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

Abstract 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-MeC 6 H 4 ( 2c ), 4-FC 6 H 4 ( 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 PPh 3 . The precursor 6-bromoquinolines were obtained from an intramolecular cyclisation reaction of ( Z )-4-[(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.