Zwitterionic pyridinium derivatives of [closo-1-CB₉H₁₀]⁻ and [closo-1-CB₁₁H₁₂]⁻ as high Δε additives to a nematic host

Abstract

Substituted closo-carbaborate–pyridinium zwitterions were prepared in 35–50% yield by reacting 1-amino-closo-1-carbaborates with 4-alkoxypyrylium triflates. Two of the new materials, 1[6]d and 2[10]b, exhibit a high temperature SmA phase, whose stability is driven by dipolar interactions. Solution studies in a nematic host, ClEster, demonstrated high positive dielectric anisotropy of these new compounds (Δε ≈ +50) resulting from a longitudinal molecular dipole moment of about 20 D.

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Introduction

Polar liquid crystals and additives enable electro-optical switching¹ and are essential components of materials for liquid crystal display (LCD) applications.² Recently, we have demonstrated that zwitterionic derivatives of the [closo-1-CB₉H₁₀]⁻ cluster (A, Fig. 1) have high dielectric anisotropy Δε and are useful additives to nematic materials for LCD. In this context, we developed a synthetic methodology and prepared 1-sulfonium^3,4 and 1-quinuclidinium,³ and also 10-sulfonium^4–6 and 10-pyridinium⁷ zwitterions, compounds of the general structure IIA and IA. Some of them exhibit nematic behavior and Δε reaching a record high value of 113.5(!) in nematic solutions.⁷ The preparation of 1-pyridinium derivatives of the [closo-1-CB₉H₁₀]⁻ (A) and [closo-1-CB₁₁H₁₂]⁻ (B) clusters however was very inefficient due to the mechanistic issue in the former,⁸ and instability of the key intermediate in the latter case.⁹ Such pyridinium derivatives IIA and IIB (Fig. 1, Q = Pyr) are predicted to have significant longitudinal dipole moments, and, consequently, high positive Δε. In addition, they are expected to have lower melting points and be more soluble than the quinuclidinium analogues.

Fig. 1 The structures of the [closo-1-CB₉H₁₀]⁻ and [closo-1-CB₁₁H₁₂]⁻ anions (A and B) and their zwitterionic 1,10-(IA, IIA) and 1,12-disubstituted (IB, IIB) derivatives. Q⁺ represents an onium fragment such as ammonium, sulfonium or pyridinium. Each vertex represents a BH fragment and the sphere is a carbon atom.

Here we demonstrate a simple method for preparation of 1-pyridinium zwitterions of anions A and B and their use as high Δε additives to liquid crystal materials for LCD applications.

Results and discussion

Synthesis

Compounds 1[n], 10-vertex derivatives of type IIA, and 2[n], 12-vertex derivatives of type IIB, were obtained by adapting a general method for converting pyrylium salts to N-substituted pyridinium derivatives¹⁰ and following a single example of using 4-alkoxypyrylium in this context.¹¹ Thus, a reaction of 1-amino derivatives 3[n] and 4[n] with 4-alkoxypyrylium salts 5 in anhydrous THF gave 1[n] and 2[n], respectively, in 35–50% yields (Scheme 1). 4-Alkoxypyryliums are very rare,¹² however triflate salts 5 were conveniently obtained by alkylation of 4H-pyran-4-one with appropriate alkyl triflate 6. In addition to three primary alkoxy derivatives 5a, 5b and 5d, the new method was demonstrated also for a secondary alkoxy derivative. Thus, (S)-2-octanol was converted to triflate 6c and subsequently to pyrylium salt 5c with apparent partial racemization (ee = 35%), as evident from the analysis of the pyridinium product 1[6]c. This indicates that the electrophilic O-alkylation of 4H-pyran-4-one with 6c proceeds through an ion pair and partial scrambling of the stereocenter. Triflate 6c was significantly more reactive than primary alkyl triflates and synthesis of 1[6]c and 2[6]c was performed at lower temperatures.

Scheme 1 Synthesis of 1[n] and 2[n]. Reagents and conditions: (i) C_nH_2n+1MgBr or C_nH_2n+1ZnCl, Pd(0); (ii) 60 °C, 1 h; (iii) THF, rt.

The requisite alkyl amines 3[n] and 4[n] were obtained by alkylation of the corresponding iodo amines 7 and 8 under Pd-catalyzed coupling conditions using either RMgBr or RZnCl reagents. The synthesis of 10-hexyl amine 3[6] was reported previously¹³ and some 4[n] are reported elsewhere.⁹ The iodo amine 8 was prepared from iodo acid 9 following the procedure described for iodo amine 7 (Scheme 2),¹³ or alternatively obtained by iodination of [closo-1-CB₁₁H₁₁-1-NH₃]⁻ (10).⁹

Scheme 2 Synthesis of iodo amine 8. Reagents and conditions: (i) (COCl)₂, CH₂Cl₂, rt; (ii) Me₃SiN₃, ZnCl₂, CH₂Cl₂, 0 °C → rt; (iii) 80 °C, MeCN; (iv) t-BuOH, MeCN, 80 °C; (v) HCl, MeOH, rt; (vi) [NMe₄]⁺ OH⁻, H₂O; (vii) ICl, AcOH, 60 °C.

Electronic absorption

Pyridinium derivatives 1[n] and 2[n] are colorless solids. Spectroscopic analysis demonstrated that the 12-vertex derivative 2[6]c is more transparent in the UV region than its analogue 1[6]c, however both compounds exhibit relatively strong π → π* absorption bands at λ_max = 265 nm (calcd at 260 nm, ƒ = 0.16)¹⁴ for 2[6]c and at λ_max = 282 nm (calcd at 309 nm, ƒ = 0.25) for 1[6]c (Fig. 2). The origin of this absorption is an efficient intramolecular charge transfer from the HOMO, localized on the cluster, to the LUMO on the pyridinium fragment as shown for 1[6]b and 2[6]b in Fig. 3.⁸ Interestingly, the HOMO of the latter has lower energy and significantly greater contribution from the B-alkyl chain than observed in the 10-vertex analogue 1[6]b.

Fig. 2 Electronic absorption spectra of 1[6]c and 2[6]c in MeCN.

Fig. 3 B3LYP/6-31G(d,p) derived contours and energies of FMOs involved in low energy excitation in 1[6]b (left) and 2[6]b (right).

Thermal analysis

All six compounds melt above 100 °C (Table 1). The lowest melting points were observed for the branched (2-octyloxy)pyridinium derivatives [6]c, which is in agreement with results obtained for bis-zwitterionic derivatives of the [closo-B₁₀H₁₀]²⁻ cluster.¹⁵ It is considered that the branching methyl group close to the pyridinium ring disrupts efficient packing in the solid state driven by coulombic interactions.¹⁵ The highest melting point among the six compounds (216 °C) is exhibited by three-ring derivative 1[6]d. Data in Table 1 also suggest that derivatives of the [closo-1-CB₉H₁₀]⁻ cluster (A) have lower melting points than the 12-vertex analogues (e.g.1[6]cvs.2[6]c), which is in agreement with general trends in mesogenic derivatives of 10- and 12-vertex carboranes.^16,17

Table 1 Transition temperatures (°C) and enthalpies (kJ mol⁻¹, in italics) for 1[n] and 2[n]^a

		n	R
a Determined by DSC (5 K min^−1) in the heating mode: Cr = crystal; SmA = smectic A; I = isotropic. b Decomp. c Cr–Cr transition at 117 °C (9.7 kJ mol^−1).
1[n]	a	6	C5H11	Cr 122 (33.7) I
	c	6	CH(Me)C6H13	Cr 101 (21.5) I
	d	6	CH2C6H10C5H11	Cr 217 (33.5) SmA > 270 I^b
2[n]	b	5	C7H15	Cr 205 (26.6) I
	b	10	C7H15	Cr^c 184 (16.2) SmA 200 (9.1) I
	c	6	CH(Me)C6H13	Cr 130 (23.7) I

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Polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) revealed that 2[10]b and 1[6]d exhibit an enantiotropic SmA phase with the clearing temperature of 202 °C and above 270 °C, respectively (Table 1 and Fig. 4).

Fig. 4 (Left) DSC trace of 2[10]b. The heating and cooling rates are 5 K min⁻¹. (Right) The optical texture of 2[10]b obtained at 190 °C on cooling from the isotropic phase.

XRD data

The formation of the SmA phase was confirmed by powder XRD measurements for 2[10]b. A diffractogram of the mesophase obtained at 195 °C showed a series of sharp commensurate reflections consistent with the lamellar structure with a layer spacing of 25.61 Å (Fig. 5). Considering the calculated molecular length of 31.75 Å,¹⁴ the observed layer spacing indicates 19% of interdigitation. The wide-angle region of the diffractogram shows an unsymmetric broad halo, which can be deconvoluted into two signals with the maxima 4.5 Å and 5.4 Å. The diffused signals correlate with the mean distance between the molten alkyl chains (former) and the mean separation between the carborane cages (latter).

Fig. 5 XRD pattern for 2[10]b at 195 °C.

Temperature dependence studies demonstrated that the SmA phase has a negative thermal expansion coefficient, κ = −0.0030 (1) Å K⁻¹, while the thermal expansion coefficient of the Cr₂ phase is positive (κ = +0.00598 (3) Å K⁻¹).¹⁴

Smectic behavior of boron cluster-derived mesogens is very rare¹⁶ even for polar compounds,^6,7 and the observed high-temperature SmA phase for 2[10]b and 1[6]d results presumably from strong lateral dipole–dipole interactions of the zwitterions. This is supported by a comparison of 2[10]b with its non-polar isosteric analogue 11[10]b,⁷ a low temperature nematogen (T_NI = 25 °C) derived from p-caraborane.

Binary mixtures

Three of the new compounds were investigated as additives to ClEster, which forms a nematic phase at ambient temperature characterized by a small negative Δε. Results demonstrated that the two-ring zwitterions are more soluble in the host than the three-ring derivative 1[6]d, and 2[10]b forms stable 6 mol% solutions in the host.¹⁴ Extrapolation of the virtual [T_NI] for 2[10]b from the solution data gives the N–I transition at 82 ± 4 °C, which is significantly lower than the SmA–I transition at 202 °C. This difference further supports the notion that SmA stability originates from dipolar interactions between the zwitterions. The branched derivative 1[6]c significantly disrupts the nematic order of the host and its extrapolated [T_NI] is below −100 °C.

Dielectric measurements

Dielectric permittivity values change non-linearly with the concentration of the additives in ClEster as shown for 2[10]b in Fig. 6. This indicates some aggregation of the polar molecules in the solution, which is similar, albeit to lesser extent, to that previously observed for sulfonium (1-Sulf) and quinuclidinium (1-Quin) derivatives of type IIA.³ Therefore, dielectric parameters for the pure additives were extrapolated from dilute about 3 mol% solutions and results are shown in Table 2.

Fig. 6 Dielectric parameters as a function of concentration of 2[10]b in ClEster.

Table 2 Extrapolated dielectric data for selected compounds^a

Compound	Mol%	Δε	ε ||
a 10 μm cell. b Ref. 3. c 4 μm cell.
1[6]c	3.7	35	49
1[6]d	2.7	54	67
2[10]b	3.0	49	60
1-Sulf	2.5^b	44^c	54^c
1-Quin	3.3^b	39^c	45^c
	2.7	43^c	48^c

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Analysis of data in Table 2 demonstrates that all three pyridinium derivatives exhibit substantial dielectric anisotropy. Zwitterions 1[6]d and 2[6]b have Δε values 54 and 49, respectively, which, for comparable concentrations, are higher by about 10 than those for the previously investigated 1-Sulf and 1-Quin derivatives.³ The value Δε = 35 extrapolated for 1[6]c with the branched alkyl chain is the lowest in the series, which is presumably related to the low order parameter as evident from dramatic destabilization of the nematic phase of the host.

Computational results for 1[6]b and 2[6]b in Table 3 indicate that the value and orientation of the calculated dipole moment for both series of pyridinium zwitterions are essentially the same: 20 D oriented about 6° relative to the long molecular axis. Consequently, assuming a typical order parameter S = 0.65 and the Kirkwood factor g = μ_eff²/μ² = 0.50, the calculated dielectric anisotropy values in ClEster are Δε = 115 (ε_|| = 140) for 1[6]b and Δε = 107 (ε_|| = 130) for 2[6]b, according to the Maier–Meier relationship between molecular and bulk parameters of the nematic phase.¹⁸ Thus, the observed differences in the extrapolated dielectric parameters for the pyridinium zwitterions in Table 2 reflect different compatibility with the host: the degree of aggregation (Kirkwood factor g) and the impact on the order parameter (S_app).

Table 3 Calculated molecular parameters for selected compounds^a

Compound	μ ∣∣/D	μ ⊥/D	μ/D	β /°	Δα/Å^3	α avrg/Å^3
a Dipole moments and polarizability obtained at the B3LYP/6-31G(d,p) level of theory in ClEster dielectric medium. Polarizability values calculated from diagonal polarizability tensors were converted from a.u. to Å^3 using the factor 0.1482. b Angle between the net dipole vectors μ and μ∣∣.
1[6]b	20.18	2.36	20.32	6.7	37.0	57.1
2[6]b	20.03	2.26	20.15	6.4	33/4	59.1
1-Sulf	16.10	2.95	16.37	10.4	24.8	53.3
1-Quin	16.77	3.31	17.09	11.2	23.4	53.3

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In comparison, the same calculations for 1-Sulf and 1-Quin, with assumed S = 0.65 and g = 0.50, give the significantly lower predicted values, Δε = 77 (ε_|| = 95) and Δε = 81 (ε_|| = 100), respectively. Trends in computational results are consistent with the extrapolated experimental dielectric parameters in Table 2.

Conclusions

We have developed a method for efficient preparation of two types of 1-pyridinium zwitterions derived from the [closo-1-CB₉H₁₀]⁻ (A) and [closo-1-CB₁₁H₁₂]⁻ anions (B) that exhibit mesogenic properties and are suitable for low concentration, high Δε additives to nematic hosts. The method appears to be general and opens access to a variety of derivatives of the general structure II where Q = 4-alkoxypyridine (Fig. 1). The method permits manipulation with the structure of the R group and the alkoxy substituent for tuning properties of the compounds, especially for improving solubility in the nematic hosts. Dielectric measurements indicate that the pyridinium zwitterions 1[n] and 2[n] are significantly more effective dipolar additives to nematic hosts than those previously investigated (1-Sulf and 1-Quin).

Computational details

Quantum-mechanical calculations were carried out using Gaussian 09 suite of programs.¹⁹ Geometry optimizations for unconstrained conformers of 1[6]b and 2[6]b with the most extended molecular shapes were undertaken at the B3LYP/6-31G(d,p) level of theory using default convergence limits. The alkoxy group was set in all-trans conformation co-planar with the pyridine ring in the input structure. The orientation of the alkyl substituents on the alicyclic ring and carborane cage in the input structure was set according to conformational analysis of the corresponding 1-ethyl derivatives. No conformational search for the global minimum was attempted.

Calculations in solvent media using the PCM model²⁰ were requested with the SCRF (solvent = generic, read) keyword and eps = 3.07 and epsinf = 2.286 input parameters.

Electronic excitation energies for 1[6]b and 2[6]b in MeCN dielectric medium were obtained at the B3LYP/6-31G(d,p) level using the time-dependent DFT method²¹ supplied in the Gaussian package. Solvent calculations using the PCM model²⁰ were requested with the SCRF (solvent = CH₃CN) keyword. Selected molecular orbitals involved in these transitions are shown in Fig. S5 and S6.†

Experimental part

General

Reactions were carried out under Ar and subsequent manipulations were conducted in air. NMR spectra were obtained at 128 MHz (¹¹B) and 400 MHz (¹H) in CDCl₃ or CD₃CN. ¹¹B chemical shifts were referenced to the solvent (¹H) or to an external sample of B(OH)₃ in MeOH (¹¹B, δ = 18.1 ppm). Optical microscopy and phase identification were performed using a polarized microscope equipped with a hot stage. Thermal analysis was obtained using a TA Instruments DSC using small samples of about 0.5–1.0 mg.

Binary mixture preparation

Solutions of the pyridinium derivatives 1[n] or 2[n] in the ClEster host (15–20 mg of the host) were prepared in an open vial. The mixture of the compound and host in CH₂Cl₂ was heated for 2 h at 60 °C to remove the solvent. The binary mixtures were analyzed by polarized optical microscopy (POM) to ensure that the mixtures were homogenous. The mixtures were then allowed to stand for 2 h at room temperature before thermal and dielectric measurements.

Dielectric measurements

Dielectric properties of solutions of selected pyridinium 1[n] or 2[n] and 1-Quin in ClEster were examined with a Liquid Crystal Analytical System (LCAS – Series II, LC Vision, Inc.) using GLCAS software version 0.13.14, which implements literature procedures for dielectric constants.⁶ The homogenous binary mixtures were loaded into ITO electro-optical cells by capillary forces with moderate heating supplied by a heat gun. The cells (about 4 μm thick, electrode area 0.581 cm² or about 10 μm thick, electrode area 1.000 cm², and anti-parallel rubbed polyimide layer) were obtained from LC Vision, Inc. The filled cells were heated to an isotropic phase and cooled to room temperature before measuring dielectric properties. Default parameters were used for measurements: triangular shaped voltage bias ranging from 0.1–20 V at 1 kHz frequency. The threshold voltage V_th was measured at a 5% change. For each mixture the measurement was repeated 10 times for two independent cells. The results were averaged to calculate the mixture's parameters. Results are shown in Tables S4–S6.† All measurements were run at 25 °C. Error in concentration is estimated at about 1.5%. The resulting extrapolated values for pure additives are shown in Table 2.

General procedure for preparation of pyridinium derivatives 1[n] and 2[n]

A mixture of amine 3[n] or 4[n] (1 mmol) and the appropriate crude pyrylium triflate 5 [freshly prepared from 4H-pyran-4-one (1.2 mmol) and alkyl triflate 6] in THF (1 mL) under Ar was stirred overnight at room temperature. The solvent was evaporated to give a dark solid. Pure product 1[n] or 2[n] was obtained as a white crystalline solid in 34–51% yield by column chromatography (CH₂Cl₂/hexane, 1 : 1) followed by recrystallization from iso-octane/toluene and then EtOH.

1[6]a . ¹H NMR (CDCl₃, 400 MHz) δ 0.3–2.8 (br m, 8H), 0.92 (t, J = 7.0 Hz, 3H), 0.99 (t, J = 7.1 Hz, 3H), 1.37–1.54 (m, 8H), 1.60 (quint, J = 14.6 Hz, 2H), 1.88–2.00 (m, 4H), 2.08 (pseudo t, J = 8.1 Hz, 2H), 4.35 (t, J = 6.5 Hz, 2H), 7.29 (d, J = 7.5 Hz, 2H), 9.14 (d, J = 7.5 Hz, 2H); ¹¹B NMR (CDCl₃, 128 MHz) δ −24.8 (d, J = 143 Hz, 4B), −15.4 (d, J = 137 Hz, 4B), 47.7 (s, 1B). Anal. calcd for C₁₇H₃₆B₉NO: C, 55.52; H, 9.87; N, 3.81. Found: C, 55.96; H, 9.91; N, 3.78%.

1[6]c . ee = 35% (AD-H Chiral, 15% EtOH in hexane), [α]²⁴_D = +8° (c = 1.0, MeCN); ¹H NMR (CDCl₃, 400 MHz) δ 0.3–2.8 (br m, 8H), 0.90 (t, J = 6.9 Hz, 3H), 0.92 (t, J = 7.0 Hz, 3H) 1.30–1.49 (m, 12H), 1.51 (d, J = 6.1 Hz, 3H), 1.60 (quint, J = 7.1 Hz, 2H), 1.74–1.81 (m, 1H), 1.86–1.96 (m, 3H), 2.08 (pseudo t, J = 8.1 Hz, 2H), 4.77 (sextet, J = 6.1 Hz, 1H), 7.27 (d, J = 6.5 Hz, 2H), 9.16 (d, J = 7.6 Hz, 2H); ¹¹B NMR (CDCl₃, 128 MHz) δ −24.9 (d, J = 143 Hz, 4B), −15.4 (d, J = 139 Hz, 4B), 47.2 (s, 1B); UV (MeCN), λ_max (log ε) 245 (4.06), 282 (4.16). Anal. calcd for C₂₀H₄₂B₉NO: C, 58.61; H, 10.33; N, 3.42. Found: C, 58.57; H, 10.26; N, 3.46%.

1[6]d . ¹H NMR (CDCl₃, 400 MHz) δ 0.3–2.8 (br m, 8H), 0.90 (t, J = 7.2 Hz, 3H), 0.92 (t, J = 7.0 Hz, 3H), 1.01 (t, J = 10.6 Hz, 2H), 1.10–1.46 (m, 15H), 1.60 (quint, J = 7.2 Hz, 2H), 1.86–1.96 (m, 7H), 2.08 (pseudo t, J = 8.0 Hz, 2H), 4.14 (d, J = 5.9 Hz, 2H), 7.30 (d, J = 7.6 Hz, 2H), 9.16 (d, J = 7.5 Hz, 2H); {¹H} ¹¹B NMR (CDCl₃, 128 MHz) δ −24.8 (4B), −15.4 (4B), 47.7 (1B). Anal. calcd for C₂₄H₄₈B₉NO: C, 62.13; H, 10.43; N, 3.02. Found: C, 62.35; H, 10.39; N, 3.03%.

2[5]b . ¹H NMR (CDCl₃, 400 MHz) δ 0.66 (br s, 2H), 0.85 (t, J = 7.0 Hz, 3H), 0.89 (t, J = 6.8 Hz, 3H), 1.0–2.8 (br m, 10H), 1.20–1.40 (m, 12H), 1.41–1.49 (m, 2H), 1.89 (quint, J = 7.0 Hz, 2H), 4.26 (t, J = 6.5 Hz, 2H), 7.10 (d, J = 7.8 Hz, 2H), 8.86 (d, J = 8.9 Hz, 2H); {¹H}¹¹B NMR (CDCl₃, 128 MHz) δ −14.2 (5B), −11.6 (5B), 4.4 (1B). Anal. calcd for C₁₈H₄₀B₁₁NO: C, 53.30: H, 9.94; N, 3.45. Found: C, 53.52; H, 9.88; N, 3.40%.

2[10]b . ¹H NMR (CDCl₃, 400 MHz) δ 0.66 (br s, 2H), 0.87 (t, J = 7.0 Hz, 3H), 0.89 (t, J = 6.8 Hz, 3H), 1.0–2.8 (br m, 10H), 1.23–1.28 (m, 16H), 1.31–1.39 (m, H), 1.42–1.49 (m, 2H), 1.89 (quint, J = 7.0 Hz, 2H), 4.26 (t, J = 6.48 Hz, 2H), 7.09 (d, J = 7.8 Hz, 2H), 8.87 (t, J = 7.8 Hz, 2H); ¹¹B NMR (CDCl₃, 400 MHz) δ −14.1 (d, J = 142 Hz, 5B), −11.6 (d, J = 137 Hz, 5B), 4.8 (s, 1B). Anal. calcd for C₂₃H₅₀B₁₁NO: C, 58.09; H, 10.60; N, 2.95. Found: C, 58.13; H, 10.45; N, 2.99%.

2[6]c . ¹H NMR (CDCl₃, 400 MHz) δ 0.66 (br s, 2H), 0.84–0.91 (m, 6H), 1.0–2.8 (br m, 10H), 1.24–1.37 (m, 16H), 1.44 (d, J = 6.2 Hz, 3H), 1.67–1.76 (m 1H), 1.79–1.84 (m, 1H), 4.67 (sextet, J = 6.1 Hz, 1H), 7.05 (d, J = 7.8 Hz, 2H), 8.84 (d, J = 7.8 Hz, 2H); {¹H} ¹¹B NMR (CDCl₃, 128 MHz) δ −14.2 (5B), −11.7 (5B), 4.34 (1B); UV (MeCN), λ_max (log ε) 264 (4.47). Anal. calcd for C₂₀H₄₄B₁₁NO: C, 55.42; H, 10.23; N, 3.23. Found: C, 55.71; H, 10.17; N, 3.28%.

General methods for preparation of pyrylium salts 5

Method A. A neat mixture of 4H-pyran-4-one (1 mmol) and alkyl triflate 6 (1 mmol) was stirred at 60 °C for 1 h under Ar resulting in brown oil. The mixture was cooled to room temperature and used without further purification.

Method B. A modified Method A by using CH₂Cl₂ (1 mL) as a solvent. After 1 h, the solvent was removed in vacuo and the product was used without further purification.

Method C. A modified Method B by conducting the reaction at 0 °C to prevent decomposition of the secondary alkyl triflate. ¹H NMR data are provided in the ESI.†

General methods for preparation of alkyl triflates 6

Method A. Following a general method for alkyl triflates,²² to a vigorously stirred solution of triflic anhydride (1.2 mmol) in CH₂Cl₂ (15 mL) at 0 °C, a solution of pyridine (1 mmol) and primary alcohol (1 mmol) in CH₂Cl₂ (10 mL) was added dropwise over a 15 min period and the mixture was stirred for an additional 1 h at 0 °C. The solution was washed with ice-cold H₂O, dried (Na₂SO₄) and evaporated to dryness to give the appropriate alkyl triflate 6 as a colorless liquid that quickly began to darken. The resulting mixture was filtered through a cotton plug and used without further purification.

Method B. To a vigorously stirred mixture of a secondary alcohol (1 mmol) and pyridine (1 mmol) at −78 °C in CH₂Cl₂ (25 mL) was added dropwise triflic anhydride (1 mmol). The mixture was stirred for 10 min at −78 °C and then kept at 0 °C until the alcohol was consumed (by TLC). The mixture was washed with ice-cold water, dried (Na₂SO₄) and the solvent was removed in vacuo at 0 °C. The resulting triflate 6 was kept at 0 °C and quickly used in the next step. ¹H NMR data are provided in the ESI.†

Preparation of 1-decyl-12-(4-heptyloxyphenyl)-p-carborane (11[10]b)

A solution of 1-decyl-12-(4-hydroxyphenyl)-p-carborane (12[10], 125 mg, 0.318 mmol), heptyl tosylate (104 mg, 0.382 mmol), K₂CO₃ (132 mg, 0.956 mmol) and NBu₄Br (10 mg, 0.032 mmol) in anhydrous CH₃CN (5 mL) was refluxed overnight. The mixture was cooled down to room temperature and filtered. The residue was washed with CH₂Cl₂ (3 × 10 mL), dried (Na₂SO₄) and evaporated to dryness. The crude product was purified by column chromatography (hexane/CH₂Cl₂, 2 : 1) to give 160 mg of 11[10]b as a colorless liquid, which was crystallized from CH₃CN containing a few drops of EtOAc at −80 °C: ¹H NMR (CDCl₃, 400 MHz) δ 0.88 (t, J = 7.1 Hz, 6H), 1.0–2.6 (br m, 10H), 1.12–1.42 (m, 24H), 1.64 (pseudo t, J = 8.1 Hz, 2H), 1.73 (quint, J = 6.6 Hz, 2H), 3.87 (t, J = 6.5 Hz, 2H), 6.65 (d, J = 9.0 Hz, 2H), 7.09 (t, J = 9.0 Hz, 2H); ¹¹B NMR (CDCl₃, 128 MHz) δ −12.3 (d, J = 164 Hz). Anal. calcd for C₂₅H₅₀B₁₀O: C, 63.25; H, 10.62. Found: C, 63.99; H, 10.73%.

1-Decyl-12-(4-methoxyphenyl)-p-carborane (11[10]e)

A solution of (4-methoxyphenyl)-p-carborane²³ (200 mg, 0.80 mmol) in Et₂O (3 mL) was treated with BuLi (0.8 mL, 1.90 mmol, 2.5 M in hexanes) at −78 °C. The mixture was stirred for 15 minutes and then warmed up to room temperature and stirred for 2 h. THF (1.0 mL) was added to the mixture and the solution was stirred for 1 h at room temperature. The reaction was cooled to 0 °C and a solution of 1-iododecane (236 mg, 0.88 mmol) in Et₂O was added to the mixture. The reaction was allowed to warm up to room temperature and stirred overnight. The solvent was removed in vacuo and the residue was dissolved in CH₂Cl₂ and passed through a short silica gel plug. The solvent was evaporated and the resulting mixture was separated by column chromatography (hexane). The desired product 11[10]e was isolated as the second fraction (122 mg, 40% yield) as a colorless film. The product was crystallized from CH₃CN containing a few drops of EtOAc and then cold EtOAc containing a few drops of hexane at −80 °C: mp 38–40 °C (no mesophase was detected upon cooling to −20 °C); ¹H NMR (CDCl₃, 400 MHz) δ 0.88 (t, J = 7.1 Hz, 3H), 1.0–2.6 (br m, 10H), 1.12–1.35 (m, 16H), 1.65 (pseudo t, J = 8.1 Hz, 2H), 3.74 (s, 3H), 6.67 (d, J = 9.0 Hz, 2H), 7.11 (d, J = 9.0 Hz, 2H); ¹¹B NMR (CDCl₃, 128 MHz) δ −12.3 (d, J = 164 Hz); anal. calcd for C₁₉H₃₈B₁₀O: C, 58.42; H, 9.81. Found: C, 58.84; H, 9.39%.

The first oily fraction (27 mg), obtained in the chromatographic separation of 11[10]e, was identified as 1-decyl-12-(3-decyl-4-methoxyphenyl)-p-carborane which apparently resulted from ortho-lithiation of (4-methoxyphenyl)-p-carborane under the reaction conditions: ¹H NMR (CDCl₃, 400 MHz) δ 0.88 (t, J = 6.9 Hz, 3H), 0.89 (t, J = 7.7 Hz, 3H), 1.05–1.35 (m, 30H), 1.44–1.52 (m, 2H), 1.64 (br t, J = 8.2 Hz, 2H), 2.48 (t, J = 7.7 Hz, 2H), 3.74 (s, 3H), 6.58 (d, J = 8.7 Hz, 1H), 6.94 (d, J = 2.6 Hz, 1H), 6.98 (dd, J₁ = 8.6 Hz, J₂ = 2.6 Hz, 1H); ¹¹B NMR (CDCl₃, 128 MHz) δ −12.3 (d, J = 163 Hz); HRMS, calcd for C₂₉H₅₉B₁₀O: m/z = 533.5496, found m/z = 533.5507.

1-Decyl-12-(4-hydroxyphenyl)-p-carborane (12[10])

A solution of 1-decyl-12-(4-methoxyphenyl)-p-carborane (11[10]e, 187 mg, 0.478 mmol) in CH₂Cl₂ (5 mL) was treated with BBr₃ (360 mg, 1.44 mmol, 1.0 M in CH₂Cl₂) at 0 °C and the reaction was allowed to warm up to room temperature and stirred overnight. Water (10 mL) was added to the mixture and the organic layer was separated, dried (Na₂SO₄) and evaporated to dryness to give the crude product as a colorless film, which was purified by column chromatography (CH₂Cl₂) to give 145 mg (78% yield) of a white solid. The product was recrystallized at −80 °C from CH₃CN containing a few drops of EtOAc and then EtOAc containing a few drops of hexane to give phenol 12[10] as a colorless film: ¹H NMR (CDCl₃, 400 MHz) δ 0.88 (t, J = 7.1 Hz, 3H), 1.0–2.6 (br m, 10H), 1.10–1.30 (m, 16H), 1.64 (pseudo t, J = 8.1 Hz, 2H), 4.71 (s, 1H), 6.59 (d, J = 8.9 Hz, 2H), 7.06 (d, J = 8.8 Hz, 2H); ¹¹B NMR (CDCl₃, 128 MHz) δ −12.3 (d, J = 164 Hz). Anal. calcd for C₁₈H₃₆B₁₀O: C, 57.41; H, 9.64. Found: C, 57.18; H, 9.33%.

Acknowledgements

This work was supported by the NSF grant DMR-1207585. We are grateful to Professor Roman Dąbrowski of Military University of Technology (Warsaw, Poland) for the gift of ClEster. We thank Ms. Amanda Doody for help with chiral HPLC analysis of 1[6]c.

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Footnote

† Electronic supplementary information (ESI) available: Additional synthesis and characterization details for intermediates 4[6], 5, 6, and 8, ee measurement details, additional XRD data, partial TD-DFT output, and archive of calculated equilibrium geometries for 1[6]b and 2[6]b. See DOI: 10.1039/c3tc32351j

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