Aryl trihydroxyborate salts: Thermally unstable species with unusual gelation abilities

Article abstract of DOI:10.1021/jo201353j

A series of aryl trihydroxyborate salts were synthesized and found to form gels in benzene. The compounds were thermally unstable and readily underwent protodeboronation in solution and the solid state. Gelation could be induced without decomposition via sonication. Subsequent characterization studies revealed an unusual dependence of gel properties on alkyl chain length.

Full text of DOI:10.1021/jo201353j

NOTE
pubs.acs.org/joc
Aryl Trihydroxyborate Salts: Thermally Unstable Species with Unusual
Gelation Abilities
Cheryl L. Moy, Raja Kaliappan,† and Anne J. McNeil*
Department of Chemistry and Macromolecular Science and Engineering Program, University of Michigan, 930 North University
Avenue, Ann Arbor, Michigan 48109, United States
S Supporting Information
b
ABSTRACT: A series of aryl trihydroxyborate salts were synthe-
sized and found to form gels in benzene. The compounds were
thermally unstable and readily underwent protodeboronation in
solution and the solid state. Gelation could be induced without
decomposition via sonication. Subsequent characterization stud-
ies revealed an unusual dependence of gel properties on alkyl chain length.
Chart 1
ryl trihydroxyborate salts have recently emerged as conve-
nient reagents for a variety of metal-catalyzed reactions,1
A
including Pd-catalyzed SuzukiꢀMiyaura cross-couplings and
Rh-catalyzed conjugate additions. Aryl trihydroxyborate salts were
first utilized by Vaultier and co-workers in solid-phase reactions in
2001.2 More recently, Cammidge and co-workers reported a simple
procedure for synthesizing aryl trihydroxyborate salts and provided
examples of their use in solution-phase reactions.3 Since these early
reports, a number of other researchers have used aryl trihydrox-
yborate salts in a variety of synthetic transformations.1 Aryl trihy-
droxyborate salts have also been postulated to be the active,
transmetalating species in base-mediated SuzukiꢀMiyaura cross-
couplings using either boronic acids4,5 or trifluoroborates6 in the
presence of water, although this hypothesis has recently come
into question.7
Given their synthetic utility, we recently prepared a series of
brominated aryl trihydroxyborates (2aꢀe) and investigated their
use as difunctional monomers for preparing π-conjugated
polymers. During these studies, we discovered that these aryl
trihydroxyborate salts are unstable to prolonged heating and
readily undergo protodeboronation. In addition, we observed
that these complexes formed gels in aromatic solvents. As such,
these compounds join a growing class of organometallic gelators.8ꢀ10
Gelation typically occurs when molecules self-assemble to form
anisotropic fibers, which entangle and entrap solvent through
surface tension and capillary forces.11,12 Because molecular
gels can be stimuli-responsive and have nano- and micro-
meter-scale architectures, they are being explored for many
different applications, including sensing,13 remediation,14 and
materials synthesis.15
trihydroxyborate salts.2 During these studies, we observed evi-
dence of decomposition, including changes in the aromatic region
of the 1H NMR spectrum and brown discoloration of the NMR
sample. Wesuspected protodeboronation was occurringbecause it
was previously reported for boronic acids under base-catalyzed
conditions.16,17 Fields and Doyle first reported on the instability of
isolated aryl trihydroxborate salts in 1974.18 They observed an
onset of weight loss for the sodium salt of benzene trihydroxybo-
rate at 170 °C using thermal gravimetric analysis. Consistent with
these previous reports, heating 1c in the solid state for 30 min at
200 °C ledtoquantitativeconversion to1,4-bis(hexyloxy)benzene
(Figure 1). To determine the decomposition rate in solution, the
formation of 1,4-bis(butyloxy)benzene was monitored via HPLC
analysis (relative to an internal standard) for samples of 1b and 2b
heated in benzene. While 2b was 70% decomposed within 1 h at
60 °C, 1b was remarkably more stable. However, heating 1b at an
elevated temperature (100 °C) for 1 h led to a 64% conversion to
1,4-bis(butyloxy)benzene. These results are consistent with Frohn
and co-workers, who observed that the rates of base-catalyzed
protodeboronation of boronic acids in solution depended on both
the number and position of fluorine atoms on the aromatic ring.16c
Compounds 1aꢀe and 2bꢀd form gels by heating and
cooling samples in benzene. However, this method was not
reproducible due to the thermal instability of these materials. To
induce gelation without heating, an alternative sonication-based
Herein, we report the synthesis, thermal instability, and unusual
gelation ability of aryl trihydroxyborate salts (Chart 1). Com-
pounds 1aꢀe and 2aꢀe were synthesized in high yields from their
corresponding boronic acids via treatment with 1 equiv of NaOH
in benzene. 11B NMR spectroscopic studies revealed the antici-
pated upfield shift for the tetravalent boron (0ꢀ5 ppm) compared
to the boronic acids (25ꢀ30 ppm), supporting formation of the aryl
Received: July 12, 2011
Published: September 02, 2011
r
2011 American Chemical Society
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|

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NOTE
Figure 1. 1H NMR spectra of compound 1c in CD3OD before (left)
and after (right) heating in the solid state at 200 °C for 30 min.
Figure 3. Plot of the cgc (in benzene) versus alkyl chain length for aryl
trihydroxyborate salts (A) 1aꢀe and (B) 2bꢀd.
Figure 2. Gels of 1c (12 mg) form in benzene (1 mL) with sonication
for 5 min at rt.
Table 1. Characterization Data for Gelators 1aꢀe and 2bꢀda
gelator
chain
cgc (mg/mL)
cgc (mM)
G0/G00 (Pa)
1a
1b
1c
1d
1e
2b
2c
2d
CH3
22 ( 2
15 ( 1
10 ( 2
26 ( 1
44 ( 2
24 ( 2
30 ( 2
32 ( 2
100 ( 10
49 ( 3
28 ( 6
67 ( 3
98 ( 4
62 ( 5
68 ( 5
68 ( 4
200/70
Figure 4. AFM images of gels from (A) 1c (12 mg/mL) and (B) 2c (32
mg/mL) in benzene.
C4H9
300/200
C6H13
C7H15
C10H21
C4H9
4000/1000
6000/1500
250/40
observed herein. However, Dey and co-workers recently re-
ported a similar trend in cgc for histidine-based gelators in
water, with a C8 chain giving the lowest cgc compared to C6 and
C10.23 They attributed this result to the hydrophilicꢀlipophilic
balance of the amphiphilic gelator. Given that the gels reported
herein are formed in benzene, the observed relationship between
cgc and chain length is not easily rationalized. It is interesting to
note that the same chain-length dependence is not observed with
brominated aryl trihydroxyborate salts 2bꢀd, suggesting that the
bromine atoms alter the molecular packing in the gel.
The strength andresilienceofa gel to deformation by an external
force is characterized by the loss (G0) and storage (G00) moduli.
The majority of the aryl trihydroxyborate gels reported herein show
relatively large values for the modulus (G0 > 1000 Pa) when
compared to other organometallic gelators.9 Strong gels typically
result from highly cross-linked microstructures. To understand this
effect, the gel structure was examined via microscopy. Scanning
electron microscope images were unreliable because the gel
samples decomposed when exposed to the electron beam (see
the Supporting Information). Instead, atomic force microscopy
(AFM) was used to generate high-resolution images of the aryl
trihydroxyborate gels. The AFM images for gels 1aꢀe and 2bꢀd
revealed that the gel microstructures contain physically cross-linked
fibers (see Figure 4 and the Supporting Information). This dense
fibrous network is consistent with the unusually strong gels
observed via rheology. Despite these results, it was surprising that
the gel-to-solution transition temperatures (measured by the
falling-ball method) had almost no correlation with the gel
strength (see the Supporting Information). These differences
are likely due to the low melting temperature of the salts and
competing decomposition reactions occurring during the Tgel
measurements.
15000/9000
1400/220
4000/900
C6H13
C7H15
a All data represent an average of three runs. The G0/G00 measurements
were performed at concentrations of 2 ꢁ cgc.
procedure was explored.19 Indeed, gels were formed by adding
solid sodium hydroxide to a vial, followed by a benzene solution
of the boronic acid precursor and sonication for 5 min at rt
(Figure 2). Using this procedure, aryl trihydroxyborate salts
1aꢀe and 2bꢀd also form gels in other solvents, including
toluene, p-cymene, styrene, and cyclohexane. Compounds 2a
and 2e did not form gels under any conditions examined.
As seen in Table 1 and Figure 3A, the critical gel concentration
(cgc), which is the minimum concentration needed to form a
stable gel, for 1aꢀe shows an unusual dependence on alkyl chain
length. For example, when the alkyl chain is changed from methyl
to hexyl, the cgc drops from 22 to 10 mg/mL. However, the trend
does not continue as further increases in chain length (decyl) led
to a substantial increase in cgc (44 mg/mL). Chain-length-
dependent cgcs have previously been observed; however, they
typically fall into one of two classes: (1) Increasing chain lengths
correlate with decreasing cgcs, which is generally attributed to
increased van der Waals interactions and/or hydrophobic
interactions.20 (2) Oddꢀeven effects of chain length are
observed.21 For example, Steed, Clarke and co-workers reported
that bis(ureas) with odd-numbered linkers did not form gels,
whereas molecules with even-numbered linkers did.21a These
results are generally attributed to differences in packing densities
for odd and even-length alkyl chains.22 Neither of these trends is
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NOTE
The 3D orientation of molecules within the gel fiber can be
elucidated if the powder X-ray diffraction (PXRD) pattern of the
gel matches a single-crystal diffraction pattern of the gelator.
Although we were unable to grow single crystals of 1aꢀe and
2bꢀd from benzene, Cammidge and co-workers obtained a
single crystal of a structurally related compound, sodium 4-meth-
oxyphenylborate salt, from H2O.3 In this compound, the sodium
ions formed a linear hydrated chain [Na(H2O)5]n wherein
significant hydrogen-bonding interactions between the H2O
and the BꢀOH were observed. We hypothesize that a similar
chainlike structure may be present in our gel fibers because the
PXRD pattern for the xerogel of 1a exhibited a Bragg reflection at
a d-spacing of 15.6 Å, which is similar to the (002) peak that
corresponds to the distance between the chains of sodium ions in
the reported structure (Supporting Information).3,24
In summary, a series of aryl trihydroxyborate salts with
increasing alkyl chain lengths were synthesized. These com-
pounds were found to be thermally unstable, undergoing quan-
titative protodeboronation with heating both in solution and the
solid state. These results suggest a limited utility for these
compounds as stoichiometric reagents in metal-catalyzed reac-
tions. Some of these salts formed gels in organic solvents. The
resulting gels are interesting because of their unusual strength
and the observed chain-length dependence on gel properties.
The origins of the chain-length dependence remain unclear at
this time. Excitingly, these novel gelators may prove useful for
preparing new materials, as the localization of difunctional
gelators 2bꢀd may lead to higher molecular weight polymers
via SuzukiꢀMiyaura cross-coupling polymerizations in the gel
state. Alternatively, free-radical polymerization of a styrene- or
divinylbenzene-based solvent could lead to porous materials after
removal of the aryl trihydroxyborate salt.
1H NMR (400 MHz, CDCl3) δ 6.83 (s, 4H), 3.90 (t, J = 6.8 Hz, 4H),
1.76 (m, 4H), 1.47ꢀ1.32 (br, 12H), 0.91 (m, 6H); 13C NMR (100 MHz,
CDCl3) δ 153.2, 115.4, 68.6, 31.6, 29.4, 25.7, 22.6, 14.0; NMR spectral
data agree well with previously reported values;26 mp 43.4ꢀ44.3 °C
(lit.27 mp 45 °C).
1,4-Bis(heptyloxy)benzene. Following the general procedure
described above, 1,4-bis(heptyloxy)benzene was obtained after purifica-
tion by recrystallization as a white crystalline solid (11.59 g, 42% yield):
HRMS (ESI) [M + H]+ calcd for C20H34O2 307.2637, found 307.2634;
1H NMR (400 MHz, CDCl3) δ 6.82 (s, 4H), 3.90 (t, J = 6.8 Hz, 4H),
1.75 (m, 4H), 1.46ꢀ1.28 (br, 16H), 0.90 (m, 6H); 13C NMR (100 MHz,
CDCl3) δ 153.2, 115.4, 68.6, 31.8, 29.4, 29.1. 26.0, 22.6, 14.1; mp
56.5ꢀ57.1 °C (lit.28 mp 57.6ꢀ58 °C).
1,4-Bis(decyloxy)benzene. Following the general procedure de-
scribed above, 1,4-bis(decyloxy)benzene was obtained after purification
by recrystallization as a white crystalline solid (27.03 g, 76% yield): HRMS
1
(ESI) [M]+ calcd for C26H46O2 390.3498, found 390.3500; H NMR
(400 MHz, CDCl3) δ 6.82 (s, 4H), 3.90 (t, J = 6.8 Hz, 4H), 1.73 (m, 4H),
1.46ꢀ1.27 (m, 28H), 0.88 (t, J = 6.4 Hz, 6H); 13C NMR (100 MHz,
CDCl3) δ 153.2, 115.4, 68.7, 31.9, 29.57, 29.55, 29.4, 29.3, 26.1, 22.7, 14.1;
some carbons on the decyl chains are unresolved; mp 68.0ꢀ69.0 °C.
General Procedure for the Synthesis of 2-Bromo-1,4-dia-
lkoxybenzene. Sequentially, 1,4-dibutoxybenzene (5.039 g, 0.0226
mol, 1.0 equiv), acetonitrile (38.5 mL), NH4NO3 (0.180 g, 0.0022 mol,
0.1 equiv), and N-bromosuccinimide (4.029 g, 0.0226 mol, 1.0 equiv)
were added to a 100 mL round-bottom flask with a stir bar. The reaction
was stirred at rt for 2 h and then quenched with water (50 mL). The
aqueous mixture was extracted with EtOAc (2 ꢁ 50 mL). The combined
organic layers were washed with water (2 ꢁ 50 mL) and brine (2 ꢁ
50mL), dried overanhydrousMgSO4, filtered, andconcentratedin vacuo.
The crude product was purified by column chromatography using 14.95/
85.00/0.05 toluene/hexanes/EtOAcastheeluent togive 3.96g ofproduct
as a clear colorless oil (58% yield): MS (EI) m/z = 300.2 [M(79Br)]+
302.2 [M(81Br)]+; HRMS (ESI) [M]+ calcd for C14H21BrO2 300.0725,
found 300.0722; 1H NMR (400 MHz, CDCl3) δ 7.11 (d, J = 2.4 Hz, 1H),
6.82 (m, 2H), 3.96 (t, J = 6.4 Hz, 2H), 3.89 (t, J = 6.4 Hz, 2H), 1.76 (m,
4H), 1.50 (m, 4H), 0.97 (m, 6H); 13C NMR (100 MHz, CDCl3) δ 153.6,
149.8, 119.5, 114.7, 114.4, 112.8, 69.9, 68.5, 31.4, 31.3, 19.24, 19.19. 13.9,
13.8; NMR spectral data agree well with previously reported values.29
2-Bromo-1,4-bis(hexyloxy)benzene. Following the general
procedure described above, 2-bromo-1,4-bis(hexyloxy)benzene was
obtained after purification by column chromatography as a clear colorless
oil (2.808 g, 35% yield): MS (EI) m/z = 356.2 [M(79Br)]+, 358.2
[M(81Br)]+; HRMS (ESI) [M]+ calcd for C18H29BrO2 356.1351, found
356.1336; 1H NMR (400 MHz, CDCl3) δ 7.11 (d, J = 2.8 Hz, 1H), 6.79
(m, 2H), 3.95 (t, J = 6.8 Hz, 2H), 3.88 (t, J = 6.8 Hz, 2H), 1.77 (m, 4H),
1.83ꢀ1.31 (br, 12H), 0.91 (m, 6H); 13C NMR (100 MHz, CDCl3) δ
153.6, 149.8, 119.5, 114.7, 114.4, 112.8. 70.2, 68.8, 31.6, 29.22, 29.23, 25.7,
22.6, 14.0. Some carbons on the hexyl chains are unresolved. NMR
spectral data agree well with previously reported values.30
’ EXPERIMENTAL SECTION
Representative Procedure for Gel Formation. An aliquot
(0.50 mL) of a NaOH solution (0.10 M in CH3OH) was added to a
4 mL vial. The CH3OH was removed in vacuo, and the resulting solid
NaOH was held under vacuum overnight. Subsequently, (2,5-bis-
(hexyloxy)phenyl)boronic acid (1.0 mL, 0.06 M in benzene) was added
to the vial. The heterogeneous mixture was sonicated for 5 min to create
a homogeneous solution of 1c. Gel formation occurred if the solution
was left undisturbed for approximately 5 min.
General Procedure for the Synthesis of 1,4-Dialkoxyben-
zene. Hydroquinone (20.02 g, 0.1818 mol, 1.0 equiv), 1-bromobutane
(48.0 mL, 0.454 mol, 2.5 equiv), and DMF (120 mL) were added under
N2 to a 500 mL flask and the mixture heated to 80 °C with vigorous
stirring. K2CO3 (62.77 g, 0.4542 mol, 2.5 equiv) was then added, and the
reaction mixture was stirred for 3 d. The reaction was cooled to rt,
filtered, and washed with hexanes. The filtrate was washed with water
(2 ꢁ 200 mL) and brine (2 ꢁ 200 mL), dried over anhydrous MgSO4,
filtered, and concentrated in vacuo. The crude product was recrystallized
from hot CH3OH to give 24.10 g of product as a white crystalline solid
(60% yield): HRMS (ESI) [M + H]+ calcd for C14H22O2 223.1698,
found 223.1696; 1H NMR (400 MHz, CDCl3) δ 6.83 (s, 4H), 3.91 (t,
J = 6.4 Hz, 4H), 1.75 (m, 4H), 1.49 (m, 4H), 0.98 (m, 6H); 13C NMR
(100 MHz, CDCl3) δ 153.2, 115.4, 68.3, 31.5, 19.2, 13.9; NMR spectral
data agree well with previously reported values;25 mp 43.8ꢀ44.4 °C
(lit.25 mp 45ꢀ47 °C).
2-Bromo-1,4-bis(heptyloxy)benzene. Following the general
procedure described above, 2-bromo-1,4-bis(heptyloxy)benzene was
obtained after purification by column chromatography as a clear color-
less oil (2.964 g, 34% yield): MS (EI) m/z = 384.3 [M(79Br)]+, 386.3
[M(81Br)]+; HRMS (ESI) [M]+ calcd for C20H33BrO2 384.1664, found
384.1677; 1H NMR (400 MHz, CDCl3) δ 7.11 (d, J = 2.8 Hz, 1H), 6.79
(m, 2H), 3.95 (t, J = 6.4 Hz, 2H), 3.88 (t, J = 6.4 Hz, 2H), 1.76 (m, 4H),
1.49ꢀ1.30 (br, 16H), 0.91 (m, 6H); 13C NMR (100 MHz, CDCl3) δ
153.6, 149.8, 119.5, 114.7, 114.4, 112.8, 70.2, 68.8, 31.77, 31.76, 29.27,
29.25, 29.0, 26.0, 22.6, 14.07, 14.06. Some carbons on the heptyl chains
are unresolved.
1,4-Bis(hexyloxy)benzene. Following the general procedure
described above, 1,4-bis(hexyloxy)benzene was obtained after purifica-
tion by recrystallization as a white crystalline solid (16.06 g, 63% yield):
HRMS (ESI) [M + H]+ calcd for C18H30O2 279.2324, found 279.2328;
2-Bromo-1,4-bis(decyloxy)benzene. Following the general
procedure described above, 2-bromo-1,4-bis(decyloxy)benzene
was obtained after purification by column chromatography using
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NOTE
19.95/80.0/0.05 toluene/hexanes/EtOAc as the eluent to give a clear
crystalline solid (1.43 g, 40% yield): MS (EI) m/z = 468.4 [M(79Br)]+,
470.4 [M(81Br)]+; HRMS (ESI) [M]+ calcd for C26H45BrO2 468.2603,
0.90 (m, 6H); 13C NMR (100 MHz, CDCl3) δ 150.1, 118.5, 111.1, 70.3,
31.9, 29.5, 29.31, 29.29, 29.1, 25.9, 22.7, 14.1; some carbons on the decyl
chains are unresolved; mp 75.0ꢀ76.5 °C.
1
found 468.2598; H NMR (400 MHz, CDCl3) δ 7.11 (d, J = 2.8 Hz,
General Procedure for (2,5-Bis(alkoxy)phenyl)boronic Acid.
An oven-dried 50 mL Schlenk flask was equipped with a stir bar and septum,
cooled to rt under vacuum, and filled with N2. 2-Bromo-1,4-dibutoxyben-
zene (1.002 g, 0.0033 mol, 1.0 equiv) and THF (32 mL) were added to
the flask. The solution was then cooled to ꢀ78 °C. Then n-BuLi (2.3 mL,
1.6 M, 1.1 equiv) was added via syringe. After 20 min, triisopropyl borate
(2.3 mL, 1.0 mmol, 3.0 equiv) was added. The reaction was gradually
warmed to rt and stirred for 24 h. The mixture was cooled to 0 °C, aqueous
HCl (5 mL, 2 M) was added, and the mixture was stirred for 20 min. The
aqueous mixture was extracted with ether (2 ꢁ 50 mL). The combined
organic layers were washed with brine (1 ꢁ 50 mL). The organic layer
was dried over anhydrous MgSO4, filtered, and concentrated in
vacuo. The crude product was purified by column chromatography
using 5/15/80 EtOAc/CH2Cl2/hexanes as the eluent to give 477 mg
of product as a white solid (53% yield): HRMS (ESI) [M]+ calcd for
C14H23BO4 266.1689, found 266.1697; 11B NMR (128 MHz, CDCl3) δ
29.24; 1H NMR (400 MHz, CDCl3) δ 7.39 (d, J = 3.2 Hz, 1H), 6.97 (dd,
J= 8.8, 3.2 Hz, 1H), 6.84 (d, J= 8.8 Hz, 1H), 6.49 (s, 2H), 4.03 (t, J=6.8Hz,
2H), 3.96 (t, J = 6.8 Hz, 2H), 1.77 (m, 4H), 1.48 (m, 4H), 0.98 (m, 6H);
13C NMR (100 MHz, CDCl3) δ 158.2, 153.3, 121.3, 119.3, 112.1, 68.7,
68.3, 31.43, 31.35, 19.3, 19.2, 14.0, 13.8; C ipso to B is not observed; mp
71.5ꢀ73.0 °C.
1H), 6.81 (m, 2H), 3.95 (t, J = 6.4 Hz, 2H), 3.88 (t, J = 6.4 Hz, 2H), 1.76
(m, 4H), 1.49ꢀ1.27 (br, 28H), 0.88 (m, 6H); 13C NMR (100 MHz,
CDCl3) δ 153.6, 149.8, 119.5, 114.7, 114.4, 112.8, 70.2, 68.8, 31.9, 29.6,
29.5, 29.4, 29.32, 29.26, 29.25, 26.0, 22.7, 14.1. Some carbons on the
decyl chains are unresolved; mp 37.0ꢀ39.0 °C.
General Procedure for the Synthesis of 1,4-Dibromo-2,5-
dialkoxybenzene. Dimethoxybenzene (2.009 g, 0.0146 mol, 1.0
equiv) was dissolved in CHCl3 and cooled to 0 °C under N2, and the
pressure was vented through a 10% aq Na2SO3 solution (∼100 mL).
Bromine (1.9 mL, 0.04 mol, 2.5 equiv) was added dropwise via syringe.
The ice bath was then removed and the reaction continued to stir at rt for
3 h. The reaction was quenched with 10% aq Na2SO3 (50 mL) and
extracted with CH2Cl2 (2 ꢁ 50 mL). The organic layer was washed with
brine (2 ꢁ 50 mL), dried over anhydrous MgSO4, filtered, and
concentrated in vacuo. The crude product was recrystallized from
CH2Cl2/MeOH to give 2.894 g of product as white crystals (67%
yield): MS (EI) m/z = 294.0 [M(79Br,79Br)]+, 296.0 [M(79Br, 81Br)]+,
298.0 [M(81Br, 81Br)]+; HRMS (ESI) [M]+ calcd for C8H8Br2O2
1
293.8891, found 293.8882; H NMR (400 MHz, CDCl3) δ 7.10 (s,
2H), 3.84 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 150.5, 117.1, 110.5,
57.0; NMR spectral data agree with previously reported values;31 mp
143.9ꢀ145.0 °C (lit.31 mp 140ꢀ143 °C).
(2,5-Bis(hexyloxy)phenyl)boronic Acid. Following the general
procedure described above, (2,5-bis(hexyloxy)phenyl)boronic acid was
obtained after column chromatography as a white solid (400 mg, 26%
yield): HRMS (ESI) [M + Na]+ calcd for C18H31BO4 345.2213, found
345.2222; 11B NMR (128 MHz, CD3OD) δ 29.67; 1H NMR (400 MHz,
CDCl3) δ 7.37 (d, J = 3.2 Hz, 1H), 6.96 (dd, J = 8.8, 3.2 Hz, 1H), 6.84 (d,
J = 8.8 Hz, 1H), 6.16 (s, 2H), 4.02 (t, J = 6.4 Hz, 2H), 3.94 (t, J = 6.8 Hz,
2H), 1.79 (m, 4H), 1.48ꢀ1.31 (br, 12H), 0.91 (m, 6H); 13C NMR (100
MHz, CDCl3) δ 158.2, 153.3, 121.4, 119.3, 112.1, 69.0, 68.6, 31.6, 31.5,
29.26, 29.29, 25.71, 25.69, 22.6, 22.5, 14.03, 13.97; C ipso to B is not
observed; mp 73.7ꢀ74.2 °C.
1,4-Dibromo-2,5-dibutoxybenzene. Following the general
procedure described above, 1,4-dibromo-2,5-dibutoxybenzene was ob-
tained after purification by recrystallization as white crystals (2.804 g,
81% yield): MS (EI) m/z = 378.1 [M(79Br,79Br)]+, 380.1 [M(79Br,
81Br)]+, 382.1 [M(81Br, 81Br)]+; HRMS (ESI) [M]+ calcd for
C14H20Br2O2 377.9830, found 377.9829; 1H NMR (400 MHz, CDCl3)
δ 7.08 (s, 2H), 3.96 (t, J = 6.8 Hz, 4H), 1.80 (m, 4H), 1.53 (m, 4H), 0.98
(m, 6H); 13C NMR (100 MHz, CDCl3) δ 150.1, 118.5, 111.1, 70.0, 31.2,
19.2, 13.8; NMR spectral data agree well with previously reported
values;25 mp 73.0ꢀ74.0 °C (lit.25 mp 76ꢀ77 °C).
1,4-Dibromo-2,5-bis(hexyloxy)benzene. Following the gen-
eral procedure described above, 1,4-dibromo-2,5-bis(hexyloxy)benzene
was obtained after purification by recrystallization as white crystals
(2.758 g, 88% yield): MS (EI) m/z = 434.3 [M(79Br,79Br)]+, 436.3
[M(79Br, 81Br)]+, 438.3 [M(81Br, 81Br)]+; HRMS (ESI) [M]+ calcd for
C18H28Br2O2 434.0456, found 434.0448; 1H NMR (400 MHz, CDCl3)
δ 7.09 (s, 2H), 3.95 (t, J = 6.4 Hz, 4H), 1.80 (m, 4H), 1.49 (m, 4H), 1.35
(m, 8H), 0.91 (m, 6H); 13C NMR (100 MHz, CDCl3) δ 150.1, 118.5,
111.1, 70.3, 31.5, 29.1, 25.6, 22.6, 14.0; NMR spectral data agree well
with previously reported values.26,32 mp 51.3ꢀ53.1 °C.
(2,5-Bis(heptyloxy)phenyl)boronic Acid. Following the gen-
eral procedure described above, (2,5-bis(heptyloxy)phenyl)boronic acid
was obtained after column chromatography as a white solid (713 mg,
62% yield): HRMS (ESI) [M]+ calcd for C20H35BO4 350.2628, found
350.2635; 11B NMR (128 MHz, CDCl3) δ 28.87; 1H NMR (400 MHz,
CDCl3) δ 7.36 (d, J = 3.2 Hz, 1H), 6.96 (dd, J = 8.8, 3.2 Hz, 1H), 6.84 (d,
J = 8.8 Hz, 1H), 5.97 (s, 2H), 4.02 (t, J = 6.8 Hz, 2H), 3.94 (t, J = 6.8 Hz,
2H), 1.77 (m, 4H), 1.48ꢀ1.30 (br, 16H), 0.89 (m, 6H); 13C NMR (100
MHz, CDCl3) δ 158.2, 153.3, 121.4, 119.3, 112.1, 69.0, 68.6, 31.8, 31.7,
29.4, 29.1, 29.0, 26.0, 22.61, 22.56, 14.08, 14.05; C ipso to B is not
observed, some carbons on the heptyl chains are unresolved; mp
74.0ꢀ76.5 °C.
1,4-Dibromo-2,5-bis(heptyloxy)benzene. Following the gen-
eral procedure described above, 1,4-dibromo-2,5-bis(heptyloxy)benzene
was obtained after purification by recrystallization as white crystals (2.835
g, 93% yield): MS (EI) m/z = 462.1 [M(79Br,79Br)]+, 464.1 [M(79Br,
81Br)]+, 466.1 [M(81Br, 81Br)]+; HRMS (ESI) [M]+ calcd for
C20H32Br2O2 462.0769, found 462.0779; 1H NMR (400 MHz, CDCl3)
δ 7.08 (s, 2H), 3.95 (t, J = 6.8 Hz, 4H), 1.79 (m, 4H), 1.48 (m, 4H),
1.38ꢀ1.31 (br, 12H), 0.90 (m, 6H); 13C NMR (100 MHz, CDCl3) δ
150.1, 118.5, 111.1, 70.3, 31.6, 29.1, 29.0, 26.0, 22.6, 14.1; NMR spectral
data agree well with previously reported values;33 mp 62.0ꢀ64.0 °C
(lit.33 mp 63 °C).
1,4-Dibromo-2,5-bis(decyloxy)benzene. Following the gen-
eral procedure described above, 1,4-dibromo-2,5-bis(decyloxy)benzene
was obtained after purification by recrystallization as white crystals
(2.557 g, 91% yield): MS (EI) m/z = 546.1 [M(79Br,79Br)]+, 548.1
[M(79Br, 81Br)]+, 550.1 [M(81Br, 81Br)]+; HRMS (ESI) [M]+ calcd for
C26H44Br2O2 546.1708, found 546.1707; 1H NMR (400 MHz, CDCl3)
δ 7.08 (s, 2H), 3.94 (t, J = 6.4 Hz, 4H), 1.80 (m, 4H), 1.54ꢀ1.27 (br, 28H),
(2,5-Bis(decyloxy)phenyl)boronic Acid. Following the general
procedure described above, (2,5-bis(decyloxy)phenyl)boronic acid was
obtained after column chromatography as a white solid (404 mg, 43%
yield): HRMS (ESI) [M + Na]+ calcd for C26H47BO4 457.3465, found
457.3460; 11B NMR (128 MHz, CDCl3) δ 29.13; 1H NMR (400 MHz,
CDCl3) δ 7.36 (d, J = 3.2 Hz, 1H), 6.97 (dd, J = 8.8, 3.2 Hz, 1H), 6.84 (d,
J = 8.8 Hz, 1H), 5.90 (s, 2H), 4.02 (t, J = 6.8 Hz, 2H), 3.94 (t, J = 6.8 Hz,
2H), 1.78 (m, 4H), 1.45ꢀ1.27 (br, 28H), 0.88 (m, 6H); 13C NMR (100
MHz, CDCl3) δ 158.2, 153.3, 121.4, 119.3, 112.1, 69.0, 68.6, 31.89,
31.87, 29.55, 29.50, 29.4, 29.34, 29.32, 29.28, 26.0, 22.7, 14.1; C ipso to B
is not observed, some carbons on the decyl chains are unresolved; mp
89.8ꢀ91.2 °C.
(2,5-Dimethoxyphenyl)boronic Acid. Following the general
procedure described above, (2,5-dimethoxyphenyl)boronic acid was
obtained after column chromatography (20/80 EtOAc/hexanes) as a
white solid (490 mg, 58% yield): HRMS (ESI) [M]+ calcd for C8H11BO4
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The Journal of Organic Chemistry
NOTE
182.0750, found 182.0757; 11B NMR (128 MHz, CDCl3) δ 29.22. 1H
NMR (400 MHz, CDCl3) δ 7.41 (d, J = 3.2 Hz, 1H), 7.00ꢀ6.97 (dd, J =
8.8, 3.2 Hz, 1H), 6.86 (d, J = 8.8 Hz, 1H), 6.72 (s, 2H), 3.87 (s, 3H), 3.81
(s, 3H); 13C NMR (100 MHz, CDCl3) δ 158.8, 153.8, 120.6, 118.6,
111.2, 56.0, 55.7; C ipso to B not observed; NMR spectral data agree
well with previously reported values;34 mp 91.2ꢀ92.7 °C (lit.35 mp
91ꢀ93 °C).
1.97 mmol, 1.0 equiv) was dissolved in benzene (10.0 mL), and then
NaOH (77.2 mg, 1.93 mmol, 0.98 equiv) was added. The reaction was
stirred for 20 h at rt, and the solution turned into a white suspension.
The mixture was concentrated in vacuo isolating 390 mg of a white
solid (89% yield): 11B NMR (128 MHz, CD3OD) δ 4.63; 1H NMR
(400 MHz, CD3OD) δ 7.12 (br, 1H), 6.74ꢀ6.65 (br, 2H) 3.74 (br,
6H); 13C NMR (125 MHz, CD3OD) δ 158.1, 154.7, 122.4, 112.3,
111.1, 56.1, 56.0; C ipso to B is not observed; mp 99 °C dec.
(2,5-Dibutoxyphenyl)boronic Acid Sodium Salt (1b). Fol-
lowing the general procedure described above, (2,5-dibutoxyphe-
nyl)boronic acid sodium salt was obtained as a white solid (405 mg,
99% yield): 11B NMR (128 MHz, CD3OD) δ 4.76; 1H NMR (400 MHz,
CD3OD) δ 7.08 (br, 1H), 6.70 (br, 2H), 3.92 (t, J = 6.4 Hz, 4H), 1.7 (m,
4H), 1.5 (m, 4H), 0.99 (m, 6H); 13C NMR (125 MHz, CDCl3) δ 157.5,
154.1, 123.2, 121.9, 113.6, 113.1, 69.7, 69.3, 32.9, 32.7, 20.4, 14.4, 14.3;
mp 80ꢀ82 °C.
(4-Bromo-2,5-dibutoxyphenyl)boronic Acid. Following the
general procedure described above, (4-bromo-2,5-dibutoxyphenyl)boro-
nic acid was obtained after column chromatography as a white solid (420
mg, 46% yield): MS (EI) m/z = 344.1 [M(79Br)]+, 346.1 [M(81Br)]+;
HRMS (ESI): [M]+ calcd for C14H22BBrO4, 344.0795; found 344.0796.
11B NMR (128 MHz, CDCl3) δ 29.05; 1H NMR (400 MHz, CDCl3) δ
7.37 (s,1H), 7.10 (s, 1H), 5.80 (s, 2H), 4.02 (m, 4H), 1.80 (m, 4H), 1.5
(m, 4H), 0.98 (m, 6H); 13C NMR (100 MHz, CDCl3) δ 158.1, 150.1,
120.7, 116.6, 116.5, 69.7, 69.0, 31.3, 31.2, 19.2, 13.9, 13.8; C ipso to B is not
observed, some carbons on the butyl chains are unresolved; mp 110.0 ꢀ
112.5 °C.
(2,5-Bis(hexyloxy)phenyl)boronic Acid Sodium Salt (1c).
Following the general procedure described above, (2,5-bis(hexyloxy)-
phenyl)boronic acid sodium salt was obtained as a white solid (450 mg,
100%yield):11B NMR(128 MHz, CD3OD) δ4.51; 1H NMR (400 MHz,
CD3OD) δ 7.08 (br, 1H), 6.71ꢀ6.65 (br, 2H), 3.90 (m, 2H), 1.71 (m,
4H), 1.47ꢀ1.34 (br, 12H), 0.91 (m, 6H);13C NMR(125 MHz, CD3OD)
δ 157.4, 154.4, 123.2, 122.2, 114.0, 113.2, 70.1, 69.7, 33.1, 33.0, 30.8,
27.1, 23.9, 14.6; some carbons on the hexyl chains are unresolved; mp
60ꢀ62 °C.
(4-Bromo-2,5-bis(hexyloxy)phenyl)boronic Acid. Following
the general procedure described above, (4-bromo-2,5-bis(hexyloxy)-
phenyl)boronic acid was obtained after column chromatography as a
white solid (525 mg, 57% yield): MS (EI) m/z = 400.1 [M(79Br)]+, 402.1
[M(81Br)]+; HRMS (ESI) [M]+ calcd for C18H30BBrO4 400.1421, found
400.1434; 11B NMR (128 MHz, CDCl3) δ 28.78; 1H NMR (400 MHz,
CDCl3) δ 7.35 (s, 1H), 7.10 (s, 1H), 5.91 (s, 1H), 4.02 (m, 4H), 1.81 (m,
4H), 1.50ꢀ1.34 (br, 12H), 0.91 (m, 6H); 13C NMR (100 MHz, CDCl3)
δ 158.1, 150.0, 120.7, 116.5, 70.0, 69.3, 31.5, 31.4, 29.20, 29.17, 25.64,
25.63, 22.6, 22.5, 14.02, 13.95; C ipso to B is not observed; mp
106.0ꢀ108.0 °C.
(2,5-Bis(heptyloxy)phenyl)boronic Acid Sodium Salt (1d).
Following the general procedure described above, (2,5-bis(heptyloxy)-
phenyl)boronic acid sodium salt was obtained as a white solid (488 mg,
80% yield): 11B NMR (128 MHz, CD3OD) δ 4.65; 1H NMR (400 MHz,
CD3OD) δ 7.08 (br, 1H), 6.69ꢀ6.62 (br, 2H), 3.94 (m, 4H), 1.71 (m,
4H), 1.47ꢀ1.27 (br, 16H), 0.90 (m, 6H); 13C NMR (125 MHz,
CD3OD) δ 157.3, 154.1, 123.4, 122.0, 113.6, 113.0, 70.0, 69.6, 33.0,
30.7, 30.5, 30.4, 30.3, 27.2, 23.7, 14.4; some carbons on the heptyl chains
are unresolved; mp 59ꢀ62 °C.
(4-Bromo-2,5-bis(heptyloxy)phenyl)boronic Acid. Following
the general procedure described above, (4-bromo-2,5-bis(heptyloxy)-
phenyl)boronic acid was obtained after column chromatography as a
white solid (315 mg, 34% yield): MS (ES) m/z = 451.2 [M + Na(79Br)]+,
453.2 [M + Na(81Br)]+; HRMS (ESI) [M + Na]+ calcd for C20H34BBrO4
451.1626, found 451.1640; 11B NMR (128 MHz, CDCl3) δ 29.04; 1H
NMR (400 MHz, CDCl3) δ 7.36 (s, 1H), 7.10 (s, 1H), 6.32 (s, 1H), 4.01
(m, 4H), 1.81 (m, 4H), 1.48ꢀ1.31 (br, 16H), 0.89 (m, 6H); 13C NMR
(100 MHz, CDCl3) δ 158.1, 150.0, 120.6, 116.5, 116.4, 69.9, 69.3, 31.8,
31.7, 29.23, 29.19, 29.0, 28.9, 25.92, 25.90, 22.60, 22.55, 14.1, 14.0; C ipso
to B is not observed; mp 90.2ꢀ92.4 °C.
(2,5-Bis(decyloxy)phenyl)boronic Acid Sodium Salt (1e).
Following the general procedure described above, (2,5-bis(decyloxy)-
phenyl)boronic acid sodium salt was obtained as a clear oil (429 mg, 97%
1
yield): 11B NMR (128 MHz, CD3OD) δ 4.41; H NMR (400 MHz,
CDCl3) δ 7.08 (br, 1H), 6.98ꢀ6.63 (br, 2H), 3.91 (t, J = 6.4 Hz, 4H),
1.73 (m, 4H), 1.46ꢀ1.29 (br, 28H), 0.89 (m, 6H); 13C NMR (125 MHz,
CDCl3) δ 157.4, 154.1, 123.7, 122.0, 113.5, 113.1, 70.0, 69.6, 33.1, 30.7,
30.6, 30.5, 27.2, 27.1, 23.8, 14.5; some carbons on the decyl chains are
unresolved.
(4-Bromo-2,5-bis(decyloxy)phenyl)boronic Acid. Following
the general procedure described above, (4-bromo-2,5-bis(decyloxy)-
phenyl)boronic acid was obtained after column chromatography as a
white solid (193 mg, 20% yield): MS (EI) m/z = 512.3 [M(79Br)]+,
514.3 [M(81Br)]+; HRMS (ESI) [M]+ calcd for C26H46BBrO4
512.2672, found 512.2665; 11B NMR (128 MHz, CDCl3) δ 26.44; 1H
NMR (400 MHz, CDCl3) δ 7.35 (s, 1H), 7.10 (s, 1H), 5.93 (s, 2H), 4.01
(m, 4H), 1.81 (m, 4H), 1.57ꢀ1.27 (br, 28H), 0.88 (m, 6H); 13C NMR
(100 MHz, CDCl3) δ 158.1, 150.0, 120.7, 116.6, 116.5, 70.0, 69.3, 31.90,
31.86, 29.56, 29.55, 29.49, 29.48, 29.4, 29.34, 29.32, 29.27, 29.25, 29.2,
26.0, 22.7, 14.1; C ipso to B is not observed, some carbons on the decyl
chains are unresolved; mp 85.0ꢀ88.2 °C.
General Procedure for (4-Bromo-2,5-dialkoxyphenyl)boro-
nic Acid Sodium Salt (2a). (4-Bromo-2,5-dimethoxyphenyl)boronic
acid (414 mg, 1.59 mmol, 1.0 equiv) was dissolved in benzene (10.0 mL)
and CH2Cl2 (5.0 mL). Then NaOH (64.4 mg, 1.61 mmol, 1.0 equiv) was
added. The reaction was stirred for 11ꢀ24 h at rt. The mixture was
concentrated in vacuo isolating 437 mg of product as a white solid (92%
1
yield): 11B NMR (128 MHz, CD3OD) δ 4.88; H NMR (500 MHz,
CD3OD) δ 7.22 (br, 1H), 6.94 (br, 1H) 3.81 (br, 3H), 3.73 (br, 3H); 13
C
NMR (125 MHz, CD3OD) δ 159.3, 151.0, 121.2, 115.5, 109.4, 57.3, 56.4;
C ipso to B is not observed; mp 134 °C dec.
(4-Bromo-2,5-dimethoxyphenyl)boronic Acid. Following
the general procedure described above, (4-bromo-2,5-dimethoxyphe-
nyl)boronic acid was obtained after column chromatography (20/80
EtOAc/hexanes) as a white solid (556 mg, 63% yield): MS (EI) m/z =
260.1 [M(79Br)]+, 262.1 [M(81Br)]+; HRMS (ESI) [M]+ calcd for
C8H10BBrO4 259.9856, found 259.9862; 11B NMR (128 MHz, CDCl3)
δ 28.96; 1H NMR (400 MHz, CDCl3) δ 7.38 (s, 1H), 7.13 (s, 1H), 6.08
(s, 2H), 3.89 (s, 3H), 3.88 (s, 3H); 13C NMR (100 MHz, CDCl3) δ
158.6, 150.5, 119.1, 116.0, 115.7, 56.7, 56.3; C ipso to B is not observed;
mp 105ꢀ109 °C.
(4-Bromo-2,5-dibutoxyphenyl)boronic Acid Sodium Salt
(2b). Following the general procedure described above, (4-bromo-2,5-
dibutoxyphenyl)boronic acid sodium salt was obtained as a white solid
(337 mg, 99% yield); 11B NMR (128 MHz, CD3OD) δ 4.57; 1H NMR
(400 MHz, CD3OD) δ 7.17 (br, 1H), 6.91 (br, 1H), 3.99 (m, 2H),
3.92ꢀ3.89 (br, 2H), 1.74 (m, 4H), 1.52 (m, 4H), 0.98 (m, 6H); 13C
NMR (125 MHz, CD3OD) δ 158.0, 150.5, 123.0, 121.7, 117.0, 110.7,
70.9, 69.7, 32.8, 32.6, 20.4, 14.3, 14.2; some carbons on the butyl chains
are unresolved; mp 98ꢀ100 °C.
General Procedure for 2,5-Dialkoxyphenylboronic Acid
Sodium Salt (1a). (2,5-Dimethoxyphenyl)boronic acid (359 mg,
(4-Bromo-2,5-bis(hexyloxy)phenyl)boronic Acid Sodium
Salt (2c). Following the general procedure described above, only
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The Journal of Organic Chemistry
NOTE
dissolved in benzene, (4-bromo-2,5-bis(hexyloxy)phenyl)boronic acid
sodium salt was obtained as a white solid (438 mg, 94% yield): 11B NMR
(128 MHz, CD3OD) δ 4.53; 1H NMR (400 MHz, CD3OD) δ 7.19 (br,
1H), 6.88 (br, 1H), 3.98 (t, J = 6.4 Hz, 2H), 3.89 (m, 2H), 1.75 (m, 4H),
1.54ꢀ1.33 (br, 12H), 0.92 (m, 6H); 13C NMR (125 MHz, CD3OD) δ
158.0, 150.4, 123.4, 121.9, 116.9, 110.2, 71.2, 70.0, 32.9, 32.8, 30.6, 30.4,
26.9, 23.7, 14.4; some carbons on the hexyl chains are unresolved; mp
35ꢀ37 °C
(4) For leading references of experimental evidence of intermediate
aryl trihydroxyborate salts, see: (a) Miyaura, N. J. Organomet. Chem.
2002, 653, 54–57. (b) Matos, K.; Soderquist, J. A. J. Org. Chem. 1998,
63, 461–470.
(5) For leading references of computational studies which implicate
aryl trihydroxyborate salts as intermediates in cross-coupling, see:
(a) Xue, L; Lin, Z. Chem. Soc. Rev. 2010, 39, 1692–1705. (b) Jover, J.;
Fey, N.; Purdie, M.; Lloyd-Jones, G. C.; Harvey, J. N. J. Mol. Catal. A:
Chem. 2010, 324, 39–47.
(4-Bromo-2,5-bis(heptyloxy)phenyl)boronic Acid Sodium
Salt (2d). Following the general procedure described above, only
dissolved in benzene, (4-bromo-2,5-bis(heptyloxy)phenyl)boronic acid
sodium salt was obtained as a white solid (310 mg, 90% yield): 11B NMR
(128 MHz, CD3OD) δ 4.49; 1H NMR (400 MHz, CD3OD) δ 7.21 (br,
1H), 6.88 (br, 1H), 3.98 (t, J = 6.8 Hz, 2H), 3.91 (t, J = 6.8 Hz, 2H), 1.75
(m, 4H), 1.53ꢀ1.27 (br, 16H), 0.90 (m, 6H); 13C NMR (125 MHz,
CD3OD) δ 158.0, 150.4, 123.6, 121.8, 116.9, 110.1, 71.1, 70.0, 33.0,
30.7, 30.3, 30.2, 27.2, 27.1, 23.7, 14.5; some carbons on the heptyl chains
are unresolved; mp 48ꢀ50 °C.
(6) For leading references, see: (a) Butters, M.; Harvey, J. N.; Jover,
J.; Lennox, A. J. J.; Lloyd-Jones, G. C.; Murray, P. M. Angew. Chem., Int.
Ed. 2010, 49, 5156–5160. (b) Molander, G. A.; Biolatto, B. Org. Lett.
2002, 4, 1867–1870.
(7) (a) Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2011,
133, 2116–2119. (b) Amatore, C.; Jutand, A.; Le Duc, G. Chem.—
Eur. J. 2011, 17, 2492–2503.
(8) For a review of metal-containing molecular gels, see: Piepen-
brock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Rev. 2010,
110, 1960–2004.
(9) For recent examples of organometallic gelators, see: (a) Strassert,
C. A.; Chien, C.-H.; Galvez Lopez, M. D.; Kourkoulos, D.; Hertel, D.;
Meerholz, K.; De Cola, L. Angew. Chem., Int. Ed. 2011, 50, 946–950.
(b) Chandrasekhar, V.; Gopal, K.; Singh, P.; Narayanan, R. S.; Duthie, A.
Organometallics 2009, 28, 4593–4601.
(4-Bromo-2,5-bis(decyloxy)phenyl)boronic Acid Sodium
Salt (2e). Following the general procedure described above, (4-bromo-
2,5-bis(decyloxy)phenyl)boronic acid sodium salt was obtained as a
white solid (193 mg, 95% yield): 11B NMR (128 MHz, CD3OD) δ 4.35;
1H NMR (500 MHz, CDCl3) δ 7.18 (br, 1H), 6.89 (br, 1H), 3.98 (m,
2H), 3.89 (br, 2H), 1.75 (m, 4H), 1.52ꢀ1.29 (br, 28H), 0.89 (m, 6H);
13C NMR (125 MHz, CDCl3) δ 158.0, 150.4, 123.4, 121.8, 116.9, 110.3,
71.1, 70.0, 33.1, 30.73, 30.68, 30.6, 30.5, 27.2, 23.7, 14.5; some carbons
on the decyl chains are unresolved; mp 46ꢀ48 °C.
(10) For examples of boron-containing gelators, see: (a) Kubo, Y.;
Yoshizumi, W.; Minami, T. Chem. Lett. 2008, 37, 1238–1239.
(b) Kimura, T.; Yamashita, T.; Koumoto, K.; Shinkai, S. Tetrahedron
Lett. 1999, 40, 6631–6634.
(11) Smith, D. K. Molecular Gels - Nanostructured Soft Materials. In
Organic Nanostructures; Steed, J. W., Atwood, J. L., Eds.; Wiley-VCH:
Weinheim, 2008; pp 111ꢀ154.
’ ASSOCIATED CONTENT
(12) For a recent review, see: Banerjee, S.; Das, R. K.; Maitra, U.
J. Mater. Chem. 2009, 19, 6649–6687.
S
Supporting Information. Spectroscopic data; micro-
b
(13) For a recent example, see: Chen, J.; McNeil, A. J. J. Am. Chem.
Soc. 2008, 130, 16496–16497.
(14) For a recent example, see: King, K. N.; McNeil, A. J. Chem.
Commun 2010, 46, 3511–3513.
scope images; rheological data; powder X-ray diffraction pat-
terns; thermal gravimetric analysis data; Tgel data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) For a recent example, see: Samanta, S. K.; Gomathi, A.;
Bhattacharya, S.; Rao, C. N. R. Langmuir 2010, 26, 12230–12236.
(16) (a) Ainley, A. D.; Challenger, F. J. Chem. Soc. 1930, 2171–2180.
(b) Kuivila, H. G.; Reuwer, J. F., Jr.; Mangravite, J. A. Can. J. Chem. 1963,
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V. F. Z. Anorg. Allg. Chem. 2002, 628, 2834–2838. (d) Crawford, C. L.;
Barnes, M. J.; Peterson, R. A.; Wilmarth, W. R.; Hyder, M. L. J. Organomet.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ajmcneil@umich.edu.
Present Addresses
†Syngene International Ltd., Bangalore, India.
(17) These results are consistent with recent theoretical studies that
found a relatively low activation barrier (ΔGq = 34 kcal/mol) for the
decomposition reaction of PhB(OH)3ꢀ. See: Glaser, R.; Knotts, N.
J. Phys. Chem. A 2006, 110, 1295–1304.
(18) Fields, C. L.; Doyle, J. R. Thermochim. Acta 1974, 8, 239–248.
(19) For recent reviews, see: (a) Cravotto, G.; Cintas, P. Chem. Soc.
Rev. 2009, 38, 2684–2697. (b) Bardelang, D. Soft Matter 2009,
5, 1969–1971.
’ ACKNOWLEDGMENT
We thank Ming Fang for acquiring the AFM images and the
Office of Naval Research (N00014-09-1-0848), the Arnold and
Mabel Beckman Foundation, 3M, and the University of Michi-
gan for support of this work. C.L.M. thanks the University of
Michigan IDEA Institute and GAANN (US-DOE 055384) for a
partial fellowship.
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