidative dimerization promoted by the TCA or TFA. The formation of products 4a-c is explained by a Wagner-Meerwein type transposition of the phenyl group from C-2 to C-3. The disproportionation, which generally occurs in the presence of acid for nitroxides having the NO function conjugated with a π-system, has been excluded even though quinoneimine N-oxides 2a-c were isolated. The structure of compounds 3a and 4a were determined by crystal X-ray analysis.