Zwitterionic pyridinium derivatives of [closo-1-CB9H 10]- and [closo-1-CB11H12] - as high Δε additives to a nematic host

Article abstract of DOI:10.1039/c3tc32351j

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.

Full text of DOI:10.1039/c3tc32351j

Journal of
Materials Chemistry C
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Zwitterionic pyridinium derivatives of [closo-1-
CB9H10]ꢀ and [closo-1-CB11H12]ꢀ as high D3
additives to a nematic host†
Cite this: J. Mater. Chem. C, 2014, 2,
1585
a
b
ac
Received 28th November 2013
Accepted 17th December 2013
*
Jacek Pecyna, Damian Pociecha and Piotr Kaszynski
´
DOI: 10.1039/c3tc32351j
www.rsc.org/MaterialsC
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 (D3 z +50) resulting from a longitudinal molecular dipole
moment of about 20 D.
ꢀ
and [closo-1-
Fig.
CB11H12
1
The structures of the [closo-1-CB9H10
]
]
ꢀ 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.
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 demon-
strated that zwitterionic derivatives of the [closo-1-CB9H10]ꢀ
cluster (A, Fig. 1) have high dielectric anisotropy D3 and are
useful additives to nematic materials for LCD. In this context,
we developed a synthetic methodology and prepared 1-sulfo-
nium3,4 and 1-quinuclidinium,3 and also 10-sulfonium4–6 and
10-pyridinium7 zwitterions, compounds of the general structure
IIA and IA. Some of them exhibit nematic behavior and D3
reaching a record high value of 113.5(!) in nematic solutions.7
lower melting points and be more soluble than the quinucli-
dinium analogues.
Here we demonstrate a simple method for preparation of
1-pyridinium zwitterions of anions A and B and their use as high
D3 additives to liquid crystal materials for LCD applications.
Results and discussion
Synthesis
The preparation of 1-pyridinium derivatives of the [closo-1-
ꢀ
CB9H10]ꢀ (A) and [closo-1-CB11H12
]
(B) clusters however was
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
very inefficient due to the mechanistic issue in the former,8 and
instability of the key intermediate in the latter case.9 Such pyr-
idinium derivatives IIA and IIB (Fig. 1, Q ¼ Pyr) are predicted to pyridinium derivatives10 and following a single example of using
have signicant longitudinal dipole moments, and, conse- 4-alkoxypyrylium in this context.11 Thus, a reaction of 1-amino
quently, high positive D3. In addition, they are expected to have
derivatives 3[n] and 4[n] with 4-alkoxypyrylium salts 5 in anhy-
drous THF gave 1[n] and 2[n], respectively, in 35–50% yields
(Scheme 1). 4-Alkoxypyryliums are very rare,12 however triate
salts 5 were conveniently obtained by alkylation of 4H-pyran-4-
one with appropriate alkyl triate 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 triate 6c and subsequently to pyry-
lium salt 5c with apparent partial racemization (ee ¼ 35%), as
evident from the analysis of the pyridinium product 1[6]c. This
aDepartment of Chemistry, Vanderbilt University, Nashville, TN 37235, USA. E-mail:
piotr.kaszynski@vanderbilt.edu; Tel: +1-615-322-3458
bDepartment of Chemistry, University of Warsaw, Zwirki i Wigury 101, 02-089 Warsaw,
Poland
c
´ ´
´ ´
Faculty of Chemistry, University of Łodz, Tamka 12, 91403 Łodz, Poland
† 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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primary alkyl triates and synthesis of 1[6]c and 2[6]c was per-
formed at lower temperatures.
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
Electronic absorption
Pyridinium derivatives 1[n] and 2[n] are colorless solids. Spec-
troscopic 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 p / p*
absorption bands at lmax ¼ 265 nm (calcd at 260 nm, f ¼ 0.16)14
for 2[6]c and at lmax ¼ 282 nm (calcd at 309 nm, f ¼ 0.25) for 1[6]
c (Fig. 2). The origin of this absorption is an efficient intra-
molecular 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 signicantly greater contribution from the
B-alkyl chain than observed in the 10-vertex analogue 1[6]b.
Scheme 1 Synthesis of 1[n] and 2[n]. Reagents and conditions: (i)
CnH2n+1MgBr or CnH2n+1ZnCl, Pd(0); (ii) 60 ꢁC, 1 h; (iii) THF, rt.
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]+ OHꢀ,
H2O; (vii) ICl, AcOH, 60 ꢁC.
Thermal analysis
All six compounds melt above 100 ꢁC (Table 1). The lowest
melting points were observed for the branched (2-octyloxy)pyr-
idinium derivatives [6]c, which is in agreement with results
obtained for bis-zwitterionic derivatives of the [closo-B10H10]2ꢀ
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 deriva-
ꢀ
tives of the [closo-1-CB9H10
]
cluster (A) have lower melting
Fig. 2 Electronic absorption spectra of 1[6]c and 2[6]c in MeCN.
points than the 12-vertex analogues (e.g. 1[6]c vs. 2[6]c), which is
in agreement with general trends in mesogenic derivatives of
10- and 12-vertex carboranes.16,17
Polarizing optical microscopy (POM) and differential scan-
ning calorimetry (DSC) revealed that 2[10]b and 1[6]d exhibit an
indicates that the electrophilic O-alkylation of 4H-pyran-4-one
with 6c proceeds through an ion pair and partial scrambling of
the stereocenter. Triate 6c was signicantly more reactive than
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).
1586 | J. Mater. Chem. C, 2014, 2, 1585–1591
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Table 1 Transition temperatures (ꢁC) and enthalpies (kJ molꢀ1, in
italics) for 1[n] and 2[n]a
n
R
1[n]
2[n]
a
c
d
b
b
c
6
6
6
5
10
6
C5H11
Cr 122 (33.7) I
CH(Me)C6H13
CH2C6H10C5H11
C7H15
C7H15
CH(Me)C6H13
Cr 101 (21.5) I
Cr 217 (33.5) SmA > 270 Ib
Cr 205 (26.6) I
Crc 184 (16.2) SmA 200 (9.1) I
Cr 130 (23.7) I
a
Determined by DSC (5 K minꢀ1) in the heating mode: Cr ¼ crystal;
b
c
SmA ¼ smectic A; I ¼ isotropic. Decomp. Cr–Cr transition at 117
ꢁC (9.7 kJ molꢀ1).
Fig. 6 Dielectric parameters as a function of concentration of 2[10]b
in ClEster.
Fig. 4 (Left) DSC trace of 2[10]b. The heating and cooling rates are 5 K
minꢀ1. (Right) The optical texture of 2[10]b obtained at 190 ꢁC on
cooling from the isotropic phase.
alkyl chains (former) and the mean separation between the
carborane cages (latter).
Temperature dependence studies demonstrated that the
SmA phase has a negative thermal expansion coefficient,
k ¼ ꢀ0.0030 (1) A Kꢀ1, while the thermal expansiꢀo1n coefficient
˚
of the Cr2 phase is positive (k ¼ +0.00598 (3) A K ).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 zwitter-
ions. This is supported by a comparison of 2[10]b with its non-
polar isosteric analogue 11[10]b,7 a low temperature nematogen
ꢁ
(TNI ¼ 25 C) derived from p-caraborane.
Fig. 5 XRD pattern for 2[10]b at 195 ꢁC.
enantiotropic SmA phase with the clearing temperature of
ꢁ
ꢁ
202 C and above 270 C, respectively (Table 1 and Fig. 4).
XRD data
Table 2 Extrapolated dielectric data for selected compoundsa
The formation of the SmA phase was conrmed by powder XRD
measurements for 2[10]b. A diffractogram of the mesophase
obtained at 195 ꢁC showed a series of sharp commensurate
reections consistent with the lamellar structure with a layer
Compound
Mol%
D3
3||
1[6]c
1[6]d
2[10]b
1-Sulf
1-Quin
3.7
2.7
3.0
2.5b
3.3b
2.7
35
54
49
67
˚
spacing of 25.61 A (Fig. 5). Considering the calculated molecular
length of 31.75 A,14 the observed layer spacing indicates 19% of
˚
49
60
44c
39c
43c
54c
45c
48c
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 A and 5.4 A. The diffused
a
b
10 mm cell. Ref. 3. c 4 mm cell.
signals correlate with the mean distance between the molten
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Table 3 Calculated molecular parameters for selected compoundsa
extrapolated dielectric parameters for the pyridinium zwitter-
ions in Table 2 reect different compatibility with the host: the
degree of aggregation (Kirkwood factor g) and the impact on the
order parameter (Sapp).
In comparison, the same calculations for 1-Sulf and 1-Quin,
with assumed S ¼ 0.65 and g ¼ 0.50, give the signicantly lower
predicted values, D3 ¼ 77 (3|| ¼ 95) and D3 ¼ 81 (3|| ¼ 100),
respectively. Trends in computational results are consistent with
the extrapolated experimental dielectric parameters in Table 2.
3
3
˚
˚
Compound
mrr/D
mt/D
m/D
bb/ꢁ
Da/A
aavrg/A
1[6]b
2[6]b
1-Sulf
1-Quin
20.18
20.03
16.10
16.77
2.36
2.26
2.95
3.31
20.32
20.15
16.37
17.09
6.7
6.4
10.4
11.2
37.0
33/4
24.8
23.4
57.1
59.1
53.3
53.3
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
3
b
˚
a.u. to A using the factor 0.1482. Angle between the net dipole
vectors m and mrr.
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 D3 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 ¼ 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 signicantly more effective dipolar
additives to nematic hosts than those previously investigated (1-
Sulf and 1-Quin).
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 D3. 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 [TNI] for
2[10]b from the solution data gives the N–I transition at 82 ꢂ
ꢁ
4 C, which is signicantly 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 signicantly disrupts
the nematic order of the host and its extrapolated [TNI] is
below ꢀ100 ꢁC.
Computational details
Dielectric measurements
Quantum-mechanical calculations were carried out using
Dielectric permittivity values change non-linearly with the Gaussian 09 suite of programs.19 Geometry optimizations for
concentration of the additives in ClEster as shown for 2[10]b in unconstrained conformers of 1[6]b and 2[6]b with the most
Fig. 6. This indicates some aggregation of the polar molecules extended molecular shapes were undertaken at the B3LYP/6-
in the solution, which is similar, albeit to lesser extent, to that 31G(d,p) level of theory using default convergence limits. The
previously observed for sulfonium (1-Sulf) and quinuclidinium alkoxy group was set in all-trans conformation co-planar with
(1-Quin) derivatives of type IIA.3 Therefore, dielectric parame- the pyridine ring in the input structure. The orientation of the
ters for the pure additives were extrapolated from dilute about 3 alkyl substituents on the alicyclic ring and carborane cage in the
mol% solutions and results are shown in Table 2.
input structure was set according to conformational analysis of
Analysis of data in Table 2 demonstrates that all three pyr- the corresponding 1-ethyl derivatives. No conformational
idinium derivatives exhibit substantial dielectric anisotropy. search for the global minimum was attempted.
Zwitterions 1[6]d and 2[6]b have D3 values 54 and 49, respec-
Calculations in solvent media using the PCM model20 were
tively, which, for comparable concentrations, are higher by requested with the SCRF (solvent ¼ generic, read) keyword and
about 10 than those for the previously investigated 1-Sulf and eps ¼ 3.07 and epsinf ¼ 2.286 input parameters.
1-Quin derivatives.3 The value D3 ¼ 35 extrapolated for 1[6]c
Electronic excitation energies for 1[6]b and 2[6]b in MeCN
with the branched alkyl chain is the lowest in the series, which dielectric medium were obtained at the B3LYP/6-31G(d,p) level
is presumably related to the low order parameter as evident using the time-dependent DFT method21 supplied in the
from dramatic destabilization of the nematic phase of the host. Gaussian package. Solvent calculations using the PCM model20
Computational results for 1[6]b and 2[6]b in Table 3 indicate were requested with the SCRF (solvent ¼ CH3CN) keyword.
that the value and orientation of the calculated dipole moment Selected molecular orbitals involved in these transitions are
for both series of pyridinium zwitterions are essentially the shown in Fig. S5 and S6.†
same: 20 D oriented about 6ꢁ relative to the long molecular axis.
Consequently, assuming a typical order parameter S ¼ 0.65 and
Experimental part
General
the Kirkwood factor g ¼ meff2/m2 ¼ 0.50, the calculated dielectric
anisotropy values in ClEster are D3 ¼ 115 (3|| ¼ 140) for 1[6]b
and D3 ¼ 107 (3|| ¼ 130) for 2[6]b, according to the Maier–Meier Reactions were carried out under Ar and subsequent manipu-
relationship between molecular and bulk parameters of the lations were conducted in air. NMR spectra were obtained at
nematic phase.18 Thus, the observed differences in the 128 MHz (11B) and 400 MHz (1H) in CDCl3 or CD3CN. 11B
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chemical shis were referenced to the solvent (1H) or to an
external sample of B(OH)3 in MeOH (11B, d ¼ 18.1 ppm). Optical
microscopy and phase identication 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.
1[6]c. ee ¼ 35% (AD-H Chiral, 15% EtOH in hexane), [a]D24
¼
+8ꢁ (c ¼ 1.0, MeCN); 1H NMR (CDCl3, 400 MHz) d 0.3–2.8 (br m,
8H), 0.90 (t, J ¼ 6.9 Hz, 3H), 0.92 (t, J ¼ 7.0 Hz, 3H) 1.30–1.49 (m,
12H), 1.51 (d, J ¼ 6.1 Hz, 3H), 1.60 (quint, J ¼ 7.1 Hz, 2H), 1.74–
1.81 (m, 1H), 1.86–1.96 (m, 3H), 2.08 (pseudo t, J ¼ 8.1 Hz, 2H),
4.77 (sextet, J ¼ 6.1 Hz, 1H), 7.27 (d, J ¼ 6.5 Hz, 2H), 9.16 (d, J ¼
7.6 Hz, 2H); 11B NMR (CDCl3, 128 MHz) d ꢀ24.9 (d, J ¼ 143 Hz,
4B), ꢀ15.4 (d, J ¼ 139 Hz, 4B), 47.2 (s, 1B); UV (MeCN), lmax
(log 3) 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%.
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.
1
1[6]d. H NMR (CDCl3, 400 MHz) d 0.3–2.8 (br m, 8H), 0.90
(t, J ¼ 7.2 Hz, 3H), 0.92 (t, J ¼ 7.0 Hz, 3H), 1.01 (t, J ¼ 10.6 Hz,
2H), 1.10–1.46 (m, 15H), 1.60 (quint, J ¼ 7.2 Hz, 2H), 1.86–1.96
(m, 7H), 2.08 (pseudo t, J ¼ 8.0 Hz, 2H), 4.14 (d, J ¼ 5.9 Hz, 2H),
7.30 (d, J ¼ 7.6 Hz, 2H), 9.16 (d, J ¼ 7.5 Hz, 2H); {1H} 11B NMR
(CDCl3, 128 MHz) d ꢀ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%.
Dielectric measurements
2[5]b. 1H NMR (CDCl3, 400 MHz) d 0.66 (br s, 2H), 0.85 (t, J ¼
7.0 Hz, 3H), 0.89 (t, J ¼ 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 ¼ 7.0 Hz, 2H),
4.26 (t, J ¼ 6.5 Hz, 2H), 7.10 (d, J ¼ 7.8 Hz, 2H), 8.86 (d, J ¼ 8.9
Hz, 2H); {1H}11B NMR (CDCl3, 128 MHz) d ꢀ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%.
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
soware version 0.13.14, which implements literature proce-
dures 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 mm
thick, electrode area 0.581 cm2 or about 10 mm thick, electrode
area 1.000 cm2, and anti-parallel rubbed polyimide layer) were
obtained from LC Vision, Inc. The lled 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 Vth 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
1
2[10]b. H NMR (CDCl3, 400 MHz) d 0.66 (br s, 2H), 0.87 (t,
J ¼ 7.0 Hz, 3H), 0.89 (t, J ¼ 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 ¼ 7.0 Hz, 2H), 4.26 (t, J ¼ 6.48 Hz, 2H), 7.09 (d, J ¼ 7.8 Hz, 2H),
8.87 (t, J ¼ 7.8 Hz, 2H); 11B NMR (CDCl3, 400 MHz) d ꢀ14.1 (d,
J ¼ 142 Hz, 5B), ꢀ11.6 (d, J ¼ 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) d 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 ¼ 6.2
Hz, 3H), 1.67–1.76 (m 1H), 1.79–1.84 (m, 1H), 4.67 (sextet, J ¼
6.1 Hz, 1H), 7.05 (d, J ¼ 7.8 Hz, 2H), 8.84 (d, J ¼ 7.8 Hz, 2H); {1H}
11B NMR (CDCl3, 128 MHz) d ꢀ14.2 (5B), ꢀ11.7 (5B), 4.34 (1B);
UV (MeCN), lmax (log 3) 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%.
ꢁ
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]
General methods for preparation of pyrylium salts 5
A mixture of amine 3[n] or 4[n] (1 mmol) and the appropriate
crude pyrylium triate 5 [freshly prepared from 4H-pyran-4-one
(1.2 mmol) and alkyl triate 6] in THF (1 mL) under Ar was
stirred overnight at room temperature. The solvent was evapo-
rated 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 chro-
matography (CH2Cl2/hexane, 1 : 1) followed by recrystallization
from iso-octane/toluene and then EtOH.
Method A. A neat mixture of 4H-pyran-4-one (1 mmol) and
alkyl triate 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 purication.
Method B. A modied Method A by using CH2Cl2 (1 mL) as a
solvent. Aer 1 h, the solvent was removed in vacuo and the
product was used without further purication.
Method C. A modied Method B by conducting the reaction
at 0 ꢁC to prevent decomposition of the secondary alkyl triate.
1H NMR data are provided in the ESI.†
1[6]a. 1H NMR (CDCl3, 400 MHz) d 0.3–2.8 (br m, 8H), 0.92 (t, J
¼ 7.0 Hz, 3H), 0.99 (t, J ¼ 7.1 Hz, 3H), 1.37–1.54 (m, 8H), 1.60
(quint, J ¼ 14.6 Hz, 2H), 1.88–2.00 (m, 4H), 2.08 (pseudo t, J ¼ 8.1
Hz, 2H), 4.35 (t, J ¼ 6.5 Hz, 2H), 7.29 (d, J ¼ 7.5 Hz, 2H), 9.14 (d, J ¼
7.5 Hz, 2H); 11B NMR (CDCl3, 128 MHz) d ꢀ24.8 (d, J ¼ 143 Hz, 4B),
ꢀ15.4 (d, J ¼ 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%.
General methods for preparation of alkyl triates 6
Method A. Following a general method for alkyl triates,22 to
a vigorously stirred solution of triic anhydride (1.2 mmol) in
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CH2Cl2 (15 mL) at 0 ꢁC, a solution of pyridine (1 mmol) and 1.12–1.35 (m, 16H), 1.65 (pseudo t, J ¼ 8.1 Hz, 2H), 3.74 (s, 3H),
primary alcohol (1 mmol) in CH2Cl2 (10 mL) was added drop- 6.67 (d, J ¼ 9.0 Hz, 2H), 7.11 (d, J ¼ 9.0 Hz, 2H); 11B NMR (CDCl3,
wise over a 15 min period and the mixture was stirred for an 128 MHz) d ꢀ12.3 (d, J ¼ 164 Hz); anal. calcd for C19H38B10O: C,
ꢁ
additional 1 h at 0 C. The solution was washed with ice-cold 58.42; H, 9.81. Found: C, 58.84; H, 9.39%.
H2O, dried (Na2SO4) and evaporated to dryness to give the
The rst oily fraction (27 mg), obtained in the chromato-
appropriate alkyl triate 6 as a colorless liquid that quickly graphic separation of 11[10]e, was identied as 1-decyl-12-(3-
began to darken. The resulting mixture was ltered through a decyl-4-methoxyphenyl)-p-carborane which apparently resulted
cotton plug and used without further purication.
from ortho-lithiation of (4-methoxyphenyl)-p-carborane under
Method B. To a vigorously stirred mixture of a secondary the reaction conditions: 1H NMR (CDCl3, 400 MHz) d 0.88 (t, J ¼
alcohol (1 mmol) and pyridine (1 mmol) at ꢀ78 ꢁC in CH2Cl2 (25 6.9 Hz, 3H), 0.89 (t, J ¼ 7.7 Hz, 3H), 1.05–1.35 (m, 30H), 1.44–
mL) was added dropwise triic anhydride (1 mmol). The mixture 1.52 (m, 2H), 1.64 (br t, J ¼ 8.2 Hz, 2H), 2.48 (t, J ¼ 7.7 Hz, 2H),
was stirred for 10 min at ꢀ78 ꢁC and then kept at 0 ꢁC until the 3.74 (s, 3H), 6.58 (d, J ¼ 8.7 Hz, 1H), 6.94 (d, J ¼ 2.6 Hz, 1H), 6.98
alcohol was consumed (by TLC). The mixture was washed with (dd, J1 ¼ 8.6 Hz, J2 ¼ 2.6 Hz, 1H); 11B NMR (CDCl3, 128 MHz) d
ice-cold water, dried (Na2SO4) and the solvent was removed in ꢀ12.3 (d,
J
¼
163 Hz); HRMS, calcd for C29H59B10O:
vacuo at 0 ꢁC. The resulting triate 6 was kept at 0 ꢁC and quickly m/z ¼ 533.5496, found m/z ¼ 533.5507.
used in the next step. 1H NMR data are provided in the ESI.†
1-Decyl-12-(4-hydroxyphenyl)-p-carborane (12[10])
Preparation of 1-decyl-12-(4-heptyloxyphenyl)-p-carborane (11
[10]b)
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
ꢁ
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 reuxed overnight. The
mixture was cooled down to room temperature and ltered. The
residue was washed with CH2Cl2 (3 ꢃ 10 mL), dried (Na2SO4)
and evaporated to dryness. The crude product was puried 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) d 0.88 (t, J ¼ 7.1 Hz, 6H), 1.0–2.6 (br m, 10H),
1.12–1.42 (m, 24H), 1.64 (pseudo t, J ¼ 8.1 Hz, 2H), 1.73 (quint,
J ¼ 6.6 Hz, 2H), 3.87 (t, J ¼ 6.5 Hz, 2H), 6.65 (d, J ¼ 9.0 Hz, 2H),
7.09 (t, J ¼ 9.0 Hz, 2H); 11B NMR (CDCl3, 128 MHz) d ꢀ12.3 (d,
J ¼ 164 Hz). Anal. calcd for C25H50B10O: C, 63.25; H, 10.62.
Found: C, 63.99; H, 10.73%.
(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 over-
night. 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 lm, which was puried 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 con-
taining a few drops of hexane to give phenol 12[10] as a colorless
lm: 1H NMR (CDCl3, 400 MHz) d 0.88 (t, J ¼ 7.1 Hz, 3H), 1.0–2.6
(br m, 10H), 1.10–1.30 (m, 16H), 1.64 (pseudo t, J ¼ 8.1 Hz, 2H),
4.71 (s, 1H), 6.59 (d, J ¼ 8.9 Hz, 2H), 7.06 (d, J ¼ 8.8 Hz, 2H); 11
B
NMR (CDCl3, 128 MHz) d ꢀ12.3 (d, J ¼ 164 Hz). Anal. calcd for
C18H36B10O: C, 57.41; H, 9.64. Found: C, 57.18; H, 9.33%.
Acknowledgements
1-Decyl-12-(4-methoxyphenyl)-p-carborane (11[10]e)
This work was supported by the NSF grant DMR-1207585. We are
grateful to Professor Roman D˛abrowski of Military University of
Technology (Warsaw, Poland) for the gi of ClEster. We thank
Ms. Amanda Doody for help with chiral HPLC analysis of 1[6]c.
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 lm. The product was crystallized from CH3CN
containing a few drops of EtOAc and then cold EtOAc contain-
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