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

Abstract

Abstract 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‐2 H ‐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.