Reactivity of aryldimesitylboranes under suzuki-miyaura coupling conditions

Article abstract of DOI:10.1021/om1006903

Sterically protected triarylboron compounds such as BMes₂(Ar) have important applications in organic optoelectronic devices and chemical sensors. Furthermore, the Suzuki-Miyaura cross-coupling reaction is the most commonly used method for building the π skeletons of such conjugated materials. We have found that BMes₂(Ar) can also be a highly active and effective coupling partner under typical Suzuki-Miyaura coupling conditions. For BMes₂(p-Br-Ph), self-coupling leads to the formation of oligomers Mes₂B-(Ph)_n-Mes, where the products with n = 1 (1), 2 (2), 3 (3), 4 (4) have been isolated and fully characterized. Examination of cross-coupling reactions of BMes₂(Ph) with various aryl bromides has established the generality of BMes₂(Ar) as a coupling partner.

Full text of DOI:10.1021/om1006903

Organometallics 2010, 29, 4007–4011 4007
DOI: 10.1021/om1006903
Reactivity of Aryldimesitylboranes under Suzuki-Miyaura
Coupling Conditions
Nan Wang, Zachary M. Hudson, and Suning Wang*
Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
Received July 13, 2010
Sterically protected triarylboron compounds such as BMes2(Ar) have important applications in
organic optoelectronic devices and chemical sensors. Furthermore, the Suzuki-Miyaura cross-
coupling reaction is the most commonly used method for building the π skeletons of such conjugated
materials. We have found that BMes2(Ar) can also be a highly active and effective coupling partner
under typical Suzuki-Miyaura coupling conditions. For BMes2(p-Br-Ph), self-coupling leads to the
formation of oligomers Mes2B-(Ph)n-Mes, where the products with n=1 (1), 2 (2), 3 (3), 4 (4) have
been isolated and fully characterized. Examination of cross-coupling reactions of BMes2(Ph) with
various aryl bromides has established the generality of BMes2(Ar) as a coupling partner.
Triarylboron compounds are an attractive class of materials
for optoelectronic applications due to the empty pπ orbital on
the boron center. This feature allows them to act as excellent
electron acceptors, and such compounds have been success-
fully developed as anion sensors,1 nonlinear optical materials,2
and luminescent or electron-transporting compounds for
organic light-emitting diodes (OLEDs).3 However, the empty
pπ orbital also leaves these compounds susceptible to nucleo-
philic attack and hydrolysis.
In 1972, Williams and co-workers reported that two mesityl
groups (Mes = 2,4,6-trimethylphenyl) were sufficient to pro-
vide an unusual degree of stability to arylboranes, preventing
the attack of most nucleophiles.4 On the basis of this design, a
great many molecular and polymeric materials have since been
synthesized containing the aryldimesitylboron moiety which
are in general very stable toward hydrolysis. Only very small
anions, such as fluoride and cyanide, are capable of overcoming
the steric bulk of the mesityl groups to bind to the boron center,
forming the basis of their use as chemical sensors.1d Hydroxide
ions have also been found to be able to bind to the boron center
of aryldimesitylboron compounds, but the binding constants
are considerably weaker than those of fluoride ions.1b
The synthesis of such π-conjugated materials commonly
makes use of metal-catalyzed carbon-carbon bond forma-
tion reactions, such as Stille, Negishi, Suzuki-Miyaura, and
Sonogashira couplings.5 Among these, the Suzuki-Miyaura
coupling reaction is arguably the most common for the for-
mation of arene-arene bonds, due to its broad scope and
lack of toxic byproducts. However, we have recently dis-
covered that under the basic conditions required for typical
Suzuki coupling reactions, the association of hydroxide with
the boron center of a triarylboron compound is significant
enough to allow this group to act as the transmetalation
coupling partner of a Suzuki-type reaction under Pd(0)
catalysis, despite the steric bulk of the mesityl groups. The
details of our findings are reported herein.
*To whom correspondence should be addressed. E-mail: suning.wang@
chem.queensu.ca.
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2001, 123, 11372. (b) Kim, Y.; Gabbaï, F. P. J. Am. Chem. Soc. 2009, 131,
3363. (c) Sun, Y.; Ross, N.; Zhao, S.-B.; Huszarik, K.; Jia, W.-L.; Wang,
R.-Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510.
(d) Hudnall, T. W.; Chiu, C.-W.; Gabbaï, F. P. Acc. Chem. Res. 2009, 388
and references therein.
(2) (a) Collings, J. C.; Poon, S. Y.; Droumaguet, C. L.; Charlot, M.;
Katan, C.; Palsson, L. O.; Beeby, A.; Mosely, J. A.; Kaiser, H. M.;
Kaufmann, D.; Wong, W. Y.; Blanchard-Desce, M.; Marder, T. B.
Chem. Eur. J. 2009, 15, 198. (b) Entwistle, C. D.; Marder, T. B. Chem.
Mater. 2004, 16, 4574. (c) Entwistle, C. D.; Marder, T. B. Angew. Chem.,
Int. Ed. 2002, 41, 2927. (d) Yuan, Z.; Collings, J. C.; Taylor, N. J.; Marder,
T. B.; Halet, J. F. J. Solid State Chem. 2000, 154, 5. (e) Yuan, Z.; Entwistle,
Results and Discussion
Self-Coupling of Bromophenyldimesitylborane. The involve-
ment of protected triarylboron in cross-coupling reactions was
first observed during our attempts to react p-bromophenyl-
dimesitylborane with p-carboxamidephenylboronic acid by
Suzuki-Miyaura coupling. Despite repeated attempts, the
desired product was never isolated. However, we reproducibly
obtained and isolated a series of oligomers by chromatography
and crystallization, containing dimesitylboron and mesityl
groups bound to an oligophenylene linker (Scheme 1).
ꢀ
C. D.; Collings, J. C.; Albesa-Jove, D.; Batsanov, A. S.; Howard, J. A. K.;
Taylor, N. J.; Kaiser, H. M.; Kaufmann, D. E.; Poon, S. Y.; Wong, W. Y.;
Jardin, C.; Fathallah, S.; Boucekkine, A.; Halet, J. F.; Marder, T. B. Chem.
Eur. J. 2006, 12, 2758 and references therein.
(3) (a) Noda, T; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714–9715.
(b) Noda, T; Ogawa, H.; Shirota, Y. Adv. Mater. 1999, 11, 283. (c) Li, F.; Jia,
W.-L.; Wang, S.; Zhao, Y.; Lu, Z.-H. J. Appl. Phys. 2008, 103, 034509/1.
(d) Zhou, G. J.; Ho, C. L.; Wong, W. Y.; Wang, Q.; Ma, D. G.; Wang, L. X.;
Lin, Z. Y.; Marder, T. B.; Beeby, A. Adv. Funct. Mater. 2008, 18, 499.
(e) Hudson, Z. M.; Sun, C.; Helander, M. G.; Amarne, H.; Lu, Z.-H.; Wang, S.
Adv. Funct. Mater., 2010, in press. (f) Hudson, Z. M.; Wang, S. Acc. Chem.
Res. 2009, 42, 1584 and references therein.
To further understand this result, a series of control
experiments were designed and investigated as shown in
Table 1. Since the coupling reactions with benzamide and
(5) (a) Miyaura, N. Adv. Met.-Org. Chem. 1998, 6, 187 and references
therein. (b) Suzuki, A. J. Organomet. Chem. 1999, 576, 147 and references
therein. (c) McGlacken, G. P.; Fairlamb, I. J. S. Eur. J. Org. Chem. 2009, 24,
4011 and references therein. (d) Hartwig, J. Organotransition Metal Chemistry:
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J. L. R. J. Organomet. Chem. 1972, 38, 229.
r
2010 American Chemical Society
Published on Web 08/25/2010
pubs.acs.org/Organometallics

4008 Organometallics, Vol. 29, No. 18, 2010
Wang et al.
Table 1. Formation of Arylborane Phenylene Oligomersa
Scheme 1
3-methylbenzamide gave the same result, it was clear that the
p-Br-B(Mes)2-benzene compound alone was responsible
for the observed coupling. Indeed, when the amide coupling
partner was removed from the reaction entirely, the same
mixture of oligomeric products was obtained. It appeared as
though the dimesitylborane was undergoing self-coupling,
with transmetalation of either a mesityl group to the bromine
site or a bromophenyl group, which allows for further
coupling to form the observed mesityl-capped oligomers.
While 2-4 were isolated only in 1-4% yields, compound 1
was isolated in moderate yield (28%, assuming that one p-Br-
BMes2-benzene produces one molecule of 1). Nonetheless, the
formation of compound 1 requires the consumption of two
molecules of Mes2B-Ph-Br, one as an aryl bromide and the other
as a transmetalation reagent, most likely in the form of a borate
formed by reaction with a hydroxyl group in the Pd -catalyzed
cycle, as shown in Scheme 2. Because of this and the fact that 1 is
not the only product, the actual isolated yield of 1 can be
considered to be at least 56%. Such significant formation of
self-coupling products from a sterically protected triarylborane
substrate is highly unusual. The low yields of the oligomers are
understandable, since they require the starting material Mes2B-
Ph-Br to undergo multiple and repetitive Suzuki-Miyaura
cycles, as illustrated in Scheme 2.
a Reagents and conditions: p-Br-(BMes2)-benzene (1 equiv), sub-
strate (1.2 equiv), Pd(PPh3)4 (5%), K2CO3 (5 equiv, 1 M in H2O) in
toluene/EtOH/H2O (2/1/1 v/v), 80 °C, reflux, 16 h.
fluorescence quantum yield markedly increases with increas-
ing conjugation length, with values of 0.22 for 1, 0.43 for 2,
and near unity (∼1) for 3. This may be attributed to the
increased π conjugation of the molecule and the increased
contribution from the polyphenyl linker to the emission.
The structures of compounds 1-3were determined by single-
crystal X-ray diffraction analysis and are shown in Figure 1.
The structure of 3 is considerably out of coplanarity, which may
be attributed to intermolecular interactions in the crystal lattice.
As shown in Figure 2, there are significant intermolecular edge-
on π-π interactions between the triphenyl linkers and the
interactions between the methyl protons and the phenyl ring
are evident. In addition, molecules of 3 are all oriented in a
parallel manner along the same direction in the lattice.
Scheme 2. Catalytic Cycles for the Formation of Compounds
1-4 and the Corresponding Intermediates (n = 1, 2, 3, ...)
The use of anionic aryl borates as coupling partners has
been reported previously, most commonly making use of
NaBPh4 due to the relatively poor stability of BPh3.6 This
reagent may be used with a palladium catalyst to transfer an
aryl group in much the same manner as, for example,
phenylboronic acid. It should also be noted that there are a
small number of examples of Suzuki coupling reactions
carried out on dimesitylboryl-containing substrates that
were indeed successful, though low yields were often pro-
blematic.7 Clearly, the reactivity of the aryl halide, aryl
borate, and catalyst together determine the product distribu-
tion in each reaction, and as such the dimesitylboron group
All four compounds were characterized by 1H NMR, mass
spectrometry, and spectroscopic analysis. Compounds 1-3
were also examined by UV-vis and fluorescence spectros-
copy. These compounds are all fluorescent in solution,
exhibiting only a small red shift in emission maximum on
lengthening of the π conjugation (λmax 376, 383, and 386 nm
for 1-3). A similar trend is observed in the absorption spec-
tra, with the wavelength of the low-energy band maxi-
mum increasing from 1 (316 nm) to 2 (331 nm) to 3 (337
nm), as expected for a more highly conjugated π-skeleton
(see the Supporting Information). Most interestingly, the
(6) (a) Qin, W.; Yasuike, S.; Kakusawa, N.; Sugawara, Y.; Kawahata,
M.; Yamaguchi, K.; Kurita, J. J. Organomet. Chem. 2008, 693, 109.
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Article
Organometallics, Vol. 29, No. 18, 2010 4009
Figure 2. (top) Diagram showing the closest intermolecular
contact distances of 3. (bottom) Crystal lattice packing diagram
showing the stacking of molecules of 3.
conversion of the starting materials, the reaction was re-
peated using 2 M NaOH as base in place of K2CO3 (entry 2).
After 16 h, this reaction produced mesitylbenzene and
biphenyl in 61% and 25% yields, respectively, indicating
that the coupling of the mesityl groups is slightly favored
under more basic conditions. To once again firmly establish
the role of each coupling partner (entry 3), this reaction was
repeated using p-bromoethylbenzene, where the coupling
products p-ethylmesitylbenzene and 4-ethylbiphenyl were
obtained in a ratio similar to those in entry 2, and 4,40-
diethylbiphenyl was not observed, establishing that cou-
pling does indeed occur between the aryl halide and aryl
borate.
Interestingly, no reaction was observed between bromo-
benzene and trimesitylborane (entry 4). This indicates that
the steric barrier offered by three pairs of o-methyl groups is
sufficient to protect these materials from reaction in this
manner, while two pairs clearly are not.
Since many triarylboron-based materials include a boron
moiety attached to a pyridine heterocycle, and the electro-
negativity of the heterocycle makes the boron center more
susceptible to nucleophilic attack by groups such as hydro-
xide, the generality of our observations was further tested
using 5-(dimesitylboryl)-2-phenylpyridine, an N,C-chelate
ligand recently reported by our group,9 as a coupling partner
(entry 5). The reaction of (dimesitylboryl)-2-phenylpyridine
with 2.2 equiv of p-bromoethylbenzene resulted in the isola-
tion of 55% p-ethylmesitylbenzene and 45% pyridine cou-
pling product (Table 2) with a high conversion yield of the
Figure 1. Crystal structures of 1 (top), 2 (middle), and 3
(bottom).
does not always participate in coupling reactions. For
example, we have recently reported a Suzuki coupling route
to dual-emissive silane compounds containing triarylboron,
which can be achieved using the highly active Pd(OAc)2/SPhos
catalyst.8 When Pd(PPh4)3 was used, however, this same
reaction was not successful.
Cross-Coupling of B(Ph)Mes2 with Aryl Bromide. In order
to more clearly demonstrate the role of the boron center in
these coupling reactions, a second series of control experi-
ments were designed in which B(Ph)Mes2, which lacks the
bromine functionality, was used as one of the coupling
partners. First, B(Ph)Mes2 was reacted with 3.2 equiv of
bromobenzene for 16 h using 1 M K2CO3 as base (Table 2,
entry 1). Mesitylbenzene and biphenyl were isolated from
this reaction in 23% and 17% yields, respectively, showing
some preference for the biphenyl product, since a 2:1 product
ratio would be expected in the absence of any selectivity.
However, in this case the total conversion of starting materi-
als (40%) was relatively low. In order to increase the
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4010 Organometallics, Vol. 29, No. 18, 2010
Wang et al.
Table 2. Suzuki-Miyaura Coupling of Aryldimesitylboranesa
For the synthesis of typical dimesitylboron compounds,
however, it is preferable to employ alternative methods such
as Stille or Negishi coupling; these methods avoid basic
conditions when a palladium catalyst is used, and numerous
successful examples have been reported for them.3d,10
Experimental Section
General Experimental Information. All reactions were carried
out under a nitrogen atmosphere with standard Schlenk tech-
niques. All starting materials were purchased from Aldrich
Chemical Co. and used without further purification. Reagent-
grade solvents were used without further purification. NMR
spectra were recorded on a Bruker Avance 400 MHz spectro-
meter. High-resolution mass spectra were obtained from a
Waters/Micromass GC-TOF EI-MS spectrometer. Quantum
yields were measured using the optically dilute method relative
to an anthracene standard (Φ=0.36). 1-Bromo-4-dimesitylbor-
on benzene,11 dimesitylboron benzene,11 and 5-(dimesityl-
boryl)-2-phenylpyridine9 were synthesized according to previ-
ously reported procedures.
X-ray Diffraction Analysis. Single crystals of the oligomers
were mounted on glass fibers for data collection. Data were
collected on a Bruker Apex II single-crystal X-ray diffract-
ometer with graphite-monochromated Mo KR radiation, oper-
ating at 50 kV and 30 mA and at 180 K. Data were processed on
a PC with the aid of the Bruker SHELXTL software package
(version 5.10) and corrected for absorption effects. All struc-
tures were solved by direct methods.
a Reagents and conditions: Pd(PPh3)4 (5%), component 1 (1 equiv),
toluene/EtOH/H2O (2/1/1 v/v), 80 °C, reflux 16 h. Legend for entries
1-5: (a) component 2 (3.2 equiv), K2CO3 (5 equiv, 1 M in H2O);
Synthesis of Arylborane Phenylene Oligomers. p-Br-BMes2-
benzene (0.60 mmol, 243 mg), Pd(PPh3)4 (0.03 mmol, 35 mg),
and K2CO3 (3.0 mmol, 414 mg) were added to a 50 mL Schlenk
flask with a stir bar and condenser. A toluene/ethanol/water
mixture (v/v/v, 6 mL/3 mL/3 mL) was stirred and purged with
nitrogen for 1 h, and then the mixed solvents were transferred to
the reaction flask via cannula. The mixture was stirred at 80 °C for
16 h and then cooled to room temperature and concentrated in
vacuo. The residue was partitioned between water and CH2Cl2
and the aqueous layer separated and extracted with dichloro-
methane (3 ꢀ 15 mL). The combined organic layers were dried
over MgSO4, concentrated, and purified on silica gel (hexanes as
the eluent).
(b) component
2 (3.2 equiv), NaOH (5 equiv, 2 M in H2O);
(c) component 2 (2.2 equiv), K2CO3 (5 equiv, 1 M in H2O).
B(Ph)Mes2 starting material. This demonstrates the higher
selectivity/reactivity of the phenylpyridine-B bond, as the
electron-withdrawing pyridine moiety results in a weaker
B-C bond; were the reactivity of the pyridyl and mesityl
groups the same, a 2:1 ratio of p-ethylmesitylbenzene and
pyridine coupling product would again be expected. The
relatively weak Mes2B-C(py) bond was also supported by
our observation that the cleavage of the Mes2B-C(py) bond
occurs usually at ∼80 °C in solution while the Mes2-
B-C(aryl) bond cleavage usually does not occur until
∼100 °C. The low stability of the B-py bond may be explained
by the decreased electron donation of py to the B center,
compared to that of a phenyl.
Several key conclusions can be drawn from these results.
First, in the presence of aqueous base and a Pd(0) catalyst,
sterically protected triarylboranes such as dimesitylarylbor-
on compounds are susceptible to reaction in much the same
manner as traditional aryl boronates and can produce
coupling products of its pendant aryl groups. This demon-
strates that, in principle, a species of the form BMes2(Ar)
may be used in a Suzuki-Miyaura coupling reaction with an
aryl halide, in much the same manner as a boronic acid or
ester could be used in a 1:1 ratio.
Perhaps more importantly, however, we have shown that
while Suzuki-Miyaura coupling reactions are one of the
most common methods for the synthesis of π-conjugated
materials, these reactions are best avoided when the sub-
strate contains a triarylboron group. We note, however, that
such reactions are possible using a sufficiently bulky or active
catalyst. Furthermore, the lack of reactivity of trimesitylbor-
ane indicates that materials which include three pairs of
o-methyl groups, such as tridurylboranes (duryl = 2,3,5,6-
tetramethylphenyl) can tolerate Suzuki coupling conditions.
1
4-Dimesitylboron-1-mesitylbenzene (1). H NMR (400 MHz,
CDCl3, 298.0 K, δ, ppm): 7.59 (d, J = 7.9 Hz, 2H), 7.16 (d, J =
7.9 Hz, 2H), 6.97 (s, 2H), 6.87 (s, 4H), 2.36 (s, 3H), 2.35 (s, 6H),
2.08 (s, 12H), 2.03 (s, 6H). 13C{1H} NMR (100 MHz, CDCl3, δ,
ppm): 144.9, 141.9, 140.8, 138.9, 138.6, 137.2, 136.7, 136.4, 135.6,
128.9, 128.2, 128.0, 23.4, 21.2, 21.0, 20.6. HRMS: calcd for
C33H37B[M]þ m/z 444.3078, found 444.2994. Yield: 28%.
4-Dimesitylboron-40-mesitylbiphenyl (2). 1H NMR (400 MHz,
CDCl3, 298.0 K, δ, ppm): 7.75 (d, J = 8.2 Hz, 2H), 7.68 (d, J =
8.2 Hz, 2H), 7.64 (d, J = 8.2 Hz, 2H), 7.25 (d, J = 8.2 Hz, 2H),
6.98 (s, 2H), 6.86 (s, 4H), 2.36 (s, 3H), 2.34 (s, 6H), 2.07 (s, 12H),
2.06 (s, 6H). 13C{1H} NMR (100 MHz, CDCl3, δ, ppm): 144.5,
144.0, 141.8, 140.8, 140.7, 138.7, 138.6, 138.5, 137.1, 136.7, 136.0,
129.8, 128.2, 128.1, 127.1, 126.4, 23.5, 21.2, 21.0, 20.8. HRMS:
calcd for C39H41B[M]þ m/z 520.3308, found 520.5031. Yield: 4%.
4-Dimesitylboron-40-mesitylterphenyl (3). 1H NMR (400 MHz,
CDCl3, 298.0 K, δ, ppm): 7.78 (s, 4H), 7.73 (d, J = 8.2 Hz,
2H), 7.69 (d, J = 8.5 Hz, 2H), 7.64 (d, J = 8.2 Hz, 2H), 7.26
(d, J = 8.2 Hz, 2H), 6.99 (s, 2H), 6.87 (s, 4H), 2.36 (s, 3H),
2.35 (s, 6H), 2.08 (s, 6H), 2.07 (s, 12H). HRMS: calcd for
C
(ca. 2%).
45H45B[M]þ m/z 596.3700, found: 596.2096. Yield: trace
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Article
4-Dimesitylboron-40-mesitylquaterphenyl (4). 1H NMR (400
Organometallics, Vol. 29, No. 18, 2010 4011
7.06 (s, 2H), 2.83 (q, J = 7.6 Hz, 2H), 2.45 (s, 3H), 2.14 (s, 6H),
1.41 (t, J = 7.6 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3, δ,
ppm): 142.2, 139.0, 138.2, 136.2, 136.0. 129.1, 128.0, 127.7, 28.5,
21.0, 20.8, 15.4. HRMS: calcd for C17H20 m/z 225.1643, found
225.1640.
MHz, CDCl3, 298.0 K, δ, ppm): 7.78 (s, 8H), 7.74 (d, J = 8.2 Hz,
2H), 7.66 (d, J = 8.2 Hz, 2H), 7.64 (d, J = 8.2 Hz, 2H), 7.27
(d, J = 8.2 Hz, 2H), 6.99 (s, 2H), 6.87 (s, 4H), 2.37 (s, 3H), 2.35
(s, 6H), 2.09 (s, 6H), 2.07 (s, 12H). HRMS: calcd for C51H49B-
[M]þ m/z 672.4014, found 672.2162. Yield: trace (ca. 1%).
Typical Suzuki-Miyaura Coupling of Dimesitylboranes.
Method 1. Dimesityl(phenyl)borane (0.6 mmol, 196 mg), bromoben-
zene (2.0 mmol, 0.21 mL), Pd(PPh3)4 (5%, 0.030 mmol, 34 mg), and
K2CO3 (3.0 mmol, 414 mg) were added to a 50 mL Schlenk flask with
stir bar and condenser. A toluene/ethanol/water mixture (v/v/v, 6
mL/3 mL/3 mL) was stirred and purged with nitrogen for 1 h, and
then the mixed solvents were transferred to the reaction flask via
cannula. The mixture was stirred at 80 °C for 16 h and then cooled to
room temperature and concentrated in vacuo. The residue was
partitioned between water and CH2Cl2, and the aqueous layer was
separated and extracted with dichloromethane (3 ꢀ 15 mL). The
combined organic layers were dried over MgSO4, concentrated,
and purified on silica gel (hexanes as eluent). The two coupling
products 2,4,6-trimethyl-1,10-biphenyl and biphenyl were identi-
fied by proton NMR.
Method 2. Aryldimesitylborane (0.6 mmol), aryl halide (2.0
mmol), Pd(PPh3)4 (5%, 0.030 mmol, 34 mg), and NaOH
(3.0 mmol, 120 mg) were added to a 50 mL Schlenk flask with
a stir bar and condenser. A toluene/ethanol/water mixture (v/
v/v, 3 mL/1.5 mL/1.5 mL) was stirred and purged with
nitrogen for 1 h, and then the mixed solvents were transferred
to the reaction flask via cannula. The mixture was stirred at
80 °C for 16 h and then cooled to room temperature and
concentrated in vacuo. The residue was partitioned between
water and CH2Cl2, and the aqueous layer was separated and
extracted with dichloromethane (3 ꢀ 15 mL). The combined
organic layers were dried over MgSO4, concentrated, and
purified on silica gel (hexanes as the eluent).
5-(p-Ethylphenyl)phenylpyridine. 5-(Dimesitylboryl)-2-phenyl-
pyridine (0.37 mmol, 150 mg), 1-bromo-4-ethylbenzene (0.77
mmol, 0.1 mL), Pd(PPh3)4 (5%, 0.02 mmol, 22 mg), and NaOH
(1.86 mmol, 74 mg) were added to a 50 mL Schlenk flask with stir
bar and condenser. A toluene/ethanol/water mixture (v/v/v,
12 mL/4 mL/4 mL) was stirred and purged with nitrogen for
1 h, and then the mixed solvents were transferred to the
reaction flask via cannula. The mixture was stirred at 80 °C
for 16 h and then cooled to room temperature and concen-
trated in vacuo. The residue was partitioned between water and
CH2Cl2, and the aqueous layer was separated and extracted
with dichloromethane (3 ꢀ 15 mL). The combined organic
layers were dried over MgSO4, concentrated, and purified
on silica gel (hexanes as the eluent). Yield: 45%. 1H NMR
(400 MHz, CDCl3, 298.0 K, δ, ppm): 8.93 (d, J = 2.2 Hz, 1H),
8.04 (d, J = 7.3 Hz, 2H), 7.94 (dd, J = 8.2 Hz, J = 2.2 Hz, 1H),
7.79 (d, J = 8.2 Hz, 1H), 7.56 (d, J = 8.0 Hz, 2H), 7.49 (t, J =
7.6 Hz, 2H), 7.42 (t, J = 7.6 Hz, 1H), 7.93 (d, J = 8.0 Hz, 2H),
2.72 (q, J = 7.6 Hz, 2H), 1.29 (t, J = 7.6 Hz, 3H). 13C{1H}
NMR (100 MHz, CDCl3, δ, ppm): 155.7, 147.8, 144.2, 139.0,
134.8, 132.4, 130.8, 128.8, 128.7, 128.6, 126.8, 126.7, 120.2,
28.5, 15.5. HRMS: calcd for C19H17N[M]þ m/z 259.1361,
found 259.1362.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council for financial support.
Supporting Information Available: Figures giving absorption
and fluorescence spectra of compounds 1-3 and CIF files giving
crystallographic data for 1-3. This material is available free of
charge via the Internet at http://pubs.acs.org.
1-Ethyl-4-mesitylbenzene. 1H NMR (400 MHz, CDCl3, 298.0
K, δ, ppm): 7.36 (d, J = 7.6 Hz, 2H), 7.18 (d, J = 7.9 Hz, 2H),