Microwave-assisted synthesis of new fluorescent indoline-based building blocks by ligand free Suzuki-Miyaura cross-coupling reaction in aqueous media

To obtain novel fluorescent building blocks, an efficient method for the preparation of an arylindoline unit containing heterocyclic derivatives was developed. The method is based on palladium-catalyzed, ligand-free, cross-coupling of 7-bromo- and 5,7-dibromoimidazo[1,2-a]indolone and 8-bromopyrimido[1,2-a]indolone derivatives with (hetero)aryl boronic acids. In the presence of the Pd(OAc)2 catalyst and the Cs2CO3 base, the microwave-assisted cross-coupling reactions of brominated indoline substrates with (hetero)aryl boronic acids proceeded smoothly in aqueous media. The required brominated substrates can be conveniently obtained with excellent yields by treatment of the corresponding fused heterocycles with NBS in acetonitrile. The optical properties of the novel prepared heterocyclic derivatives were studied by UV-vis and fluorescence spectroscopy. The obtained functionalized arylindoline derivatives, exhibit intense fluorescence with significant Stokes shifts and are characterized by high quantum yields.