Synthesis and stereochemical properties of axially chiral benzo[1,2-b:4,3-b’]dithiophene derivatives

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 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 (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. This strategy provides a convenient route to an interesting class of chiral atropisomeric heterobiaryl derivatives with C2-symmetry, which can be selectively functionalized into bromides 3. The chiroptical properties of compounds 2 and 3 have been fully elucidated by experimental and theoretical studies. Bromides 3 are expected to have potential applications in asymmetric reactions, including the enantioselective synthesis of tetrathiahelicene derivatives.