Enhancement of the Lewis acidity by substitution of sulfur-containing hetero aromatics in triarylboranes

Triarylboranes bearing two thiophene, dimethylthiophene, or benzothiophene units were synthesized. X-ray crystallographic analysis revealed the planar structure around the boron atom and non-bonding S–S interaction of the thiophene. They exhibited several intense absorption bands in the UV region. They were stable in the air for months and showed extremely strong Lewis acidities.