Efficient and convenient nonaqueous workup procedure for the preparation of arylboronic esters

Article abstract of DOI:10.1021/jo011073j

An efficient one-pot synthetic protocol for the synthesis of arylboronic esters has been established. The concentrated addition mixture of trimethylborate with aryl Grignard reagents was treated with low molecular weight diols (ethylene glycol, 1,3-propandiol) and toluene, the corresponding arylboronic esters were isolated in a convenient way with high yields. The diols not only serve as water replacement for the workup step, but also as well as the reagent for the preparation of arylboronic esters.

Full text of DOI:10.1021/jo011073j

J . Org. Chem. 2002, 67, 1041-1044
1041
Sch em e 1. On e-P ot Non a qu eou s Wor k u p
Syn th esis of Ar ylbor on ic Ester s
Efficien t a n d Con ven ien t Non a qu eou s
Wor k u p P r oced u r e for th e P r ep a r a tion of
Ar ylbor on ic Ester s
Ken-Tsung Wong,* Yuh-Yih Chien, Yuan-Li Liao,
Chang-Chih Lin, Meng-Yen Chou, and Man-kit Leung
taminated with boronic residue is not disposable, making
this procedure not environmentally benign for large-scale
preparation of the arylboronic acids. On the other hand,
arylboronic esters8 have been used as a substitution of
arylboronic acids to efficiently couple with aryl halides
or aryl triflates in the presence of Pd-catalyst. Tradition-
ally, arylboronic esters were prepared by a two-step
synthesis:9 from aryl halides to arylboronic acids, followed
by treating the arylboronic acids with corresponding
alcohol or diol in organic solvent. Alternatively, aryl-
boronic esters can be obtained by PdCl2(dppf)-catalyzed
boration of aryl halides or aryl triflates with dialkoxy-
borane.10 In this case, a small amount of the dehaloge-
nated product is obtained as the side product. Pd-
catalyzed borations of aryl halides or aryl triflates with
tetraalkoxydiboron11 are more successful, leading to the
formation of arylboronic esters with high yield. A broad
range of functional groups are tolerable in these Pd-
catalyzed borations makes these methodologies attrac-
tive. In addition, Rh-catalyzed aromatic C-H bond
activation for the regioselective boration with pinacol-
borane in an inert solvent also provides the corresponding
boronic ester efficiently.12 However, the dialkoxyborane
is hygroscopic and the tetraalkoxydiboron is expensive,
thus limiting these catalytic methods to be suitable only
for small-scale preparation. Therefore, an environment
friendly method for large-scale preparation of arylboronic
esters with high efficiency is highly desired. In this paper,
we report a convenient one-pot process for the prepara-
tion of arylboronic esters.
Arylboronic esters can be prepared by a one-pot
procedure as shown in Scheme 1. Aryl bromide and aryl
iodide are widely available from the major chemical
suppliers at reasonable prices. The transformation from
aryl halides to their Grignard reagents with magnesium
metal in ethereal solvents (e.g., THF, diethyl ether) has
been well documented. At -78 °C, the aryl Grignard
reagents formed were added into the THF solution
containing an excess amount (2 equiv) of trimethylborate,
i.e., B(OMe)3. The reaction mixture was allowed to return
to room temperature by removing the cooling bath,
leading to the formation of aryl dimethoxyboronate.
Instead of quenching the reaction mixture with acidic
aqueous solution, the solvent and the excess amount of
B(OMe)3 were removed by a rotary evaporator at ca. 20
Department of Chemistry, National Taiwan University,
Taipei 106, Taiwan
kenwong@ccms.ntu.edu.tw
Received November 14, 2001
Abst r a ct : An efficient one-pot synthetic protocol for the
synthesis of arylboronic esters has been established. The
concentrated addition mixture of trimethylborate with aryl
Grignard reagents was treated with low molecular weight
diols (ethylene glycol, 1,3-propandiol) and toluene, the
corresponding arylboronic esters were isolated in a conve-
nient way with high yields. The diols not only serve as water
replacement for the workup step, but also as well as the
reagent for the preparation of arylboronic esters.
Suzuki cross-coupling reactions of aryl halides and aryl
triflates with arylboronic acids have been widely used
for the construction of the aryl-aryl bond in modern
organic synthesis.1 Much of the recent endeavors on the
Suzuki coupling reaction have been concentrated on the
metal ligand system. In particular, new catalyst systems2
that facilitate the cross-coupling of the less reactive aryl
chlorides with boronic acids have been developed, greatly
expanding the scope of the reaction. Some efforts have
also been devoted to develop new organoboron reagents.
For instance, aryltrifluoroborate salts3 have been dem-
onstrated to be as effective (in some cases better) at
coupling with arenediazonium tetrafluoroborates,4 di-
aryliodonium salts,5 and aryl triflates6 in the presence
of Pd catalyst. Despite that, the most popular arylboron
derivatives used for Suzuki coupling reactions are aryl-
boronic acids, which could be prepared by reacting a
corresponding aryllithium or arylmagnesium complex
with trialkylborate, followed by an acidic aqueous work-
up.7 However, the aqueous workup procedure was some-
times troublesome and led to boronic acid only in
moderate yield. In addition, the aqueous solution con-
* To whom correspondence should be addressed. Fax: +886-2-2363-
6359.
(1) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (b)
Suzuki, A. J . Organomet. Chem. 1999, 675, 147.
(2) (a) Littke, A.; Fu, G. C. Angew. Chem., Int. Ed. Engl. 1998, 37,
3387. (b) Old, D. W.; Wolfe, J . P.; Buchwald, S. L. J . Am. Chem. Soc.
1998, 120, 972. (c) Wolfe, J . P.; Buchwald, S. L. Angew. Chem., Int.
Ed. Engl. 1999, 38, 2413. (d) Singer, R. A.; Buchwald, S. L. Tetrahedron
Lett. 1999, 40, 1095. (e) Zhang, C.; Huang, J .; Trudell, M. L.; Nolan,
S. P. J . Org. Chem. 1999, 64, 3804. (f) Littke, A. F.; Dai, C.; Fu, G. C.
J . Am. Chem. Soc. 2000, 122, 4020. (g) Zapf, A.; Beller, M. Chem. Eur.
J . 2000, 6, 1830.
(8) (a) Giroux, A.; Han, Y.; Prasit, P. Tetrahedron Lett. 1997, 38.
3841. (b) Wang, S.; Oldham, W. J .; Hudack, R. A.; Bazan, G. C. J . Am.
Chem. Soc. 2000, 122, 5695. (c) Kristensen, J .; Lysen, M. Vedso, P.
Begtrup, M. Org. Lett. 2001, 3, 1435.
(9) (a) Longstaff, C.; Rose, M. E. Org. Mass Spectrom. 1982, 17, 508.
(b) Diorazio. L. J .; Widdowson, D. A.; Clough, J . M. Tetrahedron 1992,
48, 8073.
(3) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf, M.
R. J . Org. Chem. 1995, 60, 3020.
(4) (a) Darses, S.; Geneˆt, J .-P.; Brayer, J .-L.; Demoute, J .-P.
Tetrahedron Lett. 1997, 38, 4393. (b) Darses, S.; Michaud, G.; Geneˆt,
J .-P. Eur. J . Org. Chem. 1999, 1875.
(5) Xia, M.; Chen, Z.-C. Synth. Commun. 1999, 29, 2457.
(6) Molander, G. A.; Ito, T. Org. Lett. 2001, 3, 393.
(7) Diorazio, L. J .; Widdowson, D. A.; Clough, J . M. Tetrahedron
1992, 48, 8073.
(10) (a) Murata, M.; Oyama, T.; Watanabe, S.; Masuda, Y. J . Org.
Chem. 2000, 65, 164. (b) Baudoin, O.; Gue´nard, D.; Gue´ritte, F. J . Org.
Chem. 2000, 65, 9268.
(11) (a) Ishiyama, T.; Murata, M.; Miyaura, N. J . Org. Chem. 1995,
60, 7508. (b) Ishiyama, T.; Itoh, Y.; Kitano, T.; Miyaura, N. Tetrahedron
Lett. 1997, 38. 3447.
(12) Tse, M. K.; Cho, J .-Y.; Smith, III, M. R. Org. Lett. 2001, 3, 2831.
10.1021/jo011073j CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/05/2002

1042 J . Org. Chem., Vol. 67, No. 3, 2002
Notes
Sch em e 2. On e-P ot Syn th esis of Ar ylbor on ic
Ta ble 1. Isola ted Yield of On e-P ot Non a qu eou s
Syn th esis of Ar ylbor on ic Ester s
Ester s w ith Va r iou s Diols
workup procedure. By taking advantage of the one-pot,
nonaqueous workup procedure, 2-biphenylboronic esters
were isolated in 89% yield as a colorless oil (entry 5). The
chemoselective Grignard reagent formation at the bromide-
substituted aryl carbon in the presence of a m-chloro
group provides a pathway to synthesize the bifunctional
reagent. Accordingly, 3-chlorophenylboronic ester was
isolated in 80% yield by using our new procedure (entry
6). The phenyl groups with strong electron-donating
substituents were also tested for the efficiency of this new
procedure, phenyl groups with a para-substituted diphen-
ylamino group or a methoxy group (entries 7 and 8) give
the corresponding boronic esters in lower yields, 79% for
4-OCH3-phenyl and 70% for 4-Ph2N-phenyl. Aryldibo-
ronic esters are important building blocks for the syn-
thesis of π-conjugated oligomers and polymers for the
optoelectronic uses.15 According to our new method, the
4,4′-biphenyldiboronic ester was isolated in 65% yield
(entry 9). Heteroaryl halides were also suitable for this
new method; 2-(5-phenylthienyl) boronic ester was iso-
lated in 85% yield (entry 10).
In addition to using ethylene glycol for this one-pot
protocol, 1,3-propanediol, pinacol, and diethyl L-tartrate
are also applicable for this new procedure to provide the
corresponding boronic ester (Scheme 2). The treatment
of mesitylenemagnesium bromide with trimethyl borate
followed by reaction with 1,3-propanediol gave the bo-
ronic ester in 80% yield. 1,3-Propanediol that is insoluble
in toluene acts as conveniently as ethylene glycol, provid-
ing a well-separated two-phase solution. Thus, the cor-
responding mesityleneboronic ester can be easily isolated
from the toluene solution with comparable yield. How-
Torr to give a yellow white solid. The solid was found to
be completely soluble in ethylene glycol, giving a clear
solution. The resulting ethylene glycol solution was added
with toluene and heated to reflux overnight. The toluene
solution was separated and concentrated to afford the
corresponding arylboronic ester as a single product.
According to this procedure, a 29 g of phenyl ethylene
glycol boronic ester,13 which corresponded to a 98% yield,
was isolated by reduced pressure distillation. To test the
scope and limitation of this protocol, a variety of aryl
halides with different structural feature were screened.
The isolated yields of arylboronic esters were usually good
to excellent (Table 1). 4-Methylphenylboronic ester was
isolated in 93% yield as a colorless oil (entry 2). This
method was also effective for the synthesis of bulky
arylboronic esters. For example, 2,4,6-trimethylphenyl-
boronic ester (mesitylene boronic ester) was isolated with
a yield of 85% (entry 3), while steric hindered 1-naph-
thylboronic ester was isolated in 85% yield (entry 4). It
is worthy to note that the 2-biphenylboronic acid14 was
isolated only in 51% yield by a conventional hydrolytic
(15) (a) Beley, M.; Chodorowski, S.; Collin, J .-P.; Sauvage, J .-P.
Tetrahedron Lett. 1993, 34, 2933. (b) Yu, W.-L.; Pei, J .; Huang, W.;
Heeger, A. J . Chem. Commun. 2000. 681. (c) Yu, W.-L.; Huang, W.;
Heeger, A. J . Adv. Mater. 2000, 12, 828.
(13) Kaminski, J . J .; Lyle, R. E. Org. Mass Spectrom. 1978, 13, 425.
(14) Aranyos, A.; Old, D. W.; Kiyomori, A.; Wolfe, J . P.; Sadighi, J .
P.; Buchwald, S. L. J . Am. Chem. Soc. 1999, 121, 4369.

Notes
J . Org. Chem., Vol. 67, No. 3, 2002 1043
trated to dryness by rotary evaporator distillation (20 Torr). To
the resulting solid were added ethylene glycol (30 mL) and
toluene (100 mL). The mixture was refluxed overnight, and the
toluene layer was separated and concentrated by simple distil-
lation under nitrogen. 2-Phenyl[1,3,2]dioxaborolane (29 g, 98%)
was isolated as a colorless oil by distillation under reduced
pressure: bp 53-54 °C (0.4 Torr) (lit.17 bp 57-59 °C); 1H NMR
(CDCl3, 400 MHz) 4.37 (s, 4H), 7.37-7.42 (m, 2H), 7.47-7.49
(m, 1H), 7.83 (d, J ) 8.0 Hz, 2 H); 13C NMR (CDCl3, 400 Hz)
66.0, 121.8, 131.4, 134.8.
ever, when the pinacol and diethyl L-tartrate were used
instead of ethylene glycol, the corresponding boronic
esters were isolated in 81% and 82% yield, respectively.
In these two cases, there are no well-separated two-layer
solutions after refluxing overnight due to the partial
solubility of pinacol and diethyl L-tartrate in toluene. The
isolation of the product was accomplished by directly
distilling the resulting mixture under reduced pressure.
To widen the scope of this new procedure, the method
used to generate the Grignard reagents is not only
limited to directly transform from the aryl halides with
magnesium metal, but can also be generated by Knochel’s
exchanging protocol.16 Accordingly, when 4-iodobromoben-
2-p-Tolyl[1,3,2]d ioxa bor ola n e.18 4-Bromotoluene (3.42 g, 20
mmol), Mg (0.53 g, 22 mmol), trimethyl borate (4.16 g, 40 mmol),
THF (20 mL), and ethylene glycol (10 mL), isolated as a white
soild (3.01 g, 93%): mp 59-60 °C (lit.17 mp 60-61 °C); 1H NMR
(CDCl3, 400 MHz) δ 2.39 (s, 3H), 4.38 (s, 4H), 7.21 (d, J ) 7.68
Hz, 2H), 7.72 (d, J ) 7.68 Hz, 1H); 13C NMR (CDCl3, 100 MHz)
δ 21.7, 65.9, 128.7, 134.9, 141.7; MS (EI, m/z) 162 (70); HRMS
(EI) m/z calcd for C9H11BO2162.0859, found 162.0852.
2-(2,4,6-Tr im eth ylp h en yl)[1,3,2]d ioxa bor ola n e. 1-Bromo-
2,4,6-trimethylbenzene (1.0 g, 5.2 mmol) , Mg (0.25 g, 10.4
mmol), trimethyl borate (1.08 g, 10.4 mmol), THF (10 mL),
ethylene glycol (5 mL), isolated as white solid (0.84 g, 85%): mp
108-109 °C; 1H NMR (400 MHz, CDCl3) δ 2.26 (s, 3H), 2.37 (s,
6H), 4.36 (s, 4H), 6.81 (s, 2H); 13C NMR (400 MHz, CDCl3)
δ21.20, 22.49, 65.56, 127.62, 139.35, 142.87; HRMS (FAB) calcd
i
zene in THF was treated with PrMgBr (2 equiv) at -40
°C for 3 h, the resulting solution was further reacted with
B(OMe)3 at -78 °C, followed by the new nonaqueous
workup procedure giving 4-bromophenylboronic ester in
62% (Scheme 2). The varieties of the methods to generate
the Grignard reagents and the different nature of the
diols suitable for subsequently workup procedure widely
expand the scope of this new procedure.
In summary, we have successfully established an
efficient and convenient nonaqueous procedure for the
large-scale preparation of arylboronic esters. In one pot,
aryl halides that can be transformed into their corre-
sponding Grignard reagents can easily be used for the
preparation of aryl boronic ester. The lowest molecular
weight diol, ethylene glycol, not only serves as a replace-
ment of water for the workup step but also acts as the
reagent for the synthesis of the corresponding boronic
esters in good to excellent yields. The boronic residue was
dissolved in ethylene glycol, leading to no boron-contami-
nated aqueous waste. Instead of using ethylene glycol,
other diols such as 1,3-propanediol, pinacol, and diethyl
L-tartrate are also effective for this new procedure.
for C11H15BO2190.1163, found 190.1165. Anal. Calcd for C11H15
BO2: C, 69.52; H, 7.96. Found: C, 69.44; H, 8.20.
-
2-(1-Na p h th yl)[1,3,2]d ioxa bor ola n e.19 1-Bromonaphtha-
lene (2.07 g, 10.0 mmol), Mg (0.27 g, 11.0 mmol), trimethyl borate
(2.08 g, 20.0 mmol), THF (10 mL), ethylene glycol (5 mL),
isolated as a white solid (1.68 g, 85%): mp 57-58 °C (lit.19 mp
1
146-149 °C); H NMR (CDCl3, 400 MHz) δ 4.49 (s, 4H), 7.48-
7.55 (m, 3H), 7.86 (d, J ) 8.8 Hz, 1H), 7.97 (d, J ) 8.2 Hz, 1H),
8.13 (d, J ) 5.8 Hz, 1H), 8.76 (d, J ) 8.9 Hz, 1H); 13C NMR
(CDCl3, 100 MHz) δ 65.9, 125.0, 125.6, 126.5, 128.3, 128.4, 132.0,
133.2, 136.1, 136.9; MS (EI, m/z) 198 (100); HRMS (EI) calcd
for C12H11BO2 198.0852, found 198.0865.
2-(2-Biph en yl)[1,3,2]dioxabor olan e. 2-Bromobiphenyl (2.79
g, 10.0 mmol), Mg (0.26 g, 11.0 mmol), trimethyl borate (2.08 g,
20.0 mmol), diethyl ether (10 mL), ethylene glycol (5 mL),
isolated as a colorless oil (2.01 g, 89%): 1H NMR (CDCl3, 400
MHz) δ 4.22 (s, 4H), 7.35-7.42 (m, 7H), 7.50 (td, J ) 7.43, 1.4
Hz, 3H), 7.83 (dd, J ) 7.37, 1.0 Hz, 1H); 13C NMR (acetone-d6,
100 MHz) δ 66.0, 126.9, 127.2, 127.5, 127.9, 128.4, 129.0, 219.4,
129.5, 131.0, 135.2, 140.7, 147.6; MS (EI, m/z) 224 (100); HRMS
(EI) calcd for C9H11BO3 178.0801, found 178.0809.
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r e. All reactions were
performed under argon and were magnetically stirred. Solvents
were distilled from an appropriate drying agent prior to use:
THF and Et2O from sodium and benzophenone, toluene from
sodium. Commercially available reagents were used without
further purification unless otherwise stated. Melting points were
measured on a Fargo MP-1D and are uncorrected. 1H NMR and
13C NMR were recorded using a Bruker (AC-400) or Varian (400
Unity Plus) spectrometer at 400 and 100 MHz, respectively. Low-
and high-resolution mass spectra were recorded using a J EOL
SX-102A spectrometer in EI mode. Microanalyses were carried
out on a Perkin-Elmer 240 analyzer.
2-(3-Ch lor oph en yl)[1,3,2]dioxabor olan e. 3-Chlorobromoben-
zene (3.83 g, 20.0 mmol), Mg (0.53 g, 22.0 mmol), trimethyl
borate (4.16 g, 40.0 mmol), THF (20 mL), ethylene glycol (10
mL), isolated as a colorless oil (2.92 g, 80%): bp 68-70 °C (0.5
Torr); 1H NMR (CDCl3, 400 MHz) δ 4.39 (s, 4H), 7.31 (t, J ) 7.7
Hz 1H), 7.44 (d, J ) 8.0 Hz, 1H), 7.67 (d, J ) 7.3 Hz, 1H), 7.78
(s, 1H); 13C NMR (CDCl3, 100 MHz) δ 66.2, 129.3, 131.5, 132.7,
134.1, 134.7; MS (EI, m/z) 182 (43), 184 (14); HRMS (EI) calcd
for C8H8B35ClO2 182.0306, found 182.0310; calcd for C8H8B37
ClO2 184.0276, found 184.0268.
-
2-(4-Meth oxylp h en yl)[1,3,2]d ioxa bor ola n e.20 4-Bromoani-
sole (3.74 g, 20 mmol), Mg (0.53 g, 22 mmol), trimethyl borate
(4.16 g, 40 mmol), THF (20 mL), ethylene glycol (10 mL), isolated
as a light yellow solid (2.80 g, 79%): mp 36-37 °C (lit.17 mp
50-52 °C); 1H NMR (CDCl3, 400 MHz) δ 3.84 (s, 3H), 4.36 (s,
4H), 6.92 (d, J ) 8.3 Hz, 3H), 7.77 (d, J ) 8.3 Hz, 1H); 13C NMR
(CDCl3, 100 MHz) δ 55.1, 65.9, 113.5, 136.6, 162.3; MS (EI, m/z)
178 (100); HRMS (EI) calcd for C9H11BO3 178.0801, found
178.0809.
2-(4-Dip h en yla m in op h en yl)[1,3,2]d ioxa bor ola n e. (4-Bro-
mophenyl)diphenylamine21 (1.0 g, 3.1 mmol), Mg (0.15 g, 6.2
mmol), trimethyl borate (0.65 g, 6.2 mmol), THF (10 mL),
ethylene glycol (1.6 mL), isolated as a waxy solid (0.68 g, 70%):
1H NMR (CDCl3, 400 MHz) δ 4.36 (s, 4H), 7.15-7.03 (m, 8H),
7.30-7.26 (m, 4H), 7.67 (d, J ) 8.6 Hz, 2H); 13C NMR (CDCl3,
Non a qu eou s P r ep a r a tion of Ar ylbor on ic Ester s. Gen -
er a l P r oced u r e. Arylmagnesium halide (1.0 M) prepared from
aryl halide (1.0 equiv) and magnesium turnings (1.1 equiv) in
THF was added dropwise into a solution of trimethyl borate (2.0
equiv) in THF at -78 °C. The reaction mixture was warmed to
room temperature and concentrated to dryness by rotary evapo-
ration distillation (20 Torr). The resulting solid was added
ethylene glycol and toluene. The mixture was refluxed overnight,
and the toluene layer was separated and concentrated in vacuo
to give the corresponding boronic ester.
Rep r esen ta tive P r oced u r e for th e Non a qu eou s P r ep a -
r a tion of Ar ylbor on ic Ester s: 2-P h en yl[1,3,2]d ioxa bor o-
la n e.17 Phenylmagnesium bromide prepared from bromobenzene
(21.0 mL, 200 mmol) and magnesium turning (4.86 g, 200 mmol)
in THF (200 mL) was added dropwise into a solution of trimethyl
borate (45.7 mL, 400 mmol) in THF (200 mL) at -78 °C. The
reaction mixture was warmed to room temperature and concen-
(18) Longstaff, C.; Rose, M. E. Org. Mass Spectrom. 1982, 17, 508.
(19) Ingham, R. K.; Huntsman, W. D. Chem. Abstr. 1963, 58, 4587e.
(20) Wallow, T. I.; Novak, B. M. J . Org. Chem. 1994, 59, 9; 17.
(21) Creson, S. C.; Wheeler, J .; Nelson, R. F. J . Org. Chem. 1972,
37, 4440.
(16) Boymond, L.; Rottlander, M.; Gahiez, G.; Knochel, P. Angew.
Chem., Int. Ed. Engl. 1998, 37, 1701.
(17) Kaminski, J . J .; Lyle, R. E. Org. Mass Spectrom. 1978, 13, 425.

1044 J . Org. Chem., Vol. 67, No. 3, 2002
Notes
100 MHz) δ 65.96, 121.43, 123.56, 125.22, 129.35, 135.93, 147.29;
MS (EI, m/z) 315 (100); HRMS (FAB) calcd for C20H18BNO2
315.1431, found 315.1427.
6H), 6.78 (s, 2H); 13C NMR (CDCl3, 100 MHz) δ 21.2, 22.2, 24.9,
83.4, 127.4, 138.9, 142.1; MS (EI, m/z) 246 (100); HRMS (EI m/z)
calcd for C15H23BO2 246.1791, found 246.1794.
Bip h en yl-4,4′-eth ylen e Glycol Dibor on ic Ester .22 4,4′-
Dibromobiphenyl (3.12 g, 10.0 mmol), Mg (0.53 g, 22.0 mmol),
trimethyl borate (4.16 g, 40.0 mmol), THF (20 mL), ethylene
glycol (5 mL), isolated as a pale yellow solid (1.91 g, 65%): mp
288-289 °C; 1H NMR (CDCl3, 400 MHz) δ 4.41 (s, 8H), 7.66 (d,
J ) 4.9 Hz, 4H), 7.90 (d, J ) 4.9 Hz, 4H); 13C NMR (CDCl3, 100
MHz) δ 66.0, 126.6, 135.3, 143.7; MS (EI, m/z) 294(100); HRMS
(EI) calcd for C16H16B2O4 294.1235, found 294.1240.
2-(5-P h en ylth ien yl)eth ylen e Glycol Bor on ic Ester . 2-Bro-
mo-5-phenylthiophene23 (1.2 g, 5.0 mmol), Mg (0.132 g, 5.5
mmol), trimethyl borate (1.04 g, 10.0 mmol), THF (10 mL),
ethylene glycol (3 mL), isolated as a white solid (0.98 g, 85%):
mp 111-112 °C; 1H NMR (CDCl3, 400 MHz) δ 4.40 (s, 4H), 7.26-
7.33 (m, 1H), 7.38-7.42 (m, 3H), 7.66 (d, J ) 3.6 Hz, 1H), 7.67
(d, J ) 6.0 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ 66.1, 124.5,
126.2, 128.0, 128.9, 134.0, 138.5, 151.7; MS (EI, m/z) 230 (100);
HRMS (EI) calcd for C12H11BO2S 230.0573, found 230.0565.
2-(2,4,6-Tr im eth ylp h en yl)[1,3,2]d ioxa bor in a n e. 1-Bromo-
2,4,6-trimethylbenzene (3.98 g, 20.0 mmol), Mg (0.53 g, 22
mmol), trimethyl borate (4.16 g, 40 mmol), THF (20 mL), 1,3-
propanediol (5 mL), isolated as a white solid (3.2 g, 80%): mp
48-49 °C; 1H NMR (CDCl3, 400 MHz) δ 2.10 (qn, J ) 5.5 Hz,
2H), 2.24 (s, 3H), 2.33 (s, 6H), 4.17 (t, J ) 5.5 Hz, 4H), 6.77 (s,
2H); 13C NMR (CDCl3, 100 MHz) δ 21.2, 21.9, 27.7, 62.0, 127.1,
137.9, 140.3; MS (EI, m/z) 204 (100); HRMS (EI m/z) calcd for
C12H17BO2 204.1322, found 204.1322.
Mesityl Dieth yl L-Ta r tr a te Bor on a te Ester . 2-Bromo-
2,4,6-trimethylbenzene (3.98 g, 20.0 mmol), Mg (0.53 g, 22
mmol), trimethyl borate (4.16 g, 40 mmol), THF (20 mL), diethyl
L-tartrate (10 mL), isolated as a colorless oil (5.47 g, 82%): 1H
NMR (CDCl3) δ 1.35 (t, J ) 2.76 Hz, 6H), 2.29 (s, 3H), 2.47 (s,
6H), 4.34 (q, J ) 2.76 Hz, 4H), 5.04 (s, 2H), 6.85 (s, 2H); 13C
NMR (CDCl3, 100 MHz) δ 14.1, 21.3, 22.4, 62.2, 127.8, 169.7,
140.2, 143.7; MS (EI, m/z) 334 (100); HRMS (EI m/z) calcd for
C17H23BO6 334.1588, found 334.1580.
2-(4-Br om op h en yl)[1,3,2]d ioxa bor ola n e.17 2-Propylmag-
nesium bromide prepared from 2-bromopropane (0.94 mL, 10
mmol) and Mg (0.243 g, 10 mmol) in THF (10 mL) was added
dropwise into a solution of 4-iodobromobenzene (1.41 g, 5.0
mmol) in THF (10 mL) at -40 °C. The mixture was stirred for
3 h and cooled to -78 °C. Trimethyl borate (1.15 mL, 10 mmol)
was added in one portion at -78 °C. After the mixture was
warmed to room temperature and following the general workup
procedure, the desired product was isolated as a white solid (0.7
g, 62%): mp 96-97 °C (lit.17 mp 97-99 °C); 1H NMR (CDCl3,
400 MHz) δ 4.37 (s, 4H), 7.54 (d, J ) 8.31 Hz, 2H), 7.67 (d, J )
8.31 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ 66.1, 126.5, 131.1,
136.3; MS (EI, m/z) 225 (100); HRMS (EI) calcd for C8H8B-
79BrO2 225.9801, found 225.9799; Calcd for C8H8B81BrO2 227.9780,
found 227.9784.
Ack n ow led gm en t. We thank the National Science
Council and Ministry of Education of Taiwan for the
financial support. We also thank Miss Sun Shu-Yun for
the HRMS analysis.
Mesitylp in a col Bor on a te Ester . 2-Bromo-2,4,6-trimethyl-
benzene (3.98 g, 20.0 mmol), Mg (0.53 g, 22 mmol), trimethyl
borate (4.16 g, 40 mmol), THF (20 mL), pinacol (7.08 g), isolated
as a colorless oil (4.0 g, 81%): bp 125 °C bulb-to-bulb (0.1 Torr);
1H NMR (CDCl3, 400 MHz) δ 1.38 (s, 12H), 2.25 (s, 3H), 2.38 (s,
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra (PDF) for all arylboronic esters. This material is
available free of charge via the Internet at http://pubs.acs.org.
(22) Beley, M.; Chodorowski, S.; Collin, J .-P.; Sauvage, J .-P. Tetra-
hedron Lett. 1993, 34, 2933.
(23) Hotta, S.; Lee, S. A.; Tamaki, T. J . Heterocycl. Chem. 2000, 37,
25.
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