Axially chiral benzo[1,2-b:4,3-b’]dithiophene derivatives: a new route to tetrathiahelicenes

Thiophene-containing fused aromatic compounds represent an interesting class of π-conjugated systems in functional organic materials [1]. Among them, benzo[1,2-b:4,3-b’]dithiophene (BDT) and its derivatives are by far the most widely studied, especially as units in mono and polydisperse oligomers in the field of the materials science [2], and as π-spacers in push-pull organic chromophores for photovoltaic applications [3]. Moreover, BDT is a key intermediate for the synthesis of inherently chiral helical systems such as tetrathia[7]helicenes 3 [4]. For all these reasons, BDT can be identified as a key starting molecule that, through a judicious functionalization of the α-positions of the thiophene rings, can allow access to more complex and interesting systems. Exploiting the experience acquired in our laboratories on the synthesis and functionalization of BDT derivatives [5,6], we have studied a novel and simple synthetic route to prepare bis(benzo[1,2-b:4,3-b’]dithiophene) systems 2, through Pd-catalyzed cross coupling reactions, starting from bromides 1 (Figure 1).This strategy provides a convenient route to an interesting class of chiral atropisomeric heterobiaryl derivatives 2 with C2-symmetry, which can be used as starting reagents for an innovative non-photochemical synthesis of tetrathiahelicenes exploiting a Suzuki-Miyaura cross coupling and a Pd-catalyzed annulation with internal alkynes as key steps.. Asymmetric versions of this synthesis is under study.