Partially Hydrogenated 7‐Oxa[5]helicenes and [5]Helicenes: Synthesis, Structures, and Dynamics

An efficient and convenient synthesis of partially hydrogenated functionalized [5] helicenes and oxa[5] helicenes, such as ( 1,2,5,6-tetrahydro-3-oxadibenzo[c,g]phenanthren-4-ylidene)acetonitriles 6 and (5,6-dihydro-3-oxadibenzo[c,g]phenanthren-4-ylidene) acetonitriles 8, has been developed through base-catalyzed ring transformation of 5,6-dihydro-4-amin-1-yl-2-oxo-2H-benzo[h] chromene-3-carbonitriles 4 with 2-tetralones 5. The molecular structures of the partially hydrogenated oxahelicenes were examined by spectroscopic methods and by X-ray crystallographic analysis. To calculate the inversion barrier of the helimeric enantiomers P-6e and M-6e and of P-8a and M-8a, the molecular geometries of both the ground and the transition states were optimized by density functional theory (DFT) using B3LYP/6-311G*. Based on these structures, RI-SCS-MP2/TZVP single-point energies (ORCA) were calculated to permit a prediction of the rotational barrier. The accuracy of the calculated value for 6e was confirmed by Dynamic Nuclear Magnetic Resonance (DNMR) experiments with line-shape analysis. Our ring-transformation protocol provides easy access to functionalized helicenes and heterohelicenes with the flexibility of substituent variations and opens new perspectives for further exploration.