Novel Discotic Boroxines: Synthesis and Mesomorphic Properties

A new synthetic approach to highly substituted triphenylboroxines 11 is described. Their mesomorphic properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (SAXS, WAXS). The tris(3,4,5-trialkyloxy)phenyl functionalized derivatives 11b–e showed broad mesophases for a minimum alkyl chain length of C9. The phase widths ranged from 110 K to 77 K near room temperature, thus decreasing with enhanced alkyl chain lengths. Textures observed under POM indicated a columnar hexagonal (Colh) mesophase symmetry that was confirmed by X-ray diffraction experiments.


Introduction
The discovery of stable liquid crystalline phases formed by disk-shaped molecules with long alkyl chains in their periphery is generally dated to the seminal work of Chandrasekar published in 1977 [1]. Since then discotic liquid crystals have attracted the attention of many research groups worldwide [2][3][4]. Due to the one dimensional charge and ion transport in the columnar mesophase, the ability of liquid crystals (LCs) to self-heal structural defects by thermal annealing and the ease of processing via spin coating, drop casting and other solution processing methods highly useful applications [5][6][7] could berealized such as organic solar cells [8], organic field effect transistors [3] and organic light emitting diodes [9].

OPEN ACCESS
From a molecular perspective, several scaffolds have turned out to be successful candidates for the nanosegregation, which ultimately led into the formation of columnar mesophases. Besides tri-and hexasubstituted benzenes, in particular triphenylenes [10], perylenes [11], hexa-peri-hexabenzocoronenes [12,13], porphyrins [14,15], phthalocyanines [15], quinoxalines and other aza analogues of polycyclic aromatic hydrocarbons were used as mesogenic subunits. However, not only disk-shaped mesogens, but also non-conventional [16] and supramolecular and hydrogen bonded liquid crystals [17] can self-assemble into columnar aggregates. While a variety of LC materials containing heterocyclic 6-membered rings have been synthesized [18], e.g., triazines such as 1 [19][20][21] or cyclotriphosphazenes such as 2 [22,23] (Figure 1), surprisingly little is known about liquid crystalline boroxines, the cyclic trimers of organoboronic acids. Aryl boronic acids are valuable reagents for a number of metal-catalyzed reactions, the most prominent one thereof is certainly the Suzuki-Miyaura cross-coupling reaction [24][25][26][27][28]. Furthermore, boroxines may serve as useful building blocks for flame retardants, lithium ion battery materials and covalent organic frameworks [29,30]. The only example considering mesomorphism was a report by Preece and coworkers who studied a series of tris(alkoxyphenyl)boroxines 3, which, however, did not reveal any mesophases ( Figure 1) [31]. Based on this precedence we anticipated that the attachment of additional alkoxy groups at the aryl rings should favor nanosegregation and thus induce mesophase formation. The results towards this goal are reported below.

Synthesis of Boroxines
First, we intended to attach alkyloxy groups with chains lengths of C12-C18 at the phenyl rings in boroxine 3 following known procedures [32][33][34][35]. However, only 3g (R 1 = C 12 H 25 ) could be isolated in 30% yield, whereas boroxines with alkyl chain lengths >12 were not obtained as a problem of poor solubility of the precursor boronic acid (for details see Supplementary Information). Derivative 3g did not show mesomorphic properties.
Following our envisioned strategy, 4-bromo-1,2-dialkoxybenzenes 4 [36,37] were lithiated with n-BuLi, THF at −78 °C, followed by treatment with B(OMe) 3 and then directly hydrolyzed by addition of aqueous HCl to the mixture. No trace of the boronic acids was detected, but the boroxines 5 were isolated (Scheme 1). Their yields decreased considerably with increasing chain lengths from 51% for 5a to 7% for 5d due to purification problems. Column chromatography led to decomposition, thus leaving recrystallization as only method to purify 5. With increasing chain length the solubility of dialkoxybromides 4, boronic acids 6 and boroxines 5 was more and more alike. Therefore, the crude product of 5e with two C 16 H 33 groups at the phenyl ring was found as an inseparable mixture of those three and no pure product could be obtained. Scheme 1. Synthesis of boroxines 5 starting from the respective dialkoxy-bromobenzenes 4. Boronic acid 6c was prepared for comparison and assignment of 1 H NMR and IR data (see Supplementary Information).
The synthesis of tris(3,4,5-trialkoxyphenyl)boroxines 11 commenced with the 5-bromo-1,3,4-trialkoxybenzenes 7 [38][39][40][41]. Their conversion into the boronic acids according to the method described in Scheme 1 turned out to be problematic, because inseparable mixtures of boronic acids and boroxines were obtained. This was also true, when the initial bromo-lithium exchange was replaced by formation of a Grignard species instead. Therefore, an alternative method was applied (Scheme 2).

Scheme 2.
Synthesis of boroxines 11a-e bearing three 3,4,5-trialkoxyphenyl substituents. For comparison, diethanolamine complex 9e was isolated. Its 1 H and 13 C NMR spectra are given together with characteristic IR bands for distinction of 10 and 11 (Table S2 in the Supplementary Information).
Bromides 7 were treated with n-BuLi, followed by addition of isopropoxypinacolborolane [42] to yield the corresponding pinacolborolanes 8 [38] which were directly treated with bisethanolamine in i-PrOH. The resulting diethanolamine complexes 9 precipitated from the solution and after filtration, they were hydrolyzed without further purification with HCl in THF giving a mixture of boroxines 11 and boronic acids 10. Refluxing this mixture with pyridine in Et 2 O [43] followed by hydrolysis with HCl in Et 2 O provided the desired free boroxines 11 quantitatively. Noteworthy, this strategy did not need purification at any step. Only pinacolborolanes 8 were deprotected to complexes 9 which precipitated in analytically pure form (shown for 9e in the Supplementary Information). The subsequent two reaction steps proceeded in quantitative yield without any impurities. The overall yields of 11 starting from 7 were in the range of 20%-40%. The solid products 11 were air-stable, however, storage in solvents containing traces of water led to slow decomposition to the corresponding boronic acids 10.
All compounds 5a-d bearing three 3,4-dialkoxyphenyl substituents did not show any mesomorphism. In contrast, boroxines 11 with additional alkoxy group turned out to be mesogenic with exception of derivative 11a with C8 alkyloxy side chains. The results obtained from DSC measurements are summarized in Table 1 and Figure 2.  Boroxines 11b-e displayed broad mesophases near room temperature. For boroxine 11b with C9 alkyloxy side chains a liquid crystalline phase between 25 °C and 135 °C was observed. With increasing chain lengths melting points continuously rose up to 49 °C (11e) and clearing points receded to 123 °C resulting in reduced phase widths from 100 K (11c) to 77 K (11e).
Considering the DSC traces of 11b-11e identical behaviour of all derivatives during heating and cooling cycle is evident. As example, DSC curves of 11e are shown in Figure 3 (for further DSC curves see Supplementary Information). Melting points were clearly visible while clearing point peaks were less intensive and very broad. Hence, peak temperatures are given in Table 1. Upon cooling both melting and clearing point displayed a supercooling of phase transitions. Clearing points were slightly affected. Boroxine 11b shows the largest shift of 6 K from 135 °C upon heating to 129 °C upon cooling. The melting points of 11b and 11d, however, were significantly shifted by 18 K from 25 °C (11b) and 42 °C (11d) to 7 °C and 24 °C, respectively. Under the microscope boroxines 11b-11e behaved similarly. Upon cooling from the isotropic liquid all derivatives formed homeotropic areas with few defects (Figure 4 top). Presumably, the boron atoms coordinate at the oxygen atoms of the glass surface, resulting in aligned mesophases perpendicular to the surface. Therefore, the glass slides were treated with trimethylchlorosilane prior to use. In this way, broken fan-shaped textures being typical for columnar mesophases could be observed for boroxines 11b-11e under the POM (Figure 4 bottom).
In order to get further insight into the phase geometry, derivatives 11b-e were investigated by XRD (SAXS and WAXS). Figure 5 shows both the SAXS profile and WAXS pattern exemplarily for boroxine 11e. In the small-angle region three reflections are visible in a ratio of 1:1/√3:1/2 which were indexed as (10), (11) and (20) [44]. This diffraction pattern is typical for columnar hexagonal (Col h ) phase geometries. In the wide-angle region a diffuse halo was observed, which is generated through the interaction of the molten-like alkyl chains. In the case of derivatives 11b-d, however, only the (10) reflection and the diffuse halo were clearly visible in the diffractograms. Due to identical molecule geometry also Col h mesophases are assumed for 11b-d (for XRD data see Supplementary Information).

Materials
All reagents were used as purchased from the suppliers without further purification. Solvents were dried and distilled under nitrogen prior to use and unless otherwise stated all reactions were carried out under nitrogen atmosphere with Schlenk-type glassware.

General Procedure for the Synthesis of Tris(3,4,5-Trialkoxyphenyl)boroxines (11)
To a solution of the appropriate 7 (4.225 mmol) in abs. THF (150 mL) at −78 °C was added n-BuLi (4.50 mL, 7.2 mmol, 1.6 M in n-hexane, Merck KGaA, Darmstadt, Germany) and the reaction mixture was stirred for 1 h. Then isopropyl pinacol borate (1.32 mL, 1.20 g, 6.34 mmol, Sigma-Aldrich, Steinheim, Germany) was added and the reaction mixture stirred for a further 1 h at −78 °C. After warming to room temperature over 24 h, the reaction was terminated by addition of NH 4 Cl (50 mL, saturated solution) and stirring for 1 h. The resulting aqueous suspension was extracted with Et 2 O (3 × 50 mL). The combined organic layers were washed with H 2 O (2 × 100 mL) and brine (80 mL) and dried (MgSO 4 ). The solvent was removed under vacuum and the crude product 8 was directly used for the next step.
The crude appropriate pinacolborolane 8 was dissolved in a minimal amount of isopropanol, diethanolamine (0.85 mL, 0.89 g, 8.45 mmol, calcd for quantitative yield in the previous step, Sigma-Aldrich, Steinheim, Germany) was added, and the mixture was stirred for 24 h at ambient temperature. The suspension was filtered giving the boronic acid diethanolamine complexes 9 as white solids. Those were dissolved in THF (15 mL) and stirred with HCl (7 mL, 2 M) for 2 h at room temperature. The resulting aqueous suspension was extracted with Et 2 O (3 × 30 mL). The combined organic layers were washed with H 2 O (2 × 500 mL) and brine (50 mL) and dried (MgSO 4 ). The solvent was removed under vacuum giving mixtures of boronic acids 10 and the appropriate boroxines 11 in good overall yield.
The respective mixture of 10 and 11 (0.3 mmol, calcd for the maximum amount of 10) was dissolved in abs. Et 2 O and, together with mol sieves (4 Å) to trap resulting water, heated to reflux. Pyridine (abs., 0.1 mL, 0.095 g, 1.2 mmol) was added and the mixture was stirred for additional 20 min.
The solution was then cooled with ice (0 °C) and HCl•Et 2 O (pH = 1) was added. After 10 min, the precipitate was collected on a glass fritted funnel and the filtrate was fully evaporated in vacuo to give the desired boroxines 11 as colorless solids in quantitative yield.

Conclusions
A new and easy approach towards discotic boroxines and study of their mesomorphic properties is reported. Pure boroxines 11 were accessible starting from 5-bromo-1,3,4-trialkoxybenzenes 7 via four-step reaction which did not need purification of intermediates.
Derivatives 5 bearing three dialkyloxyphenyl groups are found to be non-mesomorphic. In contrast, linkage of three trialkyloxyphenyl substituents gave boroxines 11 displaying liquid crystalline behaviour. A minimum of C9 alkyl chain length (11b) turned out to be necessary for mesophase formation because 11a with C8 alkyl chains is non-mesogenic. The phase widths ranged from 77 K (11e) to 110 K (11b) near room temperature, thus decreasing with enhanced alkyl chain lengths. Under POM, fan-shaped textures typical for columnar mesophases were observed for all boroxines 11b-e. The phase geometry was further supported by X-ray diffraction showing in the small-angle region the typical diffraction pattern of columnar hexagonal mesophases.