Fig. 3: Scope of the enantioselective synthesis of carbo[n]helicenes.

a, Scope of the monoarylation reaction. This method allows access to the three types of lower helicenes with various substitution patterns in high yields and enantioselectivities and is applicable to azahelicenes. Standard reagents and conditions: 1 (0.1 mmol, 1.0 equiv.), Pd₂dba₃ (5 mol%), ligand (20 mol%), Cs₂CO₃ (0.5 equiv.), CPME (1 ml), T °C, 24 h. The X-ray crystallographic structures of 2p and 2q are shown. ^aFree energy of enantiomerization computed at the B3LYP-D3(BJ)/6-311G(d,p) level of theory. ^bExperimental enantiomerization barrier measured at 120 °C. ^cThermal ellipsoids are shown at the 50% probability level. ^dThermal ellipsoids are shown at the 20% probability level. b, Synthesis of carbo[5]- and carbo[6]helicenes by double C–H arylation. This method allows a more direct access to these helicenes, albeit in lower yields. Reagents and conditions: 3 (0.1 mmol, 1.0 equiv.), Pd₂dba₃ (10 mol%), L¹ (40 mol%), Cs₂CO₃ (1.0 equiv.), CPME (1 ml), 140 °C, 24 h. The e.r. values were determined by HPLC on a chiral stationary phase. The reference racemic products were synthesized using PCy₃ instead of the chiral ligand. The absolute configurations were ascribed in analogy to 2p and 2q, and by comparing the calculated with the experimental ECD spectra for selected compounds. The red dots indicate the initial position of the bromide. a,b, Light blue highlights changes in substituents.