Difluorooxymethylene-Bridged Liquid Crystals: A Novel Synthesis Based on the Oxidative Alkoxydifluorodesulfuration of Dithianylium Salts

Abstract

Dedicated to Professor Heinz A. Staab on the occasion of his 75th birthday In our search for new, superior liquid crystals [1] for use in active-matrix liquid crystal displays [2] (AM-LCD or thin film transistor LCD, TFT-LCD) it was found that the insertion of a difluorooxymethylene bridge into a specific location of the mesogenic core structure of phenylbicyclohexyl-type liquid crystals (1) [3] results in a class of materials (2; Scheme 1) [4] that Scheme 1. The liquid crystalline basic structure 1 and its analogue 2 which is extended by a difluorooxymethylene bridge.

Cholestane-based liquid crystals containing a difluorooxymethylene bridge

“…For the design of our new class of chiral dopants, the cholesteryl moiety containing one double bond was replaced by a cholestanyl group, in order to avoid complications during the synthesis of the difluorooxymethylene bridge [7] cholestanone (3), the ketenedithioketal 4 [8] was synthesized as the central steroid building block. By a simple four-step one-pot procedure, the different phenol components could now be attached via a protonation-equilibration-addition-fluorodesulfuration reaction sequence, furnishing the liquid crystals 5-8 in good yields.…”

“…From our previous work [7] we knew that liquid crystals containing a difluorooxymethylene bridge in their mesogenic core structure do not only fulfil LCD manufacturers' stability criteria. In addition -compared to their directly or ester linked analogues -they usually show a dramatically improved solubility in combination with a significant extention of their nematic phase range.…”

“…By a simple four-step one-pot procedure, the different phenol components could now be attached via a protonation-equilibration-addition-fluorodesulfuration reaction sequence, furnishing the liquid crystals 5-8 in good yields. The exclusive formation of the desired bcholestanyl isomers was achieved by allowing the primarily formed cholestanyl dithianylium salt a/b-isomer mixture (9) to equilibrate to the thermodynamically preferred b-dithianylium salt at room temperature for a few minutes (Scheme 2) [7].…”

Fluoro-Pummerer rearrangement and analogous reactions

“…While a large array of functional groups is tolerant to the reaction, ketones required protection of the carbonyl group, usually accomplished by transforming it to the corresponding acetal [140]. In a similar 164 16 [142,143]. The reaction of ketene dithioacetals 180 with alcohols and subsequent desulfurization-fluorination of 181 with DBH and Et 3 NÁ3HF at À70 8C afforded a,adifluoroalkylethers 182.…”

Einführung von Fluor und fluorhaltigen funktionellen Gruppen

“…Aryl-a,a-difluorether kçnnen aber durch oxidative Umlagerung von Aldehyden mit Difluorarylbroman synthetisiert werden. [245] Durch Fluorierung von Carbonaten mit SF 4 werden Bis(alkoxy)difluormethanprodukte erhalten. [239] Dieser selektive Fluoridaustausch wird auf die Aktivierung von Fluorid mit Lithium zurückgeführt, die auf der Bildung von Lithiumfluorid, dem Alkalimetallhalogenid mit der hçchsten Gitterenergie, beruht.…”