Challenge to expand the concept of aromaticity to tin- and lead-containing carbocyclic compounds: Synthesis, structures and reactions of dilithiostannoles and dilithioplumbole

Abstract The aromatic compounds have for a long time played important roles in organic chemistry. The skeletons of most aromatic compounds consist mainly of carbon atoms and other second-row elements. However, a recent question is whether the concept of aromaticity can be applied when the carbon atoms of the π-frameworks are substituted by heavier group 14 atoms. In contrast to recent extensive works on the creation of silicon- and germanium-containing carbocyclic π-frameworks, which demonstrated novel heavier aromatic compounds, no one knew whether or not the concept of aromaticity could be applied to tin-containing carbocyclic compounds in 2001. We therefore targeted the synthesis of a dilithiostannole, which is a tin-analog of the cyclopentadienyl anion. Dilithiostannoles and its benzannulated derivatives were successfully synthesized by reduction of the corresponding 1,1-diphenyl derivatives. X-ray diffraction analysis of the dilithiostannoles revealed that each of the stannole rings is almost planar without bond alternation of the C–C bonds. Based on their structural features and very negative nucleus-independent chemical shift (NICS) values calculated at 1.0 A above the rings, the dilithiostannoles were concluded to be the first tin-containing carbocyclic aromatic compounds. In the benzannulated derivatives, each of the stannole rings is more aromatic than its adjacent benzene ring, as determined by theoretical calculations. The synthesis of dilithioplumbole was also accomplished and it was concluded to have considerable aromatic character, as supported by X-ray diffraction analysis and theoretical calculations. Therefore, the concept of aromaticity is now extended to lead-containing carbocyclic compounds. The reactivity of the dilithio-stannoles and -plumbole is also demonstrated.