Synthesis, structure and aromaticity of carboranefused carbo-and heterocycles

Conjugation between a 3-D icosahedral carborane and a fused 2-D pi-ring system is ambiguous. To address this issue, we prepared several carborane-fused carbo- and heterocycles. Detailed studies on their molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of (1) considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and (2) considerable conjugation between a 3-D carborane and a fused 2-D pi-ring system. These results will shed some light on the design of new carborane-based materials.