The three-dimensional structure of carbohelicenes has fascinated generations of molecular chemists and has been exploited in a wide range of applications. Their strong circularly polarized luminescence has attracted considerable attention in recent years due to promising applications in new optical materials. Although the enantioselective synthesis of fused carbo- and heterohelicenes has been achieved, a direct catalytic enantioselective method allowing the synthesis of lower, non-fused carbo[n]helicenes (n = 4–6) is still lacking. We report here that Pd-catalysed enantioselective C–H arylation in the presence of a unique bifunctional phosphine-carboxylate ligand provides a simple and general access to these lower carbo[n]helicenes. Computational mechanistic studies indicate that both the C–H activation and reductive elimination steps contribute to the overall enantioselectivity. The observed enantio-induction seems to arise from a combination of non-covalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. The photophysical and chiroptical properties of the synthesized scalemic [n]helicenes have been systematically studied.

The unique three-dimensional structure of carbohelicenes have fascinated generations of molecular chemists and has been exploited through a wide range of applications. In particular, their strong circularly polarized luminescence (CPL) has raised much attention in recent years due to promising applications in the design of new optical materials. Whereas a number of important precedents report enantioselective syntheses of fused carbo- and heterohelicenes, a direct catalytic enantioselective method allowing the synthesis of lower, nonfused carbo[n]helicenes (n = 4-6) is still lacking. We report that Pd-catalysed enantioselective C–H arylation in the presence of a unique bifunctional phosphine-carboxylate ligand provides a simple and general access to these simple carbo[n]helicenes. Computational mechanistic studies indicate that both the C–H activation and reductive elimination steps contribute to the overall enantioselectivity. In addition, the observed enantio-induction seems to arise from a combination of noncovalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. Moreover, the current method allows a comparative study of the CPL properties of lower carbo[n]helicenes, which led to the discovery that carbo[4]helicenes actually display CPL responses comparable to the higher carbo[6]helicene congeners.