Polyfluorinated Aromatic Systems for Liquid Crystal Display Applications

Abstract

This thesis is concerned with the syntheses of a range of polyfluorinated biphenyl ether derivatives for use as dopants in commercial LC materials, so that overall display performance may be improved. Reactions of hexa– and penta–fluorobenzene with sodium phenoxide in polar aprotic solvents (MeCN, THF) and under mild reaction conditions (45–50 °C, 4–65 h) afford moderate yields of pentafluorophenoxybenzene and 1,2,4,5–tetrafluoro–3–phenoxybenzene, respectively. SNAr reactions of sodium phenoxide with less electrophilic 1,2,3,4–tetra– and 1,2,3,5–tetra–fluorobenzene under microwave irradiation (150 °C, 30 mins) furnish good yields of 1,2,4–trifluoro–3–phenoxybenzene and 1,2,5–trifluoro–3–phenoxybenzene, respectively. Similar SNAr methodology is used to access a series of 1,2,4,5–tetra– and 1,2,4–tri–fluoro–3–(4–n–alkylphenoxy)benzene derivatives in good yield. A two–step SNAr–hydrodebromination procedure to access trifluorophenoxybenzene systems from highly electrophilic dibromotetrafluorobenzene derivatives is also developed, and provides a practical synthetic route to 1,2,4–trifluoro–5–phenoxybenzene. Finally, a three–step SNAr–diazotisation–reductive diazotisation pathway is developed for the syntheses of difluorophenoxybenzene derivatives, which are not accessible by direct SNAr reactions of sodium phenoxide with corresponding trifluorobenzene derivatives. The use of these polyfluorinated biphenyl ether systems as dopants for LC materials is assessed by a range of electro–optical measurements.

Highly fluorinated nitrobenzene systems are compatible substrates for palladium–catalysed Suzuki–Miyaura and Heck–type cross–coupling reactions involving C–F bond activation. Cross–coupling reactions of pentafluoronitrobenzene with a range of boronic acids and protected ester equivalents bearing electron–withdrawing or electron–donating substituents in the meta– and para– positions, with respect to the C–B bond, are described. For example, Reaction of pentafluoronitrobenzene with 5,5–dimethyl–2–phenyl–1,3,2–dioxaborinane in the presence of catalytic quantities of Pd(PPh3)4 and using KF/alumina as the preferred base (DMF, 150 °C, 15 mins, μW) affords 2,3,4,5–tetrafluoro–6–nitrobiphenyl in good yield (80 %). The nitro group is crucial to the success of the cross–coupling processes and is believed to facilitate the oxidative addition step by directing the palladium catalyst into the adjacent C–F bond by a predominantly SNAr–type mechanism.