Partially Hydrogenated 7-Oxa [5] helicenes and [5]Helicenes– Synthesis, Structures, and Dynamics[#]

Abstract

An efficient and convenient synthesis of partially hydrogenated functionalized [5]helicenes and oxa[5]helicenes, such as (1,2,5,6-tetrahydro-3-oxa-dibenzo[c,g]phenanthren-4-ylidene)acetonitriles 6 and (5,6-dihydro-3-oxa-dibenzo[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 by 2-tetralones 5. The molecular structures of the partially hydrogenated oxahelicenes were examined by spectroscopic methods and by X-ray crystallographic analysis. In order 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 states and the transition states were optimized by the 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 for the rotational barrier. The accuracy of the calculated value for 6e was confirmed by DNMR experiments with line-shape analysis. Our ring-transformation protocol provides an easy access to functionalized helicenes and heterohelicenes with the flexibility of substituent variations and opens new perspectives for further exploration.