New synthesis of 1-bromo benzo[1,2-b:4,3-b’]dithiophene through photochemical cyclization

Thiophene-containing polycondensed aromatic compounds are important source of functional organic materials1 for different applications and some of them like benzo[1,2-b:4,3-b0]dithiophene (BDT) and its derivatives are also key intermediates in the synthesis of inherently chiral helical systems such as tetrathia[7]helicenes (7-TH).2 These molecules, that are object of extensively research in our group,3 are extremely interesting conjugated systems with unique physicochemical properties provided by their helix-like structure. The search for new general and versatile synthetic strategies for the preparation of heterohelicenes is still a relevant target and recently a new synthesis of 7-TH systems has been carried out in our laboratories. The key steps for this new approach (Scheme) is the final ring closure to form the helical structure through a Suzuki type coupling with alkynes, which allows the preparation of a series of 7,8 substituted tetrathia[7]helicenes starting from a unique intermediate 3 by simply varying the alkyne counterpart. In this scheme the β-Br-BDT 1 (that is unknown in the literature) is a key starting compound but its preparation is not trivial as the direct bromination of the parent BDT gives the corresponding α-isomer. We have setup alternative synthesis for BDT 1 one of these involve the oxidative photocyclization of the parent brominated alkene. As halogens are not always stable in photochemical conditions, a systematic study of this reaction has been carried out to select the best conditions allowing the easy preparation of the target bromo BDT 1.