Thiophene-Fused Ladder Boroles with High Antiaromaticity

A series of polycyclic thiophene-fused boroles were synthesized on the basis of stepwise substitution reactions from thienylboronic ester precursors. In these ladder-type pi-conjugated systems, the thiophene-fused structure enhances the antiaromaticity of the borole ring. This trend is opposite to the conventional understanding that the arene-fused structure decreases the antiaromaticity of the 4 pi-electron ring skeletons. The ladder boroles exhibited characteristic properties such as long-wavelength absorptions and low reduction potentials.