Fluorinated liquid crystals: Evaluation of selectively fluorinated facially polarised cyclohexyl motifs for liquid crystal applications

Article abstract of DOI:10.1039/c6ob01986b

This paper explores the synthesis of a series of prototype negative dielectric liquid crystalline (LC) compounds which contain fluorinated cyclohexane motifs. The series are analogues and differ only in that they contain between one to four fluorine atoms. The stereochemistry is such that all of the fluorines/C-F bonds are on one face of the cyclohexane ring. This follows from the recent recognition that cyclohexanes with fluorines orientated in one direction, perpendicular to the ring have a strong molecular dipole, a characteristic that might be an advantage in the design of negative dielectric materials. However it is found that the increased polarity, particularly with two or more oriented C-F bonds, leads to higher melting points and poorer solubility in test matrix formulations, relative to hydrocarbon liquid crystals. This arises due to electrostatic intermolecular interactions between the polarised cyclohexyl and aryl rings. The study highlights that in order to take advantage of these polar cyclohexane motifs for liquid crystal design, appropriate scaffolds are required which are free of aromatic rings and contain peripheral solubilising motifs.

Full text of DOI:10.1039/c6ob01986b

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Fluorinated liquid crystals: evaluation of selectively
fluorinated facially polarised cyclohexyl motifs
for liquid crystal applications†
Cite this: DOI: 10.1039/c6ob01986b
Received 8th September 2016,
Accepted 30th September 2016
Nawaf Al-Maharik,a Peer Kirsch,*b Alexandra M. Z. Slawin,a David B. Cordesa and
DOI: 10.1039/c6ob01986b
David O’Hagan*a
www.rsc.org/obc
This paper explores the synthesis of a series of prototype negative of fluorine reduces intermolecular interactions and lowers
dielectric liquid crystalline (LC) compounds which contain fluori- fluxional viscosity parameters.6 Thus selectively fluorinated
nated cyclohexane motifs. The series are analogues and differ only organics such as 1 and 2 have found a role in modern liquid
in that they contain between one to four fluorine atoms. The crystal technology.1–5 In the case of 1 the fluorine atoms
stereochemistry is such that all of the fluorines/C–F bonds are on induce a dipole along the molecular axis; this is known as a
one face of the cyclohexane ring. This follows from the recent +ve dielectric anisotropic liquid crystalline material.7 An
recognition that cyclohexanes with fluorines orientated in one alternative design has the polar substituent perpendicular to
direction, perpendicular to the ring have a strong molecular the molecular axis such as that found in structure 2 (Fig. 1).
dipole, a characteristic that might be an advantage in the design of This is known as a −ve dielectric anisotropic liquid crystalline
negative dielectric materials. However it is found that the increased material, a category of LC that is attracting increasing
polarity, particularly with two or more oriented C–F bonds, leads attention.8,9
to higher melting points and poorer solubility in test matrix formu-
This paper explores compounds 3–6 (Fig. 2) in the context
lations, relative to hydrocarbon liquid crystals. This arises due to of preparing prototype −ve dielectric LC properties. In this
electrostatic intermolecular interactions between the polarised regard we have recently introduced the all-cis 1,2,4,5-tetrafluor-
cyclohexyl and aryl rings. The study highlights that in order to take ocyclohexyl 7 motif and some derivatives such as 8 containing
advantage of these polar cyclohexane motifs for liquid crystal this novel cyclohexane.10–15 A striking feature of the ring
design, appropriate scaffolds are required which are free of aro- system is that it is facially polarised. There are always two C–F
matic rings and contain peripheral solubilising motifs.
bonds running parallel and 1,3-diaxial to each other, and the
other two fluorines are equatorial, but they remain on the
same face of the cyclohexyl ring. Thus due to the electro-
negativity of fluorine, the molecule is polarised perpendicular
to the cyclohexane ring. It occurred that if the motif were
incorporated into a LC, it would introduce a dipole perpen-
dicular to the molecular axis, attractive in the design of −ve
dielectric anisotropic LC’s. In this context the tetrafluorocyclo-
hexyl LC 3 became a synthetic target. One feature which has
emerged with compounds containing this ring system is that
the high polarity of the cyclohexyl ring renders them poorly
soluble in non-polar organic solvents. Doped LC’s have to
function in a cocktail of other hydrophobic hydrocarbon com-
ponents, therefore co-solubility in the LC matrix is also a
necessary design feature, and this was required to be explored.
Thus we also set LC-s 4–6 as synthetic targets, as they progress-
ively remove a fluorine from the ring system. Although this will
reduce polarity, it was anticipated that it may increase co-solu-
bility. This paper describes the synthesis of LC’s 3–6 and
reports an initial assessment of the motif as a component in
LC design.
Introduction
Developments in organic liquid crystals (LCs) technology have
been a key element in the transformational impact of screen
displays and personal mobile devices.1,2 A key challenge is to
prepare molecules which display macromolecular organis-
ation, and which possess an orientated dipole that can orient
rapidly in electric and magnetic fields.3 Fluorinated com-
pounds can contribute some of these characteristics if cor-
rectly designed.4,5 The C–F bond is the most polar in organic
chemistry, and the high electronegativity and low polarizability
aEaStCHEM School of Chemistry, University of St. Andrews, St. Andrews,
Fife KY16 9ST, UK. E-mail: do1@st-andrews.ac.uk
bMerck KGaA, Liquid Crystals R&D Chemistry, Frankfurter Str. 250,
D-64293 Darmstadt, Germany. E-mail: peer.kirsch@merckgroup.com
†Electronic supplementary information (ESI) available. CCDC 1503420–1503427.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c6ob01986b
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Fig. 1 Selectively fluorinated liquid crystalline materials. Compound 1 is an example of a +ve dielectric anisotropic liquid crystal and 2 a −ve dielec-
tric anisotropic liquid crystal.1,2
Fig. 2 Synthetic targets 3–6 distinguished by having either four, three, two or one C–F bonds on one face of the cyclohexane ring.
To gain a deeper insight into the course of the reaction,
Results and discussion
ditriflates 16 and 17 were separated and their individual struc-
1
A synthetic route to LC-3 is shown in Scheme 1. The route tures were elucidated by H and 19F NMR and by X-ray crystal-
started with a palladium catalysed ([Pd(IPr)(cin)Cl], KOBu-t) lography (see ESI†). When 16 was treated under the same con-
Suzuki coupling reaction between phenylboronic acid 10 and ditions only the desired tetrafluorocyclohexane 3 was formed.
1-bromo-4-(trans-4-propylcyclohexyl)benzene 9,16
a reaction Conversely, treatment of ditriflate 17 with Et3N·3HF furnished
which was found to proceed well and gave biphenyl 11 in good trifluorocyclohexene 18, with only traces of 3 (≈2%) as shown
yield. Birch reduction17 of 11 followed by epoxidation of the in Scheme 1. Thus each ditriflate isomer contributes a single
product diene 12, using an excess of mCPBA.18,19 This furn- product isomer. Controlled elimination of the asymmetric
ished cis-diepoxide 13 as a single diastereoisomer and with the ditriflate 17 with DBU efficiently generated difluorocyclo-
stereochemistry as shown. This was confirmed by X-ray crystal- hexene 19.22 This was a crystalline compound and the struc-
lography (see ESI†). Nucleophilic ring-opening of the epoxide ture and stereochemistry was supported by X-ray structure ana-
rings of 13 with Et3N·3HF led to the formation of a 1 : 1.6 mix lysis (see ESI†). Palladium catalysed hydrogenation of 19 gave
of the fluorohydrin regioisomers 14 and 15, with a bias difluorocyclohexane 21 in almost quantitative yield. Treatment
towards the asymmetric product 15.14 This product was taken of 20 with Et3N·3HF under the same conditions furnished the
as a mixture and was converted to the corresponding ditriflates all-cis trifluorocyclohexane 4 as a single isomer, this time with
16 and 17.18,20 Without further purification the ditriflate an inversion of configuration as shown in Scheme 2.
mixture was treated with an excess of Et3N·3HF, under forcing Furthermore, treatment of fluorohydrin mixture 14 and 15
conditions, to generate tetrafluorocyclohexane 3.20,21 This reac- with triflic anhydride (4 equiv.) in pyridine afforded symmetric
tion however generated olefin 18 as the major product, a com- ditriflate 16 and monotriflate 19 in a 4 : 6 ratio. Separation of
pound which arises via an aryl migration after an elimination the triflates, and then treatment of 19 with Et3N·3HF, gener-
to give 19, and then via phenonium 20 rearrangement, as pre- ated the trifluorocyclohexene 18. This reaction occurs with a
viously described for closely related transformations.10,14
retention of configuration and as a single isomer (Scheme 1).
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Scheme 1 (i) [Pd(IPr)(cin)Cl], i-PrOH, KOBu-t, rt, overnight, 80%; (ii) Li, NH3, THF, −78 °C to 40 °C, 4 h, 92%; (iii) mCPBA, DCM, 0 °C, 5 h, 89%;
(iv) Et3N·3HF, 145 °C, 20 h, 2 : 3 ratio; (v) Tf2O, pyridine, DCM, 0 °C, 5 h, 41% for 16 and 56% for 17; (vi) Et3N·3HF, 100 °C, 56 h.
Scheme 2 (i) DBU, THF, 0 °C, 5 h, 98%; (ii) H2, 10% Pd/C, EtOAc, rt, 15 h, 100%; (iii) Et3N.3HF,100 °C, 56 h, 84%.
These observations are consistent with the presence of the
The synthesis of 5 and 6 started with the preparation of
double bond in 19 forcing a half-chair conformation, in which alkene 23 by sequential Miyaura borylation23 followed by a
the aryl and triflate moieties at C-6 and C-5 respectively, are Miyaura–Suzuki24–26 coupling as indicated in Scheme 3.
positioned anti-periplanar to each other, and this accommo- Palladium (Pd2(dba)3/XPhos) catalysed borylation of 9 with
dates aryl migration via a phenonium ion 20.14
bis(pinocolate)diborane gave pinacolboronate 22 in good yield.
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Scheme 3 (i) Pd2(dpa)3, XPhos, KOAc, 1,4-dioxane, 90 °C, 5 h, 89%; (ii) Pd(dba)2, KF, cyclohex-2-enyl acetate, MeOH, reflux, 20 h, 58%; (iii) mCPBA,
DCM, 0 °C, 5 h, 83%; (iv) Et3N·3HF, 145 °C, 20 h, 1 : 0.8 ratio, 98%; (v) Tf2O, pyridine, 0 °C, 5 h; (vi) Et3N·3HF, 100 °C, 56 h, 88%; (vii) H2, 10% Pd/C,
EtOAc, 99%.
Suzuki coupling of 22 with cyclohex-2-enyl acetate in MeOH orientated in the same direction. Again packing is dominated
proceeded in the presence Pd(dba)2 and KF to afford alkene by intermolecular F–C–H⋯π-interactions between the cyclo-
23, which in turn was treated with an excess of mCBPA at 0 °C hexane and the aryl ringsof adjacenet molecules (Fig. 3 and 4).
to give epoxide 24 in 82% yield.18,21 The epoxide was subjected
In the case of the difluoro compound 5, the molecules form
to ring opening/fluorination with Et3N.3HF at 140 °C giving a dimer arrangements again with the hydrogen face of the cyclo-
4 : 5 mixture of the fluorohydrins 25 and 26 respectively. hexane contacting the aryl ring of its partner through F–C–
Treatment with Tf2O in pyridine generated 3-fluoro-3-O-triflate- H⋯π-interactions. For the monofluoro system 6 however there
cyclohexane 27 and monofluorocyclohexene 28 and which are no F–C–H⋯π-interactions suggesting that the ordering of
could be separated by chromatography. Displacement of the the cyclohexane and aryl groups in this manner is reduced as
triflate moiety of 27 with fluoride was achieved using Et3N·3HF the number of fluorines is removed, presumably due to fewer
to give cis-difluorocyclohexane 5. The monofluorocyclohexane possible F–C–H⋯π-interactions (Fig. 5 and 6). This is consist-
6 was the prepared by hydrogenation of fluorocyclohexene 28 ent too with the progressive lowering of the melting point in
in good yield and high purity.
going from the tetra fluoro 3 to the mono fluoro 6 systems (see
It is interesting to compare the X-ray structures and crystal Table 1).
packing of compounds 3–6 (Fig. 3–6). The tetrafluorocyclohexane
Compounds 3–6 were assessed as for their LC properties.
3 packs as a dimer in the unit cell, with molecules orientated Unlike reference compounds 1 and 2 none of these com-
in opposing directions. Conspicuously the electropositive pounds displayed any mesogenic phase changes (see Table 1)
hydrogen face of the cyclohexane ring interacts with the most probably due to the absence of a second alkyl chain on
electronegative aryl ring of another molecule of 3 in F–C– the cyclohexane ring. The polar fluorine motifs and the
H⋯π-interactions. This has been noted for related phenyltetra- absence of a solubilising alkyl group also rendered them
fluorocyclohexane motifs27,28 and is a preferred stacking largely insoluble in the test host mixture, and they all have
mode, dictated by compensating electrostatics. For the trifluoro- melting points (crystalline (C) to isotropic(I); C → I) above that
cyclohexane crystal structure 4, the molecules are arranged for 1 and 2. Only compound 6 was sufficiently soluble to be
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Fig. 3 X-ray structure of 3 showing reciprocal F–C–H⋯π-interactions between the cyclohexane and aryl rings of each molecule within a dimer
arrangement.
Fig. 4 X-ray structure of 4 showing F–C–H⋯π-interactions between adjacent molecules.
Fig. 5 X-ray structure of 5 showing reciprocal interactions between the cyclohexane and aryl rings of each molecule within a dimer arrangement.
assessed for LC properties, and it did display a −ve dielectric the solid state consistent with this polarity. It is shown that a
anisotropy of −1.9, consistent with prediction.
regression from four to one fluorine decreases the melting
In conclusion, we report the synthesis of a series of fluori- point of these compounds. Only the monofluoro compound 6
nated cyclohexanes 3–6 as prototype negative dielectric aniso- was sufficiently soluble in the nematic liquid crystal host and
tropic liquid crystals. The structures contain one to four fluo- showed LC properties. It displayed the anticipated −ve dielec-
rine atoms which have all their fluorines on one face of the tric anisotropic tendency. It follows that if fluorinated cyclo-
cyclohexane ring. Aliphatic compounds with fluorines orien- hexanes with more than one fluorine are to be introduced into
tated perpendicular to the cyclohexane ring are polar, particu- an LC material, to capitalise on their high polarity, then the
larly if there are two diaxial fluorines. X-ray structural studies design scaffold should be devoid of aromatic rings and should
indicate significant F–C–H⋯π-intermolecular interactions in accommodate peripheral solubilising groups to lower the
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Fig. 6 X-Ray structure of monofluoro cyclohexane 6 without any obvious F–C–H⋯π-interactions, and packing is stabilised by hydrophobic
interactions.
Table 1 Mesogenic/melting point data for compounds 1–6. (C = crys-
talline, N = nematic, SB, smectic B, I isotropic). Δεvirt is the virtual dielec-
tric anisotropy, experimentally determined for 6 only. Phase transition
temperatures are noted in °C
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No.
Mesophases/mp
Δεvirt
1
2
3
4
5
6
C 25 N 54.8 I
C 68 SB 120 I
C 214 I
C 175 I
C 128 I
C 94 I
−1.1929
melting points and counter the poor solubility created by the
polar motif.
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Acknowledgements
The authors thank the European Research Council for and
Advanced Grant and DO’H acknowledges the Royal Society for
a Wolfson Merit Award. We also thank the EPSRC mass
spectrometry service at the University of Swansea.
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L. Cavallo, C. S. J. Cazin and S. P. Nolan, Chem. – Eur. J.,
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