Completely Solvent-free Synthesis of Double Heterohelicenes and Their Further Ring Fusion Using Mechanochemical Reaction

In this study, we developed a simple and efficient method for synthesizing double hetero[5]helicenes composed of two heteroacenes bearing an NH group, such as benzo[b]phenoxazine (BPO) and dibenzo[b,i]phenoxazines (DBPO), using mechanochemical oxidative C–N coupling reactions, allowing the complete solvent-free synthesis from commercially available compounds. Our new synthetic method afforded over 1 g of double hetero[5]helicene, which has a high dissymmetry factor for circularly polarized luminescence (gCPL) of > 1 × 10−2, in a one-pot mechanochemical reaction using BPO as a reactant. In addition, the mechanochemical oxidative coupling also allowed for further fusion reactions of double hetero[5]helicenes, leading to semi- or fully planarized molecules, which have not been previously achieved through solution-phase reactions. We isolated the semi-planarized heterohelicenes 5 and 6 and determined their structures. The enantiomers of 6 exhibited a clear CPL emission with a |gCPL| value of 2 × 10−3. The magnitudes of the transition magnetic dipole moment (TMDM) of 5 and 6 increased from those of 3 and 4. Transition moment density analysis revealed that large TMDM densities appeared on the newly formed C–C bonds, providing a unique molecular design guideline for enhancing the magnitude of the TMDM without expanding the molecular structure.