Highly Electron-Deficient and Air-Stable Conjugated Thienylboranes

Introduced herein is a series of conjugated thienylboranes, which are inert to air and moisture, and even resist acids and strong bases. X‐ray analyses reveal a coplanar arrangement of the thiophene rings, an arrangement which facilitates p–π conjugation through the boron atoms despite the presence of highly bulky 2,4,6‐tri‐tert‐butylphenyl (Mes*) or 2,4,6‐tris(trifluoromethyl)phenyl (FMes) groups. Short B⋅⋅⋅F contacts, which lead to a pseudotrigonal bipyramidal geometry in the FMes species, have been further studied by DFT and AIM analysis. In contrast to the Mes* groups, the highly electron‐withdrawing FMes groups do not diminish the Lewis acidity of boron toward F− anions. These compounds can be lithiated or iodinated under electrophilic conditions without decomposition, thus offering a promising route to larger conjugated structures with electron‐acceptor character. The bigger the better: Sterically demanding 2,4,6‐tri‐tert‐butylphenyl (Mes*) or 2,4,6‐tris(trifluoromethyl)phenyl (FMes) groups do not prevent but rather promote coplanarity and enhance electronic communication within conjugated thienylboranes. FMes exerts a strong electron‐withdrawing effect which results in significant lowering of the LUMO energy level and high Lewis acidity toward fluoride anions, while ensuring stability in air and towards acid or base.