Metal-Catalyzed Cross-Coupling Reactions for the Synthesis of Functionalized 1,4,5,8-Tetraazafulvalenes

Abstract

Tetraazafulvalenes can be cross-coupled (palladium-catalyzed) with alkenes or acetylenes. Although Suzuki conditions fail, the Heck and the Sonogashira methods are useful tools for functionalizing tetraazafulvalenes. Starting from the tetraiodoaryl derivative 2b, four arylic iodine atoms could be replaced by acrylic acid esters, various styrenes or cylohexenes thus furnishing the new heterofulvalenes 4a-d. Under analogous conditions, the tetraaldehyde 4e could be obtained by employing allylic alcohol as the coupling partner. Cross coupling reactions using the Sonogashira method yielded a series of highly functionalized tetraazafulvalenes 6a-p, 7a-d. In an analogous manner, deeply colored pyrazino-fused tetraazafulvalenes 9a-c as well as imidazo-fused derivatives 10b-d, which show strong fluorescence, were prepared. First steps towards dendritic heterofulvalenes have been realized as illustrated by the synthesis of derivative 6q.