Synthesis of organic liquid crystals containing selectively fluorinated cyclopropanes

Abstract

This paper describes the synthesis of a series of organic liquid crystals (LCs) containing selectively fluorinated cyclopropanes at their termini. The syntheses used difluorocarbene additions to olefin precursors, an approach which proved straightforward such that these liquid crystal candidates could be efficiently prepared. Their physical and thermodynamic properties were evaluated and depending on individual structures, they either displayed positive or negative dielectric anisotropy. The study gives some guidance into effective structure-property relationships for the design of LCs containing selectively fluorinated cyclopropane motifs.

Keywords: dielectric anisotropy; difluorocarbene; organic liquid crystals; selectively fluorinated cyclopropanes.

Figure 1

Examples of liquid crystal candidates with negative values for dielectric anisotropy (Δε) [–10].

Figure 2

Synthetic candidate LC targets 8–11.

Scheme 1

Synthesis of 8. Reagents and conditions: a) TMSCF₃, NaI, THF, reflux, 55% [–14].

Scheme 2

Synthesis of 9. Reagents and conditions: a) NBS, HF·Py, DCM; b) t-BuOK, DCM, 42% in two steps [15]; c) TMSCF₃, NaI, THF, reflux, 46%.

Scheme 3

Synthesis of compound 10. Reagents and conditions: a) NaBH₄, MeOH, rt, 45%; b) C₄H₉OCH=CH₂, Pd(TFA)₂, BPhen, Et₃N, 75 °C, 62% [16]; c) TMSCF₃, NaI, THF, reflux, 50%.

Scheme 4

Synthesis of compounds 11. Reagents and conditions: a) PPh₃CH₃Br, t-BuOK, diethyl ether, 0 °C to rt, 72% [17]; b) TMSCF₃, NaI, THF, reflux; c) recrystallization, 27%.

Figure 3

Theory study exploring the relative energies for different conformers of 11a.