An Efficient Synthesis of Tetraazapentacenes

摘要

Azapentacenes, or better their N,N-dihydrocompounds, have been known since the end of the 19th century and they were first synthesized by Fischer and by Hinsberg. Hinsberg's synthetic approach, that is, condensation of an aromatic diamine with a suitable catechol or an activated ortho-dihaloarene (1,2-dichloroquinoxaline or 1,2-dichloropyrazine) is still used today. The starting materials are reacted in the melt at 130-160 °C to give the desired dihydrodiazaacenes, such as 3, in surprisingly good yields. This method is however unsuitable for the pair 7 plus 2, as substituents ortho to the amino groups apparently severely curtail reactivity. The condensation of 4 with 5a,b gives the desired dihydrodiazapentacenes, however in relatively low yields (10-23%). Oxidation with MnO2 quantitatively transforms these products into the diazaacenes 6a,b.