A C-H Activation-Based Enantioselective Synthesis of Lower Carbo[n]helicenes (n = 4-6)

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.