Controllable 1,4-Palladium Aryl to Aryl Migration in Fused SystemsApplication to the Synthesis of Azaborole Multihelicenes

Herein, we report the first 1,4-Pd aryl to aryl migration/Miyaura
borylation tandem reaction in fused systems. The Pd shift occurred
in the bay region of the dibenzo[g,p]chrysene building blocks, giving rise to a thermodynamically controlled
mixture of 1,8- and 1,9-borylated compounds that allowed the preparation
of regioisomeric azaborole multihelicenes from the same starting material.
The outcome of this synthesis can be controlled by the choice of reaction
conditions, allowing the migration process to be turned off in the
absence of an acetate additive and the target multiheterohelicenes
to be prepared in a regioselective manner. The target compounds show
bright green fluorescence in dichloromethane with emission quantum
yields (Φ) of up to 0.29, |glum|
values up to 2.7 × 10–3, and green or green-yellow
emission in the solid state, reaching Φ of 0.22. Single crystal
X-ray diffraction analyses gave insight into their molecular structures
and the packing arrangement. Evaluation of aromaticity in these multihelicenes
revealed a nonaromatic character of the 2H-1,2-azaborole
constituent rings.