Microwave-promoted suzuki-miyaura cross-coupling of aryl imidazolylsulfonates in water

Article abstract of DOI:10.1002/adsc.201200364

Aryl imidazol-1-ylsulfonates are efficiently cross-coupled with potassium aryl- and alkenyltrifluoroborates in neat water under microwave heating (40 W, 110°C) using 0.5 mol% of oxime palladacycle 1a, hexadecyltrimethyl ammonium bromide (CTAB) as additive, and triethylamine as base. Under these simple phosphane-free reaction conditions a wide array of biaryl, stilbene and styrene derivatives has been prepared in good to high yields and with high regio- and diastereoselectivities in only 30 min.

Full text of DOI:10.1002/adsc.201200364

FULL PAPERS
DOI: 10.1002/adsc.201200364
Microwave-Promoted Suzuki–Miyaura Cross-Coupling of Aryl
Imidazolylsulfonates in Water
Josꢀ F. Cꢁvicos,a Diego A. Alonso,a,* and Carmen Nꢂjeraa,*
a
Departamento de Quꢁmica Orgꢂnica and Instituto de Sꢁntesis Orgꢂnica (ISO), Facultad de Ciencias, Universidad de
Alicante, Apartado 99, 03080 Alicante, Spain
Fax : (+34)-96-590-3549; e-mail: diego.alonso@ua.es or cnajera@ua.es
Received: April 26, 2012; Revised: June 14, 2012; Published online: September 28, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200364.
Abstract: Aryl imidazol-1-ylsulfonates are efficiently array of biaryl, stilbene and styrene derivatives has
cross-coupled with potassium aryl- and alkenyltri- been prepared in good to high yields and with high
fluoroborates in neat water under microwave heating regio- and diastereoselectivities in only 30 min.
(40 W, 1108C) using 0.5 mol% of oxime palladacycle
1a, hexadecyltrimethyl ammonium bromide (CTAB) Keywords: microwave chemistry; palladacycles; po-
as additive, and triethylamine as base. Under these tassium organotrifluoroborates; Suzuki–Miyaura re-
simple phosphane-free reaction conditions a wide action; water
Introduction
tosylates, these electrophiles are typically unreactive
towards Pd catalysis, with phosphane-derived nickel
In the past several decades, the Suzuki–Miyaura complexes, such as NiCl2ACTHNUGTRENNUG(PCy3)2 being the most active
cross-coupling has become one of the most useful catalysts, usually working in organic or aqueous sol-
tools to construct carbon-carbon bonds in both labo- vents (Scheme 1). Very recently, aryl imidazolylsulfo-
ratory and industry.[1] Although aryl and alkenyl hal- nates have been demonstrated as efficient electrophil-
ides are most commonly employed as the electrophilic ic coupling partners in the Suzuki reaction with aryl-
partner in the Suzuki reaction, much effort has been boronic acids employing bidentate phosphane ligands,
directed to search for alternative electrophiles. such as 1,1’-bis(diphenylphosphino)ferrocene (dppf)
Oxygen-based electrophiles are particularly attractive and
partners in cross-coupling reactions due to their high
stability and the ubiquitous presence of the O-based
starting materials both in nature and in synthetic sys-
tems. Phenolic derivatives offer a valuable alternative
given that phenols are typically inexpensive and read-
ily available materials.[2,3] The Suzuki–Miyaura cross-
coupling reaction of enols, phenols, and hydroxylated
arenes usually involves their transformation into tri-
flates due to the superior performance of these deriv-
atives as electrophilic partners.[4] However, triflates
are substrates with limited stability, so recent studies
have focused on the development of less common
phenol-based electrophiles,[5] such as mesylates,[6] to-
sylates,[6e,7] ethers,[8] esters,[6c,9] carbamates,[10] carbon-
2,2’-bis(diphenylphosphino)-1,1’-binaphthalene
ACHTUNGTRENNUNG
ates,[10b] phosphonium salts,[11] phosphoramides and
phosphates,[12] N,N-dialkyl O-sulfamates,[6f,10b,f,13] and
borates[14] since these substrates are usually more
robust and allow functionalization by regioselective
À
C H activation and directed ortho metalation proces-
ses.[10a,13a,15] With the exception of aryl mesylates and Scheme 1. Suzuki coupling of phenol-derived electrophiles.
Adv. Synth. Catal. 2012, 354, 2771 – 2776
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Josꢀ F. Cꢁvicos et al.
FULL PAPERS
(BINAP), under high catalyst loadings (5–10 mol% ylation of 2aa in the presence of TEA as base using
Pd).[16,17]
different boron nucleophiles 3 and surfactants 4
In our group, we have disclosed a phosphane-free (Table 1). As shown in entries 1–4 for the best results,
oxime-palladacycle[18,19]-catalyzed Suzuki reaction of the Suzuki cross-coupling of the imidazolylsulfonate
aryl imidazolylsulfonates with aryl- and alkenylboron- 2aa afforded compound 5a in good to excellent yields
ic acids and potassium trifluoroborates in aqueous (53–91%) irrespective of the electrophile used, potas-
methanol, a reaction which can be performed using sium phenyltrifluoroborate and CTAB (4e) being the
conventional or microwave heating, under low load- match combination (Table 1, entry 4). The cross-cou-
ing conditions (1 mol% Pd).[20] In this work, the lack pling reaction was not so effective when it was carried
of efficiency in the Suzuki coupling under aqueous out with conventional heating at 1108C for 24 h,
À
conditions of other C O electrophiles, such as carbo- yielding compound 5a in a 38% isolated yield
nates, carbamates, phosphates, and N,N-dialkylsulf- (entry 6).
A
Regarding the catalyst, a 66% yield of compound
Despite the significant advances in the pool of 5a was obtained under the optimized reaction condi-
available electrophiles and their Suzuki cross-cou- tions when oxime palladacycle 1b (1 mol% Pd) was
pling, much effort remains to generalize the transfor- employed (Table 1, entry 7). Other Pd sources such as
mation. To date, no examples of Pd- or Ni-catalyzed Pd
(OAc)2 and Pd
E
À
C O activation in water have been reported shown in entries 8–10. Phosphane-derived nickel com-
(Scheme 1).[21] In this work, we present the first plexes, such as NiCl
Suzuki–Miyaura coupling of aryl imidazolylsulfamates
with potassium aryl- and alkenyltrifluoroborates using
oxime palladacycle 1a as precatalyst in water under
microwave irradiation and phosphane-free conditions.
Table 1. Suzuki coupling of 2 in water. Reaction conditions
study.
Results and Discussion
Initially, we investigated the microwave-promoted
Suzuki coupling of naphthalen-1-yl 1H-imidazole-1-
sulfonate (2aa, 1 equiv.) with phenylboronic acid
(1.5 equiv.) catalyzed by palladacycle 1a (1 mol% Pd)
at 1108C in water. A first base optimization study,[22]
which was performed in the presence of the surfactant
polyoxyethanyl-a-tocopheryl sebacate (PTS) (4a,
15% w/w),[23] showed triethylamine (TEA) as the
most effective base, affording 5a in a 56% isolated
yield (Scheme 2). Under these conditions, a subse-
quent surfactant study[22] using other non-ionic [poly-
oxyethylene lauryl ether (Brij 35, 4b), 4-(1,1,3,3-tetra-
methylbutyl)phenylpolyethylene glycol (Triton X100,
4c), and macrogol (25)-cetostearyl ether (Cremophor
A25, 4d)], as well as ionic [hexadecyltrimethylammo-
nium bromide (CTAB, 4e) and tetrabutylammonium
bromide (TBAB, 4f)] additives, did not improve this
result, affording 5a in much lower yields than for 4a
(7–39%).[22]
Entry
2
Xn
Pd catalyst
4
Yield [%][a]
1
2
3
4
5
6
7
8
2aa (OH)2
2aa [OC(Me2)]2
2aa
1a
1a
4a 56
4e 76
4c 53
4e 91
4e 39
4e 38[b]
4e 66
ACHTUNGTRENNUNG
(OCOCH2)2NMe 1a
2aa F3K
2aa (OH)2
2aa F3K
2aa F3K
2aa F3K
2aa F3K
2aa F3K
2aa F3K
2ab F3K
2ac F3K
2ad F3K
2ae F3K
2af F3K
1a
1a
1a
1b
Pd
Pd
Pd
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
9
10
11
12
13
14
15
16
2A
4e <5
4e <5
4e <5
4e <5
4e <5
4e <5
4e <5
[d]
–
Next, an exhaustive nucleophile/surfactant screen-
1a
1a
1a
1a
1a
ing[22] was carried out for the microwave-promoted ar-
[a]
Isolated yield after flash chromatography.
Reaction performed with conventional heating at 1108C
for 24 h.
Reaction performed in the presence of [HP
(2 mol%).
Reaction performed using 1 mol% of NiCl2ACTHUNGTRENNU(G PCy3)2.
[b]
[c]
[d]
ACHTUNGRTEN(NUNG t-Bu)3]BF4
Scheme 2. Suzuki coupling of 2aa with PhB(OH)2 in water.
Reaction conditions study
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Microwave-Promoted Suzuki–Miyaura Cross-Coupling of Aryl Imidazolylsulfonates in Water
Table 2. Suzuki arylation and alkenylation of 2aa.[a]
mitted to the coupling with naphthalen-1-yl 1H-imida-
zole-1-sulfonate under the optimized reaction condi-
tions. As shown in Table 2, entries 4 and 5, these com-
pounds afforded the corresponding adducts 5d and 5e
in 55 and 60% isolated yields, respectively. Good
yields were also observed for the coupling of 2aa with
potassium trans-2-phenylvinylboronic acid (entry 6)
and
potassium
trans-1-decen-1-yltrifluoroborate
(entry 7), reactions that afforded compounds 5f and
5g in 84 and 62% yield, respectively. These reactions
were highly regioselective, and no ipso coupling was
observed, a process often found in Pd-catalyzed alke-
nylations of aryl chlorides.[26]
We next extended the substrate scope of the reac-
tion to the arylation and alkenylation of other differ-
ent aryl imidazolylsulfonates (Table 3). Concerning
arylation, electron-rich, electron-poor, sterically hin-
dered, and heterocyclic electrophiles showed good re-
activity
when
coupling
with
potassium
phenyltrifluoroAHCTUNGTRbEUNNNG orate, affording the corresponding bi-
phenyl derivatives in good to excellent yields
(Table 2, entries 1–4). ortho-Cyano-substituted elec-
Table 3. Substrate scope of the Suzuki reaction.
[a]
[b]
Reaction conditions:
A
mixture of 2aa (0.055 g,
0.2 mmol), RBF3K (0.3 mmol), Et3N (0.056 mL,
0.4 mmol), 4e (0.030 g, 15% w/w in water), 1a (0.0006 g,
1 mol% Pd) in H2O (1.7 mL) was heated in air at 1108C
for 30 min with the aid of an initial 40 W irradiation.
Isolated yield after flash chromatography.
as the most effective catalysts to carry out the Suzuki
coupling of phenol derivatives in organic solvents.[6c]
However, the coupling between imidazolylsulfonate
2aa and potassium phenyltrifluoroborate catalyzed by
NiCl2ACHTUNGTRENNUNG(PCy3)2, led to a negligible reaction conversion
(Table 1, entry 11). Finally, we also tested the reactivi-
ty of the electrophiles 2ab, 2ac, 2ad, 2ae, and 2af in
the process under the optimized reaction conditions.
As depicted in entries 12–16, none of these deriva-
tives showed any reactivity and only starting material
was recovered from the crude reaction mixture.[24,25]
To test the effectiveness of the catalytic system in
the Pd-catalyzed Suzuki reaction in water, a range of
potassium aryl- and alkenyltrifluoroborates was exam-
ined in the coupling with imidazolylsulfonate 2aa
under the optimized reaction conditions (Table 2).
Regarding aryltrifluoroborates, both activated and
deactivated nucleophiles afforded the corresponding
cross-coupled products in good yields as demonstrat-
ed with the coupling of potassium 4-tolyltrifluorobo-
rate and potassium 4-(trifluoromethyl)phenyltrifluoro-
ACHTUNGTRENNUNGborate (Table 2, entries 2 and 3). Heterocyclic nucleo-
philes such as, potassium pyrid-3-yltrifluoroborate
and potassium thien-3-yltrifluoroborate were also sub-
[a]
Isolated yield after flash chromatography.
80 W, 1508C, 1 h.
[b]
Adv. Synth. Catal. 2012, 354, 2771 – 2776
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Josꢀ F. Cꢁvicos et al.
FULL PAPERS
vent under microwave irradiation. This reaction is car-
ried out in the presence of hexadecyltrimethylammo-
nium bromide as additive and is catalyzed by only
1 mol% Pd of bench stable oxime palladacycle 1a.
Further studies to demonstrate the ability of imidazo-
lylsulfonates as electrophiles in cross-coupling reac-
tions in water are underway in the group.
Experimental Section
Typical Procedure for the Suzuki Coupling in Water
under MW Irradiation Conditions
A 10-mL MW vessel was charged with 1-naphthalen-1-yl-
1H-imidazole-1-sulfonate (2aa) (0.055 g, 0.2 mmol, 1 equiv.),
potassium 4-(trifluoromethyl)phenyltrifluoroborate (0.079 g,
0.3 mmol), Et3N (0.056 mL, 0.4 mmol), CTAB (0.030 g, 15%
w/w in water), catalyst 1a (0.6 mg, 1 mol% Pd), and H2O
(1.7 mL). The vessel was sealed with a pressure lock, and
the mixture was heated in air at 1108C for 30 min with the
aid of an initial MW irradiation of 40 W in a CEM Discover
MW reactor. Then, after cooling to room temperature, the
reaction mixture was extracted with EtOAc (3ꢄ10 mL), and
the organic layers were washed with H2O (3ꢄ10 mL), dried
over MgSO4, and concentrated under reduced pressure. The
crude residue was purified by flash chromatography
(hexane/EtOAc: 4/1) to afford 5c; yield: 0.033 g (68%);
white solid; mp 48–498C (hexane); Rf 0.34 (hexane). IR
(KBr): n=3068, 3045, 1616, 1404, 1323, 1166, 1123, 1069,
Scheme 3. Suzuki coupling of bromo-containing imidazolyl-
sulfonate 2bg in water.
trophile 2be reacted very efficiently with potassium p-
toluenetrifluoroborate, producing 2-cyano-4’-methyl-
biphenyl (5l), a key intermediate in the synthesis of
angiotensin II receptor antagonists that are used for
the treatment of hypertension,[27] with an 86% yield
(Table 3, entry 5).
With respect to alkenylations, slightly lower yields
were obtained in the coupling reaction of different
imidazolylsulfonates 2 with potassium trans-2-phenyl-
vinyltrifluoroborate and potassium trans-1-decen-1-yl-
trifluoroborate, which regioselectively led to stilbene
derivatives 5m–5p in 55 to 84% isolated yields
(Table 3, entries 6–9).
1
1019, 850, 804, 700 cmÀ1; H NMR (CDCl3, 400 MHz): dH =
8.09–7.99 (m, 3H), 7.90 (d, J=8.4 Hz, 2H), 7.72 (d, J=
8.4 Hz, 2H), 7.69–7.57 (m, 3H), 7.54 (d, J=8.4 Hz, 1H);
13C NMR (CDCl3, 100 MHz): dC =144.9, 138.7, 133.8, 131.3,
130.4, 129.5 (q, J=32.3 Hz), 128.44, 128.39, 127.0, 126.4,
126.0, 125.5, 125.3, 125.2 (q, J=3.3 Hz), 124.4 (q, J=
270.5 Hz); MS: m/z (%)=273 (M+ +1, 18), 272 (M+, 100),
271 (M+-1, 23), 251 (15), 203 (50), 202 (54).
Finally, concerning the Suzuki coupling of halogen-
À
ated imidazolylsulfonates, the C Cl bond did not par-
ticipate in the coupling event as demonstrated in the
arylation of p-chlorophenyl imidazolylsulfonate (2bc)
with potassium phenyltrifluoroborate, a reaction that
afforded 4-chlorobiphenyl (5j) in
a
74% yield
À
(Table 3, entry 3). However, the weaker C Br bond
did react in the process, and the reaction of electro-
phile 2bg (1 equiv.) with potassium phenyltrifluorobo-
rate (1.5 equiv.)[28] afforded as a major product
Acknowledgements
Financial support from the MICINN (Projects CTQ2007-
1,1’:4’,1’’-terphenyl (6) in 55% yield, together with 62771/BQU, CTQ2010–20387), and Consolider INGENIO
2010 (CSD2007-00006), FEDER, the Generalitat Valenciana
(PROMETEO/2009/038), and the University of Alicante is
acknowledged.
a 28% yield of 1,1’-biphenyl-4-yl 1H-imidazole-1-sul-
fonate (2bh) (Scheme 3). On the other hand, only
(E)-4-styrylphenyl 1H-imidazole-1-sulfonate (2bi) was
isolated, from the 1a-catalyzed alkenylation of 2bg
with potassium trans-2-phenylvinyltrifluoroborate
(Scheme 3).[29]
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