Development of synthetic approaches towards 2- substituted 3- and 4-fluorothiophene building blocks and their application in liquid crystal synthesis

Abstract

Thiophene-containing liquid crystals (LCs) often exhibit attractive physical characteristics for use as ferroelectric materials, which offer several advantages for display applications. In addition, fluorine substituents on the aryl core of LCs are known to confer favorable LC properties; they often suppress or eliminate higher ordered smectic mesophases and improve dielectric anisotropy, viscosity and melting points. However, the development of smectic ferroelectric LC devices has proven challenging due to chevron defects. Our goal is to further understand structure-mesophase property relationships in LCs that bear S-heterocycle cores and to begin to understand those structural features that favor chevron defect-free ferroelectric LCs. In this dissertation, we present an exploration into the effect of the fluorothiophene core unit on the mesogenic properties of LCs. We thus synthesized three fluorothiophene-2-carboxylate-based targets (2.2, 2.3 and 2.4), a novel series of 2-alkoxy-3-fluorothiophene-based (4.2a-4.2e and 4.3) and 5-alkoxy-3-fluorothiophene-based (5.1a, 5.1b and 5.2) LCs. We also present the synthesis of thienyl- O-carbamate and phosphoramide precursors with a view to investigating the directing ability of the carbamate and phosphoramide groups in electrophilic fluorination reactions. In the synthesis of the three 3-fluorothiophene-2-carboxylate-based targets (compounds 2.2-2.4), all of which did not display the desired SmC* phases, sequential Balz-Schiemann (thermal-/microwave-assisted), fluorination, bromination, and regioselective Suzuki coupling reactions were utilized. Fluoro-alkoxy thiophene-based compounds are largely unknown in the literature. We investigated several strategies as potential pathways for generating fluoro-alkoxythiophenes. Using a halogen/lithium exchange-fluorination approach, the synthesis of the first series of 3-fluoro-2-alkoxythiophene-based LCs (compounds 4.2a-4.2e and 4.3) was accomplished in good yields. Notably, these reactions gave high fluorination to protodemetalation ratios. A practical solution to remove the protodemetalated byproduct by mercuration was found, thus making the purification/isolation of the fluoro-product easy. No synthetic approaches to 5-alkoxy-substituted 3-fluorothiophenes have previously been reported. 5-Alkoxy-3-fluorothiophene-based LCs (compounds 5.1a , 5.1b and 5.2) were synthesized using a bromine/lithium exchange-fluorination protocol. In a final study, we have developed an efficient approach to 2-thienyl-O-carbamate, and 2-thienyl-O- phosphorodiamidate derivatives. In this dissertation we present in detail a review of the relevant literature, the synthesis of the above materials, and the results of mesophase evaluation of these novel materials.