Preparation, Structures, and Theoretical Calculations of Novel Silacalixarene Derivatives:  Silacalix[4]quinone and Silacalix[4]hydroquinone Octamethyl Ether

A novel silicon-bridged calix[4]arene derivative, 2,8,14,20-tetrasilacalix[4]hydroquinone octamethyl ether (6), has been successfully prepared. Cerium ammonium nitrate oxidation of 6 gave the first heteroatom-bridged calixquinone derivative, 2,8,14,20-tetrasilacalix[4]quinone (5). The structures of 5 and 6 were determined by 1H, 13C, and 29Si NMR spectroscopy and mass spectroscopy. X-ray crystallographic analysis confirmed the structures of 5 and 6. X-ray crystal structures revealed that 5 and 6 adopted 1,3-alternate conformations. The conformations of 5 and 6 were also studied by theoretical calculations. The optimized 1,3-alternate conformer of 5 is at least 4.9 kcal/mol more stable than the other conformers at the B3LYP/6-31G(d) level. This finding is in agreement with the solid-state structure. In contrast, the optimized 1,3-alternate conformer of 6 is 1.3 kcal/mol less stable than the corresponding 1,2-alternate conformer. The result suggests that 6 has significant conformational flexibility.