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.

The Cu(I)-catalyzed N-arylation reaction of indole (or carbazole) with 6-bromo-1,3-dimethyl-1H-perimidin-2(3H)-one utilizing diamine ligands was unexpectedly accompanied by 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). π-Excessive aryl bromides were found to be suitable substrates for a similar transformation. In contrast, 1-bromonaphthalene and π-deficient 4-bromo-N-butyl-1,8-naphthalimide exhibited reactivity of another type, affording a product of double N-arylation of 1,2-diamine. The X-ray structures and 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 showed strong solvatochromic and pH-dependent fluorescence, with quantum yields ranging from 0.45 to 0.97.