Controllable 1,4-Palladium\nAryl to Aryl Migration\nin Fused SystemsApplication to the Synthesis of Azaborole\nMultihelicenes

Herein, we report the first 1,4-Pd aryl to aryl migration/Miyaura\nborylation tandem reaction in fused systems. The Pd shift occurred\nin the bay region of the dibenzo[<i>g</i>,<i>p</i>]chrysene building blocks, giving rise to a thermodynamically controlled\nmixture of 1,8- and 1,9-borylated compounds that allowed the preparation\nof regioisomeric azaborole multihelicenes from the same starting material.\nThe outcome of this synthesis can be controlled by the choice of reaction\nconditions, allowing the migration process to be turned off in the\nabsence of an acetate additive and the target multiheterohelicenes\nto be prepared in a regioselective manner. The target compounds show\nbright green fluorescence in dichloromethane with emission quantum\nyields (Φ) of up to 0.29, |<i>g</i><sub>lum</sub>|\nvalues up to 2.7 × 10<sup>–3</sup>, and green or green-yellow\nemission in the solid state, reaching Φ of 0.22. Single crystal\nX-ray diffraction analyses gave insight into their molecular structures\nand the packing arrangement. Evaluation of aromaticity in these multihelicenes\nrevealed a nonaromatic character of the 2<i>H</i>-1,2-azaborole\nconstituent rings.