Axially chiral benzo[1,2-b:4,3-b’]dithiophene derivatives as key intermediates for enantiopure tetrathia[7]helicenes

Thiophene-containing fused aromatic compounds are an interesting class of π-conjugated systems with applications in functional organic materials.1 Among them, benzo[1,2-b:4,3-b’]dithiophene (BDT) and its derivatives are widely studied, for instance as units in mono and polydisperse oligomers in the field of materials science,2 and as π-spacers in push-pull organic chromophores for photovoltaic applications.3 Furthermore, BDT is a key intermediate for the synthesis of inherently chiral helical systems such as tetrathia[7]helicenes.4 Thus, BDT can be identified as a key starting molecule, which can allow access to more complex and interesting systems through a judicious functionalization of the α-position of the thiophene rings. In our ongoing studies on the synthesis and functionalization of BDTs,5 we have developed a novel synthesis to prepare systems 2, starting from bromides 1 (Figure 1). Compounds 2 are an interesting class of chiral atropisomeric heterobiaryl derivatives with C2-symmetry, which can be selectively functionalized into bromides 3, starting reagents for an innovative non-photochemical synthesis of tetrathiahelicenes through Pd-catalysed annulation with internal alkynes as key step. Asymmetric versions of this synthesis is under study thanks to the chiroptical properties of 3, which represent useful intermediates for the enantioselective synthesis of the corresponding tetrathiahelicene derivatives.