Supramolecular structures and spontaneous resolution: The case of ortho-substituted phenylboronic acids

Article abstract of DOI:10.1039/b719282g

The solid state structures of a number of ortho-substituted arylboronic acids, ortho-bromophenyl, ortho-phenylphenyl, pentamethylphenyl, and 10-bromo-9-anthryl, were determined by X-ray diffraction techniques. All boronic acids investigated form dimers in the solid state, but the interconnection of dimers to ribbons differs from that of the parent phenylboronic acid. Pentamethylphenylboronic acid only uses one hydrogen bond but an additional OH-π interaction for connection of dimers, while all others investigated employ two hydrogen bonds for interconnection of dimers to ribbons. 10-Bromo-9-anthrylboronic acid is found to undergo spontaneous resolution of its enantiomers to a racemic conglomerate upon crystallization. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719282g

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Supramolecular structures and spontaneous resolution: the case of
ortho-substituted phenylboronic acids†
Matthias Filthaus,a Iris M. Oppelb and Holger F. Bettinger*a
Received 13th December 2007, Accepted 28th January 2008
First published as an Advance Article on the web 28th February 2008
DOI: 10.1039/b719282g
The solid state structures of a number of ortho-substituted arylboronic acids, ortho-bromophenyl,
ortho-phenylphenyl, pentamethylphenyl, and 10-bromo-9-anthryl, were determined by X-ray
diffraction techniques. All boronic acids investigated form dimers in the solid state, but the
interconnection of dimers to ribbons differs from that of the parent phenylboronic acid.
Pentamethylphenylboronic acid only uses one hydrogen bond but an additional OH–p interaction for
connection of dimers, while all others investigated employ two hydrogen bonds for interconnection of
dimers to ribbons. 10-Bromo-9-anthrylboronic acid is found to undergo spontaneous resolution of its
enantiomers to a racemic conglomerate upon crystallization.
hydrogen bonding, which is not available to carboxylic acid dimers.
Introduction
In prototypical phenylboronic acid 1, the structure of which has
been known since 1977,15 each dimer unit is interacting with four
other dimers by hydrogen bonds (Scheme 2, I-A). Here, the planes
defined by the hydrogen bonding network within the four dimers
are almost orthogonal to the plane of the central dimer.
Boronic acids are invaluable reagents for transition metal catalyzed
cross-coupling reactions1 and thus comprise an important class of
auxiliary compounds in modern organic synthesis.2 Along with
the boroxins, their cyclic trimeric anhydrides, boronic acids also
are of current interest in supramolecular chemistry and crystal
engineering.3–8 The use of boronic acids as building blocks in the
assembly of discrete supermolecules has been motivated by the
formation of self-complementary dimers (I), akin to carboxylic
acids (II; Scheme 1).9
Scheme 1 Homodimeric units of boronic acids (I) and carboxylic acids
(II).
The formation of dimers is generally observed in the crystal
structures of arylboronic acids, but there are some examples which
do not crystallize within motif I.10–13 Most of these examples
have in common a hydrogen bond acceptor on the phenyl ring,
which interacts with one of the B(OH)2 hydroxyl groups of
another molecule. Another exception is 4-methoxyphenylboronic
acid where one molecule is hydrogen bonding to four other
monomers.14a
Scheme 2 Schematic representation of the interaction of homodimeric
arylboronic acid dimers in the solid state. In I-A a central dimer interacts
with four other dimers (only monomers are shown for clarity) which are
oriented almost perpendicularly to the central dimer. In I-B each dimer is
interacting with two other dimers resulting in ribbons.
The two additional hydrogen atoms in boronic acid dimers allow
for interaction of homodimeric units in the solid state through
However, another connectivity of the homodimeric units
was observed for
a
few arylboronic acids, like 2,6-
aOrganische Chemie II, Ruhr-Universita¨t Bochum, Universita¨tsstr. 150,
44780, Bochum, Germany. E-mail: Holger.Bettinger@rub.de; Fax: +49
234/322-4529
difluorophenyl (2),16 2,6-bis(trifluoromethyl)phenyl (3)17 and 2,2ꢀ-
ethynylenedibenzeneboronic acid (4; Scheme 3).18 In the solid state
structures of these compounds each dimer is bound to only two
other homodimers by hydrogen bonds (Scheme 2, I-B). This con-
nectivity results in eight-membered rings with an almost coplanar
arrangement of oxygen and hydrogen atoms. Compounds 2–4
bAnalytische Chemie, Ruhr-Universita¨t Bochum, 44780, Bochum, Germany
† Electronic supplementary information (ESI) available: Graphical visu-
alization of crystal structures using displacements ellipsoids; crystallo-
graphic data in CIF or other electronic format (CCDC reference numbers
670992–670996). See DOI: 10.1039/b719282g
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Scheme 3
Scheme 4
have in common non-hydrogen substituents in positions ortho
to the boronic acid functionality, and it is tempting to assume
that the steric requirements of the ortho substituent might be
responsible for the preference of structural motif I-B in the solid
state. Establishing such an influence of the molecular structure on
the crystal structure would be very valuable for crystal engineering
applications utilizing arylboronic acid synthons.4 However, the
crystal structures of three other arylboronic acids indicate that the
absence or presence of ortho substituents not necessarily results
in the preferred formation of motif I-A or I-B, respectively. For
example, p-styrylboronic acid (5) and 1,4-phenylenediboronic acid
(6) also crystallize within motif I-B, but do not have any non-
hydrogen groups in the ortho position.19 On the other hand, in
the crystal structure of pentafluorophenylboronic acid motif I-B
is not observed in spite of the two fluorine atoms.20
Scheme 5 General scheme for synthesis of substituted phenylboronic
acids.
This brief survey of the crystal structures of arylboronic acids
shows that it is difficult to clearly establish a link between
molecular and crystal structures, even for such seemingly simple
compounds. In view of the future utilization of arylboronic
acids in crystal engineering, additional structural information
is required to gain a deeper understanding of the influences
of substituents at the arene ring on the crystal structures of
phenylboronic acids. We have therefore investigated the solid
state structures of a-substituted arylboronic acids 7–10 and 9-
anthracenylboronic acid dimethyl ester (11; Scheme 4), which have
not been described previously, and present our results here. We
have selected these compounds because they are of interest in our
ongoing research, which is aimed at the synthesis of kinetically
stabilized benzoborirenes.
be difficult. This is most likely due to the equilibrium between the
boronic acids and their cyclic anhydrides.21,22
Solid state structures
Relevant crystallographic details for all compounds are compiled
in Table 1. The molecular parameters (Table 2) are as expected
˚
for arylboronic acids. For example, the BC bond in 7, 1.58 A,
23,24
˚
is longer than in the meta or para isomer (1.53 A),
but it
˚
is well within the range (1.57–1.59 A) typically observed for a-
substituted phenylboronic acids.16–18,20,25–29 The tilt angles, which
describe the relative rotations of the aryl rings and the BO2 planes,
range between 34–75◦ for the boronic acids 7–10b. The BO bonds
have similar lengths and do not deviate significantly from values
in other phenylboronic acids.
Results and discussion
As expected, all acids 7–10 form dimers, but they differ in
the way dimers are interconnected in the solid state. Among
the series investigated, the geometrically least hindered 2-
bromophenylboronic acid (7) forms the motif I-B. Within each
dimeric unit, the boron and oxygen atoms are coplanar. Each
dimer in turn is connected by hydrogen bonds to two other dimers,
one on either side, thereby creating a ribbon-like infinite structure
(Fig. 1).
Synthesis and spectral properties
The phenylboronic acids 7, 8, and 10a have been reported earlier as
intermediates in complex syntheses, but their detailed preparations
and spectroscopic properties have not yet been described. All
phenylboronic acids studied in the present paper, including the
novel compound 9, were synthesized from the corresponding
bromoarenes as depicted in Scheme 5.
The boron and oxygen atoms within this ribbon are almost co-
planar and the hydrogen atoms are only slightly above and below
this plane. The aryl rings are tilted in the opposite direction,
All compounds showed the required 1H, 13C, and 11B NMR and
mass spectra, but obtaining acceptable elementary analyses can
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Table 1 Crystallographic information for 7–11
7
8
9
10b
11a
Empirical formula
Molecular weight
Temperature/K
Wavelength/A
Crystal system
C6H6BBrO2
200.83
108(2)
C12H11BO2
198.02
108(2)
C11H17BO2
192.06
108(2)
0.71073
Orthorhombic
Aba2, 16
31.624(4)
15.161(1)
9.156(1)
90.0
90.0
90.0
4390(1)
1.162
2.68, 27.49
0.0406
C14H10BBrO2
300.94
113(2)
0.71073
Orthorhombic
P21212, 4
8.264(1)
17.094(1)
4.9200(2)
90.0
90.0
90.0
1216.8(1)
1.643
2.82, 27.49
0.0240
C16H15BO2
250.09
294(2)
0.71073
Monoclinic
C2/c, 8
31.502(3)
7.6126(6)
12.1156(7)
90.0
110.10(1)
90.0
2728.5(4)
1.218
2.76, 25.23
0.0530
0.1597
˚
0.71073
Triclinic
0.71073
Triclinic
¯
¯
Space group, Z
P1, 2
P1, 4
˚
a/A
4.957(8)
7.056(1)
10.757(2)
95.77(1)
92.22(1)
96.61(1)
371.4(1)
1.796
8.264(1)
9.289(1)
14.548(2)
98.51(1)
105.00(1)
99.90(1)
1040.7(2)
1.264
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
Volume/A
Density/g cm−3
◦
hmin, hmax
/
3.32, 27.49
0.0241
0.0595
2.95, 25.00
0.0615
0.1590
R1 (observed)
wR2 (all)
0.1014
n. a.
0.0490
−0.014(8)
Flack parameter
n. a.
n. a.
n. a.
˚
Table 2 Geometrical parameters (in Angstroms and degrees) of molecules 7–11 as determined by crystal structure analysis
Parameter
7
8
9
10b
11a
r(CB)
1.583(3)
1.368(3); 1.369(3)
34.0
1.556(5)
1.364(4); 1.374(4)
50.9
1.574(4)
1.350(3); 1.377(3)
74.7
1.580(3)
1.355(3); 1.359(3)
52.5
1.581(3)
1.344(2); 1.346(2)
90.0
r(BO)
Averaged tilt angle
Fig. 1 Two different views of the arrangement of dimers of 2-bromophenylboronic acid (7) in the solid state.
if viewed along the B–C bond, giving rise to a symmetric
arrangement of boronic acid monomers with respect to inversion
hydrogen atoms in the hydrogen bonding network are disordered
(Fig. 3).
Most importantly, the aromatic rings are tilted in the same
in the center of the dimer. The compound crystallizes in the space
ꢀ
¯
group P1, as does 1,1 -biphenyl-2-yl-boronic acid 8. The latter
direction, looking along the B–C bond, on either side of the
ribbon. Therefore, the inversion center is lost, and the compound
crystallizes in the Sohncke space group P21212. The molecule 10b
does not have an element of symmetry, and is thus dissymmetric,
i.e., it is not superimposable with its mirror image.30 However,
the barrier for rotation around the C–B bond is expected to be
too low to allow separation of enantiomers at room temperature
and thus a racemate of two very quickly equilibrating optical
isomers exists in solution. Only one of the two enantiomers is
found in the single crystal we have analyzed. Thus the crystal
structure of 10b provides an example of spontaneous resolution
upon crystallization under formation of a racemic conglomerate.
also forms ribbons of dimer units in the solid similar to 7 and
thus displays the hydrogen bonding motif I-B. The boron and
oxygen atoms are no longer coplanar in the dimer units and
these are strongly tilted with respect to each other (Fig. 2). It
appears reasonable to ascribe these distortions compared to 7 to
the sterically more demanding ortho-phenyl group in 8.
An aggregation of dimer units into ribbons with hydrogen bond-
ing motif I-B is also obtained in (10-bromo-9-anthracenyl)boronic
acid (10b). As with 8 coplanarity of the boron, oxygen, and
hydrogen atoms is lost and the dimer units are tilted against
each other, but the distortions are not as severe. Also some of the
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Fig. 2 Two views of the arrangement of dimeric units of 1,1ꢀ-biphenyl-2-yl-boronic acid (8) in the solid state.
Fig. 3 Two views of the arrangement of dimeric units of 10-bromo-9-anthracenylboronic acid (10b) in the solid state.
While this is not an uncommon phenomenon, it is less often
observed in apparently achiral compounds.31–36 The reason why
homochiral chains are formed in 10b, but not in the structurally
closely related 7 and 8, is unclear, as is so often the case
when molecules resolve spontaneously upon crystallization.34,37
Molecules with large permanent dipole moments appear to have
a somewhat higher tendency towards conglomerate formation,38
and the bromine substituent should cause an increased dipole
moment in 10b.
In an effort to learn if the formation of a racemic conglomerate
is unique to the bromo derivative 10b, parent compound 10a was
synthesized and crystals were grown from chloroform solution.
Unfortunately, we could not obtain a satisfactory crystal structure.
In order to grow better crystals, we transformed boronic acid
10a into the novel dimethyl ester 11a. It is obtained easily upon
heating of 10a in methanol, and during slow cooling needles of
11a crystallize. Crystal structures of esters of arylboronic acids
and small alcohols are rather scarce, possibly due to the fact that
they are usually liquid and quickly hydrolyze. The large anthryl
group efficiently protects the boron atom from attack of water as
11a is stable under atmospheric conditions. Compound 11a is no
longer dissymmetric as the tilt angle is 90◦ (Fig. 4, Table 2). It
appears that the lack of hydroxyl groups, which are necessary for
formation of the supramolecular ribbon structure in 10b, results
in a packing which is dominated by weaker non-covalent interac-
tions.
Fig. 4 Arrangement of two molecules of 9-anthracenylboronic acid
dimethyl ester (11a) in the solid state with face to face interaction of
the anthryl groups.
Lastly, a different packing mode than in boronic acids 7, 8,
and 10b is observed in (2,3,4,5,6-pentamethylphenyl)boronic acid
(9), which also forms dimers in the solid state. Again, the boron
and oxygen atoms are almost coplanar within a dimeric boronic
acid unit (Fig. 5). But in contrast to the earlier examples, dimers
of 9 are interconnected by only one hydrogen bond.
Which factors are responsible for giving up one hydrogen bond
in 9 compared to having two such interactions between two dimeric
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Fig. 5 Two views of the arrangement of dimeric units of (2,3,4,5,6-pentamethylphenyl)boronic acid (9) in the solid state.
◦
1
building blocks in 7, 8, and 10b? We observe the close proximity
of the OH group, which is not engaged in a hydrogen bond,
to the pentamethylphenyl ring. The OH group is not exactly
located above the center of the aryl ring, resulting in three shorter
values, C) were obtained with open capillary tubes. H and 13C
NMR spectra were recorded with a Bruker DRX 400 calibrated
on TMS (tetramethylsilane) as an internal standard (d = 0.0 ppm),
and 11B NMR spectra with a Bruker DPX 250 calibrated on
BF3·OEt2 (d = 0.0 ppm). All spectra were measured in acetone-
d6. Mass spectrometry (EI, 70 eV) was performed with a VG
Autospec. The intensities are reported as a percentage relative
to the base peak after the corresponding m/z value. Elemental
analyses were determined with a Carlo-Erba Elemental Analyser
1106. FT-IR spectroscopy was performed with a Bruker Equinox
55 (KBr, m in cm−1). UV/Vis spectra were recorded with a Cary
Varian 1 (kmax. in nm, e in m2 mmol−1) in CH2Cl2.
˚
(O · · · Caryl: 3.05–3.22 A) and three longer contacts (O. . .Caryl: 3.45–
39–41
˚
3.58 A). Such an attractive OH–p interaction
could partially
compensate the loss of a more stable hydrogen bond.
Conclusions
Four structurally related arylboronic acids have been synthesized
and their solid state structures have been analyzed by single-crystal
X-ray crystallography. We find that all of them prefer the formation
of dimeric units in the solid state, but the interconnection of
dimeric units differs from that in the parent phenylboronic acid.
Infinite ribbons are formed by 7, 8, and 10b, in which the
dimeric units are interconnected by two hydrogen bonds. In the
sterically most crowded pentamethylphenylboronic acid (9), the
two dimeric units within the ribbons are only connected by one
hydrogen bond. We have found an example of a dissymmetric,
but achiral molecule under ambient conditions, 10-bromo-9-
anthrylboronic acid (10b), to undergo spontaneous resolution
upon crystallization.
X-Ray analysis
Intensity data for 7, 8, 9, 10b and 11a were collected on an Oxford
Diffraction Xcalibur2 CCD using MoKa radiation and employing
the x scan method. The data were corrected for Lorentz, polar-
ization and absorption (multi-scan) effects. All structures were
solved by using direct methods (SHELXS-97)42 and refined by
using a full-matrix least-squares refinement procedure (SHELXL-
97).43 The protons bonded to carbon were placed at geometrically
estimated positions while in all cases the proton bonded to oxygen
was found in the Fourier difference synthesis and refined freely
with only the distance fixed to the literature value. In 10b the
protons had to be split into two positions due to a disorder
(2-fold axis). Compound 9 crystallizes in the polar space group
Aba2. As 9 contains no heavy atom the absolute structure could
not be determined. Therefore the Friedel pairs have been merged
resulting in a meaningless Flack parameter of −10(10). In contrast
10b contains a bromine atom. Here (space group P21212) the
absolute structure could be determined as 68% of the Friedel pairs
had been measured. The Flack parameter refines to −0.014(8).
Crystallographic data for the structure reported in this paper are
available as ESI† (CIF format).
Experimental
General
All reactions were carried out under a dry argon atmosphere
in oven dried Schlenk type glassware with magnetic stirring.
Temperatures are reported as bath temperatures. Et2O and THF
were continuously refluxed and freshly distilled from sodium using
˚
benzophenone as indicator. MeOH was dried with activated 3 A
molecular sieves. All commercially available reagents were used
without further purification, and were purchased from Aldrich
Chemical Co., Acros Organics or Merck. t-BuLi was used as a
15% solution in pentane (1.48 M), and n-BuLi was used as a
15% solution in hexane (1.59 M). TLC was performed on Al-
backed plates coated with silica gel with F254 indicator (Polygram
SIL G/UV from Macherey-Nagel); the chromatograms were
visualized under UV light (254 nm). Rf values refer to silica gel
(eluent: pentane–ethyl acetate (4 : 1)). Melting points (uncorrected
General procedure for the preparation of boronic acids (8, 9, 10).
A solution of the arylbromine in THF was cooled to −90 ◦C with
a nitrogen/isopropanol cooling bath and the lithium compound,
t-BuLi (2.1 eq.) or n-BuLi (1.1 eq.), were added quickly. The
coloured reaction mixture was stirred for 45 min, while the
temperature rose to −65 ◦C. The reaction was treated with an
excess of B(OMe)3, warmed slowly to room temperature and
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stirred overnight. After the addition of water (80 cm3) the reaction
mixture was acidified with 0.5 M hydrochloric acid (pH ∼ 3–5) and
extracted with Et2O (2 × 70 cm3). The organic layer was washed
with brine and water (2 × 25 cm3) and dried over MgSO4. The
solvents of the organic layer were removed by rotary evaporation.
The residue was refluxed in water for 2 h, and filtered scalding hot
(recrystallization). The filtrate was cooled to 0 ◦C for 10 min and
the white product was isolated by filtration, washed with pentane
and water (2 × 20 cm3).
H 8.92, found: C 68.53, H 8.96. HRMS calc. 192.13303, found:
192.13290.
9-Anthracenylboronic acid (10a). 44,45 According to the general
procedure, 9-bromoanthracene (3.5 g, 13.61 mmol) in THF
(140 cm3), n-BuLi (10.0 cm3, 15.9 mmol) and B(OMe)3 (10 cm3,
88 mmol), gave 10a (1.94 g, 64%) as a pale yellow solid after
recrystallization from water (650 cm3), mp (from water) 217 ◦C. Rf
0.30; dB 32.5; dH 7.40–7.46 (4 H, m), 7.90 (2 H, s, OH), 7.97–8.00
(2 H, m), 8.12–8.14 (2 H, m), 8.42 (1 H, s); dC 125.01, 125.04,
126.60, 128.55, 129.04, 131.31, 133.47; MS (EI): m/z (%): 222
2-Bromophenylboronic acid (7).
A
solution of ortho-
•
(7)[M+ ], 204 (7), 178 (100), 151 (9), 139 (2), 126 (3), 89 (14), 76
dibromobenzene (0.5 cm3, 4.15 mmol), THF–Et2O (25 cm3
:
(13), 63 (3), 51 (2), 40 (2); IR (KBr) in cm−1: 3268, 3047, 1621, 1558,
1511, 1485, 1415, 1353, 1303, 1181, 1069, 888, 849, 834, 728, 607;
UV/Vis kmax. (log e) = 244 (4.17), 329 (3.44), 345 (3.72), 363 (3.88),
382 (3.85); calc. for C14H11BO2: C 75.73, H 4.99, found: C 75.62,
H 5.00. HRMS calc. 222.08495, found: 222.08482. Hydrolysis and
recrystallization of 11a in hydrochloric acid (1000 cm3, 0.5 M) for
2 h at 100 ◦C gave 10a as pale yellow needles of higher purity.
◦
25 cm3) was cooled to −115 C with a nitrogen/ethanol cooling
◦
bath and pre-cooled (−78 C) t-BuLi (5.9 cm3, 8.73 mmol) was
added very quickly. The yellow reaction mixture was stirred for
35 min at −115 ◦C, and the reaction was treated with a pre-
◦
cooled solution (−78 C) of B(OMe)3 (5 cm3, 44 mmol) in Et2O
(10 cm3) and stirred for two hours at −115 to −105 ◦C. The
solution was quenched with 5 cm3 EtOH–conc. HCl (8 : 2) at
◦
−115 C and warmed slowly to room temperature. Workup (see
general procedure) and recrystallization from water (150 cm3) gave
7 (0.55 g, 66%) as a white solid, mp 183 ◦C (from water), Rf 0.35.
Single crystals (colourless needles) were obtained from chloroform
solution by slow evaporation. dB 28.8; dH 7.25–7.35 (2 H, m), 7.51–
7.54 (2 H, m); dC 126.38, 126.94, 131.16, 132.31, 135.09; MS (EI):
(10-Bromo-9-anthracenyl)boronic acid (10b). According to the
general procedure, 9,10-dibromoanthracene (4.0 g, 11.90 mmol)
in THF (180 cm3), n-BuLi (8.24 cm3, 13.1 mmol) and B(OMe)3
(15 cm3, 132 mmol), gave 10b (2.48 g, 69%) as a pale yellow
solid after column chromatography on silica (eluent: pentane–
ethyl acetate (4 : 1)) and washing with pentane (50 cm3), mp (ethyl
acetate) 179 ◦C. Rf = 0.3; dB 32.0; dH 7.55–7.68 (4 H, m), 7.95
(2 H, s, OH), 8.15–8.17 (2 H, m), 8.50–8.52 (2 H, m); dC 123.58,
127.06, 128.93, 129.11, 131.38, 131.64, 135.65; MS (EI): m/z (%):
•
m/z (%): 547 (39) 469 (6), 410 (13), 281 (8), 200 (91)[M+ ], 156 (57),
130 (42), 103 (7), 77 (100), 50 (33); IR (KBr) in cm−1: 3300, 1586,
1556, 3017, 2360, 1594, 1558, 1466, 1376, 1291, 1165, 1122, 1007,
822, 770, 736, 589; UV/Vis (CH2Cl2) kmax. (log e) = 237 (3.76), 283
(3.02); calc. for C6H6BBrO2: C 35.88, H 3.01; found: C 36.03, H
3.06. HRMS calc. 199.96442, found 199.96438.
•
300 (8)[M+ ], 282 (6), 256 (100), 208 (11), 176 (54), 151 (22), 128
(16), 88 (37), 75 (13), 63 (3), 43 (3); IR (KBr) in cm−1: 3298, 1954,
1928, 1620, 1546, 1483, 1413, 1298, 1261, 1149, 1036, 947, 883,
828, 753, 655, 618; UV/Vis kmax. (log e) = 246 (4.14), 338 (3.45),
355 (3.73), 374 (3.87), 395 (3.85); calc. for C14H10BBrO2: C 55.87,
H 3.35, found: C 56.11, H 3.19. HRMS calc. 298.99771, found:
298.99710.
1,1ꢀ-Biphenyl-2-yl-boronic acid (8). According to the general
procedure, 2-bromo-1,1ꢀ-biphenyl (0.8 cm3, 4.63 mmol) in THF
(50 cm3), n-BuLi (3.2 cm3, 5.09 mmol) and B(OMe)3 (5 cm3,
44 mmol), gave 8 (0.74 g, 81%) as colourless needles after
recrystallization from water (150 cm3). Mp (from water) 194–
196 ◦C, Rf 0.35. dB 30.6; dH 6.89 (2 H, s, OH), 7.34–7.54, (8 H,
m), 7.67–7.71 (1 H, m); dC 126.43, 127.18, 128.47, 128.86, 129.10,
9-Anthracenylboronic acid dimethyl ester (11a). Recrystalliza-
tion of dry 10a (5.2 g, 23.42 mmol) from sparse MeOHabs. under
arg◦on gave 11a (3.26 g, 56%) as pale yellow needles, mp (MeOH)
81 C. Rf 0.35; dB 32.3; dH 3.61 (6 H, s, O-CH3), 7.43–7.53 (4 H,
m), 7.86–7.89 (2 H, m), 7.99–8.08 (2 H, m), 8.52 (1 H, s); dC 52.70,
126.09, 126.59, 128.08, 128.91, 129.74, 132.15, 134.32; MS (EI):
•
133.38, 143.80, 145.81; MS (EI): m/z (%): 540 (19) 198 (100)[M+ ],
180 (43) (29), 154 (58), 128 (5), 77 (8), 51 (4); IR (KBr) in cm−1:
3456, 3222, 3058, 3017, 2360, 1594, 1558, 1499, 1476, 1433, 1370,
1152, 1115, 1093, 1073, 1009, 824, 781, 742, 698; UV/Vis kmax.
(log e) = 250 (3.90); calc. for C12H11BO2: C 72.78, H 5.60, found
C 72.17, H 5.53. HRMS calc. 198.08562, found: 198.08542.
•
m/z (%): 250 (100)[M+ ], 234 (4), 219 (5), 204 (41), 192 (78), 178
(52), 165 (6), 151 (9), 139 (2), 126 (3), 88 (4), 73 (11), 43 (5); IR
(KBr) in cm−1: 3301, 2950, 2871, 1622, 1473, 1452, 1415, 1414,
1329, 1266, 1165, 1049, 1003, 888, 839, 736, 663; UV/Vis kmax.
(log e) = 245 (3.83), 315 (3.13), 329 (3.40), 346 (3.58), 364 (3.64),
382 (3.61); calc. for C16H15BO2: C 76.84, H 6.05, found: C 76.69,
H 5.88. HRMS calc. 250.11798, found: 250.11733. Hydrolysis and
recrystallization of 11a in hydrochloric acid (1000 cm3, 0.5 M) for
2 h at 100 ◦C gave 10a as pale yellow needles of higher purity.
(2,3,4,5,6-Pentamethylphenyl)boronic acid (9). According to
the general procedure, 1-bromo-2,3,4,5,6-pentamethylbenzene
(3.0 g, 13.21 mmol) in THF (180 cm3), t-BuLi (18.7 cm3,
27.7 mmol) and B(OMe)3 (10 cm3, 88 mmol) afforded 9 after
recrystallization from water (◦400 cm3) as colourless needles (2.21 g,
87%), mp (from water) 185 C, Rf 0.35. dB 31.8; dH 2.19 (6 H, s,
meta-H), 2.23 (3 H, s, para-H), 2.30 (6 H, s, ortho-H), 4.45 (2 H,
br s, OH); dC 20.16 (meta-C), 20.58 (para-C), 25.16 (ortho-C),
Acknowledgements
•
131.53, 131.70, 134.20; MS (EI): m/z (%): 192 (94)[M+ ], 174 (29),
159 (37), 148 (30), 133 (100), 117 (15), 105 (13), 91 (19), 77 (9), 65
(6), 53 (6), 41 (8); IR (KBr) in cm−1: 3488, 3408, 3229, 2924, 1574,
1477, 1224, 1152, 1086, 1022, 995, 941, 827, 746, 565; UV/Vis
kmax. (log e) = 229 (3.37), 273 (2.25); calc. for C11H17BO2: C 68.79;
This work was supported by Deutsche Forschungsgemeinschaft
and Fonds der chemischen Industrie. HFB is grateful for support
through a Heisenberg Fellowship by the DFG, and to Prof. Dr
W. Sander for his support and interest.
1206 | Org. Biomol. Chem., 2008, 6, 1201–1207
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