Unexpected pyrazine ring annulation in the Cu-catalyzed reaction of aryl bromides with 1,2-diamines. Synthesis of novel pyrazino[2,3-f]perimidine fluorophores

Abstract

Cu(I)-catalyzed N-arylation reaction of indole (or carbazole) with 6-bromo-1,3-dimethyl-1H-perimidin-2(3H)-one utilizing diamine ligands is unexpectedly accompanied with the pyrazine ring annulation. The optimal conditions and scope of the latter process were studied. It was shown that in the presence of copper catalysts, 6-bromo-1,3-dimethyl-1H-perimidin-2(3H)-one reacts with aliphatic and aromatic 1,2-diamines, forming polynuclear pyrazine-fused compounds (pyrazino[2,3-f]perimidine derivatives, their benzo and naphtho analogs). pi-Excessive aryl bromides were found to be suitable substrates for similar transformation. On the contrary, 1-bromonaphthalene and pi-deficient 4-bromo-N-butyl-1,8-naphthalimide demonstrate reactivity of another type, affording a product of double N-arylation of 1,2-diamine. The X-ray structures, UV-vis and emission spectra of the synthesized compounds were investigated. Novel 6-hetaryl-1,3-dimethyl-1H-perimidin-2(3H)-ones and pyrazino[2,3-f]perimidine derivatives display strong solvatochromic and pH-dependent fluorescence, with quantum yields ranging from 0.45 to 0.97.