Site-Selective Sonogashira Reactions of 1,4-Dibromo-2-(trifluoromethyl)benzene: Synthesis and Properties of Fluorinated Alkynylbenzenes

Site-selective Sonogashira cross-coupling reactions of 1,4-dibromo-2-(trifluoromethyl)benzene have been performed, affording a variety of functionalized alkynyl-substituted (trifluoromethyl)benzenes. The site selectivity, in favor of position C-4, was confirmed by C-13 NMR and H-1-C-13 HMBC NMR measurements, X-ray crystal structure analysis as well as DFT calculations. Furthermore, the mesomorphic properties and optical textures were examined by differential scanning calorimetry and polarized optical microscopy. Several dialkynyl-substituted (trifluoromethyl)benzenes showed nematic liquid-crystalline properties over a long phase range. The influence of terminal and lateral substituents on the formation and stability of the molecular network within the liquid-crystalline phase is explained by the results of DFT calculations. Additionally, the absorption and fluorescence properties were analyzed in detail. The intramolecular influence of electron-donating or electron-withdrawing groups and also of solvatochromatism are considered. In this context, time-dependent B3LYP/6-31G* (TD-B3LYP) calculations were carried out to characterize the main absorption bands.