Thermal decomposition of o-azidobithienyls

Thermal decomposition of the six possible o-azidobithienyls (1)—(6) has been investigated as a possible source of the hitherto unknown dithienopyrrole ring system. It was found that the nature and yield of products and the temperature required for decomposition are strongly dependent on the structure of the starting azides. 3-Azido-2,2′-bithienyl (1) and 3-azido-2,3′-bithienyl (2) gave 4H-dithieno[3,2-b;2′,3′-d]pyrrole (7) and 4H-dithieno[2,3-b;2′,3′-d]pyrrole (8), respectively, in very good yields. 4-Azido-3,3′-bithienyl (3) and 4-azido-2′,3-bithienyl (4) were relatively stable under the same conditions, but polymeric materials were obtained when the thermal decomposition was performed under more vigorous conditions. By contrast, 2-azido-3,3′-bithienyl (5) and 2-azido-2′,3-bithienyl (6) extrude nitrogen at room temperature resulting in ring-opening fragmentation. Thermolyses of 3-azido-2-cyclohex-1-enylthiophen (11) and 4-azido-3-cyclohex-1-enylthiophen (12) were also investigated to elucidate the different behavioural patterns exhibited by the 3-azidothienyl derivatives.