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

Abstract

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.

Solvent-free synthesis and chiroptical properties of a C–N axially chiral cruciform dimer of benzo[b]phenoxazine

“…The dimerization of tBuBPO was also performed using a solvent-free mechanochemical oxidative coupling. [21][22][23] In a 50 mL stainless-steel jar of a mixer mill, tBuBPO (500 mg, 1.73 mmol) and DDQ (236 mg, 0.6 eq.) were placed with twelve stainless steel balls (10 mm f) and milled at 30 Hz for 30 min.…”