A water-soluble pyridyl-triazole ligand for aqueous phase palladium catalyzed Suzuki-Miyaura coupling

Article abstract of DOI:10.1039/c3ra44740e

An environmentally friendly water-soluble ligand has been prepared by "clicking" 2-(azidomethyl)pyridine with but-3-ynyl sodium sulphate. In situ combination of the new ligand with [Pd(η³-C ₃H₅)Cl]₂ (Pd:ligand =

Full text of DOI:10.1039/c3ra44740e

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A water-soluble pyridyl-triazole ligand for aqueous
phase palladium catalyzed Suzuki–Miyaura coupling†
Cite this: DOI: 10.1039/c3ra44740e
*
Emanuele Amadio, Alberto Scrivanti, Valentina Beghetto, Matteo Bertoldini,
Md. Mahbubul Alam and Ugo Matteoli
An environmentally friendly water-soluble ligand has been prepared by “clicking” 2-(azidomethyl)pyridine
with but-3-ynyl sodium sulphate. In situ combination of the new ligand with [Pd(h3-C3H5)Cl]2 (Pd : ligand ¼
1 : 1) provides a highly active catalytic system for the Suzuki–Miyaura reaction. Coupling of phenylboronic
acid with a variety of aryl or heteroaryl bromides is carried out at catalyst loadings of 0.01% or lower under
conventional heating. The aqueous catalytic phase may be used three times without loss of activity.
Received 18th July 2013
Accepted 10th September 2013
DOI: 10.1039/c3ra44740e
www.rsc.org/advances
Besides employing green reaction media, the use of catalysts
and ligands prepared according to atom economical reactions is
Introduction
The Suzuki–Miyaura cross-coupling reaction of aryl halides with
arylboronic acids is one of the most powerful and versatile
methods for the synthesis of biaryls.1 Usually the reaction is
carried out in an organic solvent employing a transition metal-
catalyst and a base. Water is an attractive alternative to the
traditional organic solvents employed in S–M coupling because
it is inexpensive, nontoxic and non-ammable; furthermore, a
catalytic process carried out in water allows an effortless
recovery of the insoluble reaction products and a prompt recycle
of catalyst. Water was rst employed as the solvent for S–M
coupling by Beletskaya2 in 1989 and nowadays water-soluble
catalysts are increasingly employed either in water alone or in
water–organic solvent biphasic mixtures. The advancement of
the research in this eld has been the topic of a number of
reviews.3,4
advisable in order to improve the sustainability of a process.
Recently, taking advantage of the copper-catalyzed azide–alkyne
[3 + 2] cyclization (see Scheme 1), we have synthesized a small
library of N–N or N–S ligands which were successfully employed
in conventional S–M reactions.5 The ultimate aim of our studies
is to take advantage of the modular nature of the copper-cata-
lyzed Huisgen reaction in order to nely tune the electronic and
steric properties of N-donor ligands. As an extension of this
work we were intrigued by the possibility of modifying one of
the most catalytically active triazolyl ligands to make it water-
soluble. Accordingly, we wish to report herein the synthesis of
the new ligand 1 and the study of its activity in palladium
catalyzed S–M coupling.
Results and discussion
Ligand 1 is obtained in high yield by coupling 2-(azidomethyl)
pyridine with but-3-ynyl sodium sulfate in t-BuOH using
Cu(OAc)2 (see Scheme 1) according to the protocol developed by
Zhu6 which does not require the presence of a reductant. 1
displays good solubility in water, methanol and DMSO, and its
identity was conrmed by elemental analysis, the ESI-MS
1
spectrum in the negative mode and H and 13C NMR spectros-
copy; the relevant data (see the Experimental and the ESI†) are
in close agreement with those of other pyridyl-triazole ligands
and do not deserve particular comments.5b,6
In order to develop a very simple and general protocol for the
reaction, we devised to prepare the catalyst in situ by combining
1 and a suitable source of palladium in water. Early experiments
carried out employing PdCl2 or Na2PdCl4 led to not well
reproducible catalytic systems, thus we settled to prepare the
catalyst by mixing a methanol solution of allyl complex [Pd(h3-
C3H5)Cl]2 with an aqueous solution of 1 (Pd/1 ¼ 1 : 1). This
approach was indeed successful allowing to prepare stock
Scheme 1 Synthesis of the water-soluble pyridyl-triazolyl ligand 1.
`
Department of Molecular Science and Nanosystems, Universita Ca' Foscari Venezia,
Venice, Italy. E-mail: scrivanti@unive.it; Fax: +39 041 2348917; Tel: +39 041 2348903
† Electronic supplementary information (ESI) available: Details of ligand
synthesis and characterization. See DOI: 10.1039/c3ra44740e
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Table 2 Catalyst recycle experimentsa
solutions of the catalytic system which showed to be shelf-stable
for weeks.
For a preliminary assessment of the catalyst efficiency and to
optimize the reaction conditions, we initially investigated the
reaction of 4-bromoacetophenone with phenylboronic acid (see
Table 1).
The experiments were carried out in the presence of tetrabu-
tylammonium bromide (TBAB) as phase transfer catalyst (PTC) in
order to ensure a good migration of the organic substrate into the
aqueous phase3b employing K2CO3 as the base. At 110 ꢀC the
reaction can be carried out with catalyst loadings as low as 0.001
mol% giving the expected coupling product in almost quantita-
Catalytic run
Conv.b (%)
Yieldb (%)
1
2
3
4
99
99
95
40
99
99
76
26
a
Under aerobic conditions: Pd/substrate
¼
1 : 100 000; 4-bromo-
acetophenone: 2.0 mmol; phenylboronic acid: 2.4 mmol; K2CO3: 3.0
mmol; TBAB: 3.0 mmol; 10 mL of H2O; reaction time ¼ 2 h; T ¼ 110
ꢀC. b By GLC using n-undecane as internal standard.
tive yield by only two hours (entry 1 of Table 1). The high catalytic instead of TBAB. While TBAAc appears almost equivalent to
efficiency of 1 is demonstrated by the fact that almost total TBAB, the chloride appears signicantly more effective under the
substrate conversion is obtained in two hours even when the same reaction conditions, in agreement with previous results
reaction temperature is lowered to 70 ꢀC (entries 2 and 3). On the obtained by us employing other triazolyl-based ligands in palla-
other hand, when the reaction temperature is further lowered to dium catalyzed S–M reaction.5c Even if these results reveal that
50 ꢀC a sudden signicant decrease of the reaction rate is TBAC is more effective than TBAB, in all the following experi-
observed. It should be pointed out that the reduced yield is not ments we preferred to employ this latter owing to its inexpen-
due to catalyst deactivation since the reaction conversion can be siveness which makes the process more cost-effective.
increased by increasing the reaction time (entry 5).
The use of water as the solvent in catalysis offers as addi-
The inuence of the presence of TBAB is highlighted by data tional advantage an effortless catalyst recycle and a prompt
ꢀ
in entries 6–8 of Table 1. While at 110 C the reaction goes to recovery of the products either by ltration or by extraction with
completion even when no phase transfer catalyst is present, at an appropriate organic solvent. To verify if at the end of the
lower temperatures a small, but signicant decrease of the reaction the aqueous phase is still catalytically active, we per-
reaction rate is observed in the absence of the ammonium salt. formed some catalyst recycle experiments using the model 4-
To better understand the inuence played by the anion of the bromoacetophenone–phenylboronic acid coupling. The
quaternary ammonium salt, experiments were carried out product was isolated by simple ltration of the reaction
employing tetrabutylammonium chloride (TBAC) or tetrabuty- mixture, then fresh substrates and base were added to the
lammonium acetate (TBAAc) (entries 9 and 10 of Table 1) recovered aqueous phase for another catalytic run. According to
the data gathered in Table 2, the catalyst can be used in three
consecutive reactions (two recycles) without signicant loss of
efficiency; only when the catalytically active aqueous phase is
employed in a fourth experiment a substantial decrease in
substrate conversion is observed.
Table 1 Influence of the temperature on the coupling of phenylboronic acid
with 4-bromoacetophenonea
In this connection, it is worth to note that the raw product
recovered by ltration contains about 10% of TBAB. Since
during the recycling experiments the TBAB level in the aqueous
phase was not restored, we are brought to believe that the
decrease in catalytic activity aer the second recycle is due to
loss of the phase transfer agent combined with accumulation of
the saline by-products. Most importantly, the experiments
showed that, our catalytic system maintains its efficiency for
three consecutive catalytic runs. This is a reasonably good result
which in our opinion suggests that the catalyst is actually
working in the aqueous phase.7
Entry T (ꢀC) PTC (mmol) Conv.b (%) Yieldb,c (%) TONd
1
2
3
4
5
6
7
8
9
10
110
90
70
TBAB (3)
TBAB (3)
TBAB (3)
TBAB (3)
TBAB (3)
—
99
99
99
5
25
99
88
67
45
29
99 (92)
97
97
5
25
90
77
62
40
105
105
105
50
5 ꢁ 103
2.5 ꢁ 104
105
With optimized reaction conditions in hand, we tested the
catalyst system with a variety of aryl bromides having different
steric and electronic parameters (Table 3).
50e
110
90
—
—
8.8 ꢁ 104
6.7 ꢁ 104
4.5 ꢁ 104
2.9 ꢁ 104
As expected the electron-neutral non-activated bromo-
benzene is less reactive than 4-bromoacetophenone, the reac-
tion rate being approximately one third that of the model
substrate under the same reaction conditions (compare entry 1
in Table 3 with entry 3 in Table 1); nevertheless, complete
substrate conversion can be obtained either by increasing the
reaction temperature to 90 ꢀC or by decreasing the substrate to
palladium molar ratio to 10 000 : 1. Using this catalyst loading,
70
50e
50e
TBAC (3)
TBAAc (3)
24
a
Under aerobic conditions: Pd/substrate
¼
1 : 100 000; 4-bromo-
acetophenone: 2.0 mmol; phenylboronic acid: 2.4 mmol; K2CO3: 3.0
b
mmol; 10 mL of H2O; reaction time ¼ 2 h. By GLC with n-undecane
c
d
as internal standard. In parentheses: isolated yield. TON: turnover
e
number ¼ mol of substrate converted/mol of catalyst. Reaction time
¼ 18 h.
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Table 3 Scope of the catalysta
the reactions of 4-bromobenzaldehyde or 1-bromo-4-uoro
benzene (two activated substrates, entries 4 and 5) with phe-
nylboronic acid go to almost complete conversion in only two
hours at 70 ꢀC, while with electron-rich deactivated aryl
bromides such as 4-methoxybromobenzene and 4-methyl-
bromobenzene it is necessary to increase the reaction temper-
Entry
1d
Aryl-Br
T (ꢀC)
Conv.b (%)
36
Yieldc (%)
24
ꢀ
ature to 90 C to obtain almost complete conversion (entries 6
and 7). Also the moderately sterically demanding 2-methyl-
bromobenzene (entry 8) was efficiently coupled with phenyl-
boronic acid, and only when the substrate is the highly
deactivated and hindered 2,4,6-trimethylbromobenzene the
reaction rate undergoes a signicant decrease (entry 9). Never-
theless, the product yield can be increased either increasing the
reaction time (entry 10) or the temperature at 100 ꢀC, at this
latter temperature a reasonably good 40% substrate conversion
is obtained by only 2 hours (entry 11). Excellent results were
obtained also with 1-bromonaphtalene (entry 12).
70
2d
3
90
70
98
98
70
65
4
5
6
7
70
70
90
90
99
99
99
78
81
86
75
66
To further explore scope and limitations of the catalyst, we
carried out experiments with some heteroaryl bromides. As
expected, signicantly lower reaction rates are observed with
heteroaromatics substrates. In fact, the reaction of 3-bromo-
pyridine with phenylboronic acid at 90 ꢀC proceeds to only 33%
conversion by 2 hours (entry 13). Nevertheless, good conver-
sions can be obtained either by increasing the reaction time or
the temperature (entries 14 and 15). The catalytic system
showed to be by far less efficient with 2-bromopyridine; in fact,
only traces of the coupling product were obtained under the
optimized conditions (entry 16). The difference in reactivity
between 3- and 2-bromopyridine may be ascribed to the differ-
ences in the capability of the two substrates to approach the
metal centre.8 Likely, coordination of the more basic 3-bromo-
pyridine9 to the metal centre is favoured when compared to the
less basic 2-bromo derivative.10 According to examples in liter-
ature,11a,b oen the coupling of halopyridines with phenyl-
boronic acid is successful when carried out employing alcohols–
water mixtures as the reaction solvent. This approach revealed
to be benecial since in an experiment carried out under the
same conditions of entry 16, but using an EtOH–H2O mixture
(1 : 1, v/v) as the solvent, we obtained a substrate conversion of
21% by 2 hours. Albeit modest, this conversion arises from a
reasonably good reaction rate of about 1000 catalytic cycles per
hour. The coupling of 5-bromopyrimidine with phenylboronic
acid is efficiently promoted by the catalytic system. Although
the substrate conversion is only 25% by 2 hours when the
reaction is carried out in water at 90 ꢀC (entry 18), almost
complete conversion can be obtained extending the reaction
time at 22 hours (entry 19); it is worth to note that a signicant
increase in the reaction rate can be obtained by setting the
reaction temperature to 100 ꢀC (entry 20).
8
9
90
90
95
15
95
13e
10 f
11
90
100
30
40
22e
35e
12
90
90
99
80
13
33
27
14 f
15
90
100
76
60
67
55
16
90
90
90
2
21
25
—
14
20
17g
18
19 f
20
90
100
98
61
80
40
21
90
98
87
22
100
8
—
At 90 ꢀC, the reaction of 2-bromothiophene with phenyl-
boronic acid proceeds smoothly affording the desired coupling
product in almost complete yield by only 2 hours (entry 21 of
Table 3). On the contrary, the catalyst is unable to activate 3-
bromothiophene, which reaction with phenylboronic acid
affords only small amounts of the sought coupling product even
when the reaction temperature is increased to 100 ꢀC (entry 22).
In contrast to that observed with 2-bromopyridine no
a
Under aerobic conditions: Pd/substrate ¼ 1 : 10 000; aryl bromide: 2.0
mmol; phenylboronic acid: 2.4 mmol; K2CO3: 3.0 mmol; TBAB: 3.0
b
mmol; 10 mL of H2O; reaction time: 2 h. By GLC using n-undecane
c
d
as internal standard. Isolated yield. Pd/substrate ¼ 1 : 100 000.
e
g
GLC yield. f Reaction time: 22 h. In EtOH–H2O 1 : 1 (v/v).
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improvement was achieved by running the coupling of 3-bro- (2.0 mmol), phenylboronic acid (2.4 mmol), potassium
mothiophene in water–ethanol mixtures (experiment not carbonate (3.0 mmol) and tetrabutylammonium bromide (3.0
reported in Table 3).
mmol) were added into a glass reactor (50 mL of volume) and
Finally, the catalyst efficiency was tested in the coupling of suspended in 10 mL of water. To the stirred suspension, 100 mL
some aryl chlorides. The reactions were carried out employing a of the 10ꢂ2 M catalyst stock solution were added and the
ꢀ
substrate to catalyst ratio of 10 000 : 1 and a TBAB/substrate mixture was heated to 90 C. Aer 2 hours the suspension was
ꢀ
molar ratio of 1 : 1; the temperature was set to 100 C and the rapidly cooled to room temperature and extracted with ethyl
reaction time was of 2 hours. Good results (90% conversion) acetate (3 ꢁ 5 mL). The organic phases were reunited and aer
were obtained only with the activated 4-chloroacetophenone, addition of n-undecane (internal standard) analyzed by GLC.
while with non-activated (chlorobenzene) or deactivated
substrates (e.g. 4-chloroanisole and 4-chlorotoluene) only
modest conversions in the 5–10% range were achieved. No
Conclusions
substantial improvement in product yield was obtained by We have prepared a new water-soluble nitrogen ligand and
increasing the reaction time suggesting that the catalyst has a developed a simple protocol for the Suzuki–Miyaura reaction.
limited longevity when employed with aryl chlorides. Experi- The coupling of a variety of aryl bromides and heteroaryl
ments carried out at higher catalyst loadings afforded no better bromides with phenylboronic acid has been successfully carried
yields and resulted in extensive precipitation of palladium out in reasonable reaction times using very low catalyst load-
black.
ings. The catalytic system works under conventional heating, is
Summing up, the catalytic activity of our system appears air stable and can be employed for three successive runs with
comparable to that of other catalysts11–15 able to efficiently negligible loss of efficiency.
promote the coupling of aryl bromides but displaying inade-
It is nally worth to note that 1 must be considered the
quate activities with aryl chlorides. It is to emphasize that our prototype of a new family of water-soluble ligands since both its
catalyst, based on a simply designed ligand, shows an impor- steric and electronic characteristics can be easily tuned taking
tant longevity so that the reaction can be carried out employing advantage of the modular nature of the copper-catalyzed [3 + 2]
as little as 0.001–0.01 mol% of palladium. This feature Huisgen cycloaddition.
combines favourably with catalyst reusability, and the positive
impact on the economical and practical aspects of the catalytic
process is obvious. In this connection, it is worth to note that
Notes and references
there is a limited number of catalysts able to activate aryl
chlorides in water.16
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Experimental
Preparation of sodium 2-(1-((pyridin-2-yl)methyl)-1H-1,2,3-
triazol-4-yl)ethyl sulfate (1)
To a suspension of but-3-ynyl sodium sulfate17 (455 mg, 2.6
mmol) in a mixture of t-BuOH (15 mL) and H2O (1 mL) were
added a solution of 2-(azidomethyl)pyridine (3.57 mg, 2.7
mmol) in t-BuOH (5 mL), and, nally,
a solution of
Cu(OAc)2$H2O (26 mg, 0.13 mmol) in water (0.5 mL). The
mixture was stirred under inert atmosphere for 48 hours, then
the liquid phase was taken to dryness to give a green solid.
Flash-chromatography (silica gel, tetrahydrofuran–methanol
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¨
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Preparation of the catalyst stock solutions
10ꢂ2 M catalyst stock solutions were prepared by diluting with
water in a 10 mL volumetric ask a methanol solution (3 mL)
containing 19.5 mg (0.05 mmol) of [Pd(h3-C3H5)Cl]2 (ref. 18) and
30.8 mg (0.1 mmol) of 1.
Typical procedure for the cross-coupling reactions
All manipulations were carried out under aerobic conditions. In
a typical experiment (entry 2 of Table 1), 4-bromoacetophenone
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¨
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