1‑Heteroaromatic-Substituted Tetraphenylboroles: π–π Interactions Between Aromatic and Antiaromatic Rings Through a B–C Bond

A series of 2,3,4,5-tetraphenylboroles substituted with
different
aromatic heterocycles (thiophene, furan, pyrrole, and dithiophene)
in the 1-position were synthesized and characterized by means of NMR,
elemental analysis, and X-ray crystallography. In contrast to known
2,3,4,5-tetraphenylboroles, X-ray diffraction revealed a nearly coplanar
arrangement of the aromatic heterocycles and the antiaromatic borole
scaffold as a result of π-conjugation, which could be substantiated
by DFT calculations. Furthermore, the 2,2′-dithiophene-bridged
bisborole (14) exhibits a large bathochromic shift in
the absorption spectrum, demonstrating the exceptional Lewis acidity
of the nonannulated borolyl moiety.