Unexpected Increase in Antiaromaticity through Heteroannulation of Borole

Significance

The authors describe the syntheses and properties of thiophene-fused borole compounds. The synthesis of compound 2 features an intramolecular nucleophilic substitution of the boron center via lithiation of the bromo­thiophene ring. Compounds 3 and 4 result from 5-endo-dig cyclization. The resulting compounds exhibit high antiaromaticity.