Novel 2-phenyl-6-phenylethynyl-4-(trifluoromethyl)quinolines: Synthesis by Sonogashira cross-coupling reaction and their evaluation as liquid crystals

This work reports the synthesis of a new series of 2-aryl-6-(phenylethynyl)-4-(trifluoromethyl) quinolines (2), where aryl = 4-(phenylethynyl)phenyl (2a), Ph (2b), 4-MeC6H4(2c), 4-FC6H4(2d). These were obtained by the Sonogashira coupling reaction producing 42-88% yield, utilizing 6-bromoquinolines and phenylacetylene as precursors in the presence of catalyst systems of Pd(OAc)(2) and PPh3. The precursor 6-bromoquinolines were obtained from an intramolecular cyclisation reaction of (Z)-4-1(4-bromophenyl)amino]-1,1,1-trifluorobut-3-en-2-ones, which were previously prepared though the reaction of 4-methoxy 4 aryl 1,1,1-trifluoroalk-3-en-2-ones with 4-bromoaniline. Liquid crystal (LCs) properties were evaluated using thermal data and mesophase texture. Only compound 2a displayed monotropic nematic mesophase with very narrow liquid crystal window. Compounds 2b, 2c and 2d only displayed transition between the crystal phase to a liquid state. Nematic behavior was mainly dependent on the molecular length, as proved for 2a, which has the longest molecular length. Torsional angles between quinoline and aryl moieties at the 2-position were significant on hysteresis observed during the crystallization process. (C) 2019 Elsevier B.V. All rights reserved.