Phosphane-free suzukimiyaura coupling of aryl imidazolesulfonates with arylboronic acids and potassium aryltrifluoroborates under aqueous conditions#

Article abstract of DOI:10.1246/cl.2011.907

Aryl imidazole-1-sulfonates are efficiently cross-coupled with arylboronic acids and potassium aryltrifluoroborates using only 0.5 mol% of oxime palladacycles 1 under aqueous conditions at 110 °C. Under these simple phosphane-free reaction conditions a wide array of biaryl derivatives has been prepared in high yields. This methodology allows in situ phenol sulfonation and one-pot Suzuki arylation as well as the employment of microwave irradiation conditions.

Full text of DOI:10.1246/cl.2011.907

doi:10.1246/cl.2011.907
Published on the web September 5, 2011
907
Phosphane-free Suzuki-Miyaura Coupling of Aryl Imidazolesulfonates with Arylboronic Acids
and Potassium Aryltrifluoroborates under Aqueous Conditions#
José Francisco Cívicos, Mohammad Gholinejad, Diego A. Alonso,* and Carmen Nájera*
Departamento de Química Orgánica and Instituto de Síntesis Orgánica, Universidad de Alicante,
Apdo. 99, E-03080, Alicante, Spain
(Received March 8, 2011; CL-110199; E-mail: diego.alonso@ua.es, cnajera@ua.es)
Aryl imidazole-1-sulfonates are efficiently cross-coupled
with arylboronic acids and potassium aryltrifluoroborates using
only 0.5 mol % of oxime palladacycles 1 under aqueous
conditions at 110 °C. Under these simple phosphane-free
reaction conditions a wide array of biaryl derivatives has been
prepared in high yields. This methodology allows in situ phenol
sulfonation and one-pot Suzuki arylation as well as the
employment of microwave irradiation conditions.
and potassium trifluoroborates catalyzed by oxime palladacycles
1 under aqueous conditions.
Initial Suzuki cross-coupling studies were carried out
over 1-naphthol derivatives 2 (Table 1). The cross-coupling of
compounds 2 (1 equiv) with phenylboronic acid (1.5 equiv) was
initially performed using catalyst 1a16 (1 mol % Pd) and KOH
(2 equiv) as base in a mixture MeOH/H2O: 3/1 as solvent in the
presence of tetrabutylammonium bromide (TBAB) (20 mol %)
as phase transfer agent at 110 °C (Table 1, Entries 1-5). As
depicted in Table 1, only the imidazolesulfonate derivative 2ae
showed reactivity in the process affording 1-phenylnaphthalene
(3a) in a 67% isolated yield.
The Suzuki-Miyaura reaction constitutes one of the most
powerful carbon-carbon bond-forming transformations.1 Due to
the broad existence of O-containing organic compounds in
nature there has been a tremendous interest on the transition-
metal-catalyzed cross-coupling reactions of this type of com-
pounds.2 The palladium-catalyzed Suzuki-Miyaura cross-cou-
pling reaction of enols, phenols, and hydroxylated arenes
usually involves their transformation into triflates due to the
superior performance of these derivatives as electrophilic
partners.3 However, triflates are substrates with limited stability.
Thus, different studies have shown the ability of other func-
tional groups, such as mesylates,4 tosylates,4d,5 carbamates,6
carbonates,6a carboxylates,4b,7 ethers,8 phosphonium salts,9
phosphoramides,10 and N,N-dialkyl O-sulfamates,6a,11 to per-
form in the Suzuki reaction usually under palladium or nickel
catalysis in the presence of phosphane ligands. Very recently,
aryl imidazolesulfonates have been demonstrated as efficient
electrophilic coupling partners in the Suzuki reaction with aryl
boronic acids employing bidentate phosphane ligands, such
as dppf[1,1¤-bis(diphenylphosphano)ferrocene] and BINAP,
under high catalyst loadings (5-10 mol % Pd).11-13
Once demonstrated that the imidazole-1-sulfonate was the
best electrophile in the process, an optimization of the reaction
Table 1. Reaction conditions studya
Pd catalyst
(1 mol% Pd)
+ PhBX
TBAB, KOH
110 °C, 1d
OR
Ph
2aa, R = CO2t-Bu
2ab, R = CONMe2
3a
2ac, R = P(O)(2-oxooxazolidin-3-yl)
2ad, R = SO2NMe2
2ae, R = SO2(imidazol-1-yl)
Entry
2
BX
Catalyst
Solventb Yield/%c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2aa
B(OH)2 1a
A
A
A
A
A
B
H2O
A
A
A
A
A
A
A
A
<5
<5
<5
<5
67
<5
<5
84
2ab B(OH)2 1a
2ac B(OH)2 1a
2ad B(OH)2 1a
In our research group, the Suzuki-Miyaura reaction has
been studied during the past decade using oxime palladacycles 1
(Figure 1) as efficient precatalysts for the coupling of vinyl- and
aryl halides with boronic acids and potassium trifluoroborates in
organic and aqueous solvents.14,15 Herein we report the use of
imidazolesulfonates as effective electrophilic coupling reagents
in a phosphane-free Suzuki-Miyaura reaction with boronic acids
2ae
2ae
2ae
2ae
2ae
2ae
2ae
2ae
2ae
2ae
2ae
B(OH)2 1a
B(OH)2 1a
B(OH)2 1a
B(OH)2 Pd(OAc)2
B(OH)2 1b
B(OH)2 1b
>99
>99d
89
BF3K
BF3K
BF3K
BXe
1a
1b
Pd(OAc)2
1a
1b
45
R1
10
50f
39f
BXg
N
Pd
OH
)2
aReaction conditions: Pd catalyst (1 mol % Pd), 2 (0.1 mmol),
PhBX (0.15 mmol), TBAB (20 mol %), KOH (0.2 mmol) under
R2
Cl
b
A or B solvent conditions. A: MeOH/H2O: 3/1; B: Me2CO/
1a, R1
1b, R1
= Me, R2 = OH
= 4-ClC6H4, R2 = Cl
c
d
H2O: 3/2. Isolated yield after flash chromatography. Reac-
tion performed under microwave irradiation conditions (40 W,
e
f
110 °C, 30 min). BX = B[(OCOCH2)2NMe]. Reaction time:
2 days. BX = B[(OC(Me)2)]2.
g
Figure 1. Oxime palladacycles.
Chem. Lett. 2011, 40, 907-909
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

908
Table 2. Substrate scope
1 (1 mol% Pd),
TBAB, KOH
MeOH/H2O: 3/1
110 °C, 24 h
Ar1OSO2
N
Ar2BX
Ar1-Ar2
+
N
3
Product
Yield/%a
Entry
Ar1OSO2Im
No.
Ar2BX
Palladacycle
No.
3b
1
2
3
4
5
6
7
2ae
2ae
4-MeC6H4B(OH)2
4-MeC6H4B(OH)2
4-MeC6H4BF3K
4-MeC6H4BF3K
4-MeOC6H4B(OH)2
4-CF3C6H4B(OH)2
4-CF3C6H4BF3K
1a
1b
1a
90
76
61
72
75
66
64
3b
OSO2Im
OSO2Im
2ae
2ae
2ae
2ae
3b
3b
3c
3d
1b
1b
1b
2ae
1b
3d
2be
4-MeC6H4B(OH)2
PhB(OH)2
1b
3e
86
90
8
9
OSO2Im
2ce
2de
1b
1b
3e
3f
Me
Me
OSO2Im
OSO2Im
10
PhB(OH)2
85
Me
11
12
2ee
2fe
PhB(OH)2
PhB(OH)2
1b
1b
3g
3h
81
78
F3C
OSO2Im
CF3
Me
OSO2Im
2ge
1b
3i
13
PhB(OH)2
53
Me
aIsolated yield after flash chromatography.
conditions was carried out. No reaction took place when the
coupling was performed in aqueous acetone or in neat water
(Table 1, Entries 6 and 7). Regarding catalysts, Pd(OAc)2
afforded a higher (84%) yield in the reaction (Table 1, compare
Entries 5 and 8). To our surprise, the yield improved
substantially (99%) when palladacycle 1b, catalyst which
usually performs better in organic solvents,15 was used (Table 1,
Entry 9). In the presence of catalyst 1b the reaction could also
be performed under microwave irradiation conditions (40 W,
110 °C), affording compound 3a in quantitative yield after only
30 min (Table 1, Entry 10). The optimized results obtained
with different boron electrophiles are shown in Entries 11-15.
Potassium phenyltrifluoroborate afforded compound 3a in a
good 89% isolated yield using catalyst 1a (Table 1, Entry 11).
With this nucleophile, catalyst 1b and Pd(OAc)2 were much less
reactive than 1a affording compound 3a in a poor 45 and 10%
yield, respectively (Table 1, Entries 12 and 13). Low yields were
also obtained after two days when imidazolesulfonate 2ae
reacted with phenylboronic acid MIDA ester (50%, Table 1,
Entry 14) and phenylboronic acid pinacol ester (39%, Table 1,
Entry 15).
The scope of this methodology was then examined under the
optimized reaction conditions. Cross-coupling of naphthalen-
1-yl 1H-imidazole-1-sulfonate (2ae) with diverse arylboronic
acids and potassium trifluoroborates, afforded the corresponding
biaryl derivatives 3b-3d in good to excellent yields (Table 2,
Entries 1-7). Generally better yields were observed for boronic
acids than for potassium trifluoroborates being the lowest yields,
Chem. Lett. 2011, 40, 907-909
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

909
OH
Ar
References and Notes
1. Im2SO2, Cs2CO3, THF
1b
#
1
Dedicated to Prof. Alfredo Ricci on the occasion of his retirement.
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2.
(1 mol% Pd), TBAB
ArB(OH)2, KOH,
MeOH/H2O, 110 °C,
24 h
3a, Ar = Ph, 71%
3b, Ar = 4-MeC6H4, 65%
Scheme 1. One-pot sulfonation/cross-coupling sequence.
2
For recent reviews on the use of new C-O electrophiles in cross-
coupling reactions, see: a) D.-G. Yu, B.-J. Li, Z.-J. Shi, Acc. Chem.
Res. 2010, 43, 1486. b) B.-J. Li, D.-G. Yu, C.-L. Sun, Z.-J. Shi,
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those obtained when electron-deficient nucleophiles were used
(Table 2, Entries 6 and 7). Also, catalyst 1b performed usually
better than 1a, though no clear correlation between the catalyst
and the boron source could be established from parallel
experiments. Thus, further studies on the reaction scope and
utility of the method were always optimized with respect to the
catalyst, the best results being shown in Table 2. Regarding the
electrophilic component (Table 2, Entries 8-13), high isolated
yields (81-90%) were obtained in the 1b-catalyzed cross-
coupling of arylboronic acids with neutral, electron-rich, and
electron-poor phenyl imidazolesulfonates (Table 2, Entries 8-
11). The process was also effective for the coupling of sterically
hindered electrophiles such as 2fe and 2ge (Table 2, Entries 12
and 13), though a moderate 53% isolated yield was observed for
the more sterically demanding derivative 2ge.
As the synthesis of imidazolesulfonates typically proceeds
quantitatively and with minimal by-product formation, we
carried out a one-pot sulfonation/Suzuki cross-coupling se-
quence of 1-naphthol with phenyl- and 4-tolylboronic acids. As
depicted in Scheme 1, the one-pot conversion of 1-naphthol
to biaryl adducts 3a and 3b was performed by reaction of
1-naphthol with 1,1¤-sulfonyldiimidazole in THF using cesium
carbonate as base and subsequent 1b-catalyzed Suzuki cross-
coupling of the in situ generated imidazole-1-sulfonate with the
corresponding arylboronic acid, to afford the desired products in
a 71% and 65% isolated yield, respectively.
In conclusion, we have disclosed a general phosphane-free
oxime-palladacycle-catalyzed Suzuki cross-coupling reaction of
electron-rich, electron-poor, and hindered aryl imidazole-
sulfonates with arylboronic acids and potassium aryltrifluoro-
borates under aqueous conditions. The reaction, which can be
performed under conventional heating or microwave irradi-
ation, has been optimized for use with only 1 mol % Pd of
bench stable palladacycles 1. Furthermore, this methodology
allows in situ phenol sulfonation and one-pot Suzuki arylation.
Currently, further studies are underway addressing extension of
this methodology to other palladium-catalyzed transforma-
tions.17
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Financial support from the MICINN (Projects CTQ2007-
62771/BQU, CTQ2010-20387, and Consolider INGENIO 2010
CSD2007-00006), FEDER, the Generalitat Valenciana (Projects
GV/2007/142 and PROMETEO/2009/038), and the University
of Alicante is acknowledged. M. G. thanks Shiraz University
and University of Alicante for grants.
14 Palladacycles: Synthesis, Characterization and Applications, ed.
by J. Dupont, M. Pfeffer, Wiley-VCH, Weinheim, 2008.
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precatalysts in cross-coupling reactions, see: a) E. Alacid, D. A.
Alonso, L. Botella, C. Nájera, M. C. Pacheco, Chem. Rec. 2006, 6,
117. b) D. A. Alonso, C. Nájera, Chem. Soc. Rev. 2010, 39, 2891.
16 Catalyst 1a has shown to be very effective for cross-coupling and
Heck reactions under aqueous conditions. See ref. 15.
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
17 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2011, 40, 907-909
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/