Aromatization-driven Grob-fragmentation approach toward polyazaacenes. Synthesis and amination of multi-functionalized diazapentacenes

A general approach toward the synthesis of multi-functionalized diazapentacene derivatives 1, using 1,2,3,4-tetrachloro-5,5-dimethoxycyclopentadiene (TDCp, 2), a substituted benzene-1,2-diamine (ADA, 6), and a naphthalene-1,4-dione (BQ, 3) as the building units, is described. The synthesis basically entails three operations: (i) oxidation of the dichloroetheno-bridge in the Diels-Alder cycloadduct 4 of TDCp and 3, (ii) condensation of the 1,2-diketone 5 thus generated with an ADA to give quinoxaline-fused polycyclic compounds 7, followed by (iii) an one-pot, three-reaction process keyed upon the base- or acid-catalyzed aromatization-driven Grob-type fragmentation to produce quinoxaline ring-embedded diazapentaceneesters 1. The diazapentacene derivative 1a underwent the nucleophilic aromatic ipso-amination with primary and secondary amines to afford the amino-substituted derivatives 12, which tend to self-assemble in solid state driven by the cofacial π-stacking interactions, demonstrated by the crystal packing structures of 12a and 12f.