Integration of borylation of aryllithiums and Suzuki-Miyaura coupling using monolithic Pd catalyst

Article abstract of DOI:10.1039/c5cy02098k

Integration of the preparation of arylboronic esters via aryllithiums and Suzuki-Miyaura coupling using monolithic Pd catalyst without an intentionally added base was achieved. A continuous operation has been done successfully for over 21 hours.

Full text of DOI:10.1039/c5cy02098k

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Technology
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Integration of borylation of aryllithiums and
Suzuki–Miyaura coupling using monolithic Pd
catalyst†
Cite this: DOI: 10.1039/c5cy02098k
Received 4th December 2015,
Accepted 5th January 2016
a
a
a
b
b
c
A. Nagaki,* K. Hirose, Y. Moriwaki, K. Mitamura, K. Matsukawa, N. Ishizuka
a
and J. Yoshida*
DOI: 10.1039/c5cy02098k
www.rsc.org/catalysis
1
0
Integration of the preparation of arylboronic esters via aryllithiums
and Suzuki–Miyaura coupling using monolithic Pd catalyst without
an intentionally added base was achieved. A continuous operation
has been done successfully for over 21 hours.
magnetic nanoparticle supports. Among them, monolith-
based devices have good flow characteristics when coupled
with the highly controlled surface properties associated with
the formation of nano-, micro- and mesoporous structures,
and they therefore represent ideal supports for reagents and
catalysts where contact time and temperature can be spatially
1–3
Chemical synthesis in flow microreactor systems
has
received significant research interest from both academia and
industry. Recent investigations revealed significant features of
flow microreactor systems involving fast mixing stemming
from short diffusion paths and fast heat transfer by virtue of
high surface-to-volume ratios, which are advantageous to in-
crease the selectivity of chemical reactions. A short residence
time in a microchannel is beneficial for controlling highly re-
active intermediates. By taking advantage of such features of
flow microreactor systems, various chemical reactions for or-
11
and temporally mediated.
12
Suzuki–Miyaura coupling of arylboronic acids and their
derivatives has been extensively used because of their air and
moisture stability. Although some arylboronic acids are com-
mercially available, it is often necessary to prepare appropri-
13
ate arylboronic acids for a desired transformation, and
their purification often causes great difficulties. Therefore,
integration of borylation and Suzuki–Miyaura coupling is
strongly needed to improve the efficiency of their overall
transformation. Recently, Buchwald et al. reported boronic
esters that were synthesized via lithiation in flow, which were
subsequently used for Suzuki–Miyaura coupling. We have
also reported that a wide range of arylboronic esters bear-
ing electrophilic functional groups can be synthesized based
on flash chemistry using a flow microreactor and that the
reaction can be integrated with Suzuki–Miyaura coupling of
aryl halides having electrophilic functional groups leading to
the crosscoupling of two aryl halides bearing electrophilic
functional groups. However, these flow methods are based
on homogeneous Pd catalysts. The use of heterogeneous Pd
catalysts should serve as a more environmentally benign pro-
cess because there is no need for easy separation and reuse
4
ganic synthesis have been developed so far. Crosscoupling
14
5
reactions that serve as a powerful method for carbon–carbon
bond formation are also a fascinating field in the applications
of flow microreactor systems. Especially, flow reactions with
the use of recyclable catalytic materials became an innovative
synthetic methodology that perfectly fits the current need for
more environmentally friendly procedures. In fact, recently,
successful examples using heterogeneous palladium catalysts
have been reported and they serve as useful and practical pro-
cedures for the recovery and reuse of the catalysts while
obtaining the desired product with minimal costs in terms of
15
16
17
6
time and waste.
Several examples of supported palladium catalysts have
been reported for use in flow microreactors, using mono-
7
8
9
liths, polyurea-encapsulated PdIJOAc)
2
(PdEnCat), silica and
a
Department of Synthetic Chemistry and Biological Chemistry, Graduate School
of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615–8510, Japan.
E-mail: anagaki@sbchem.kyoto-u.ac.jp, yoshida@sbchem.kyoto-u.ac.jp
b
Emaus Kyoto Inc. R&Ds, 26 Nishida-cho, Saiin, Ukyo-ku, Kyoto 615–0055, Japan
c
Osaka Municipal Technical Research Institute, Electronic Material Research
Division, 1-6-50, Morinomiya, Joto-ku, Osaka 536–8553, Japan
Fig. 1 Flow microreactor system for lithiation, borylation, Suzuki–
Miyaura coupling (micromixers: M1 and M2, microtube reactors: R1
and R2).
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy02098k
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1
8
of catalysts. Herein, we report the space integration of the
preparation of arylboronic esters via aryllithiums and
Suzuki–Miyaura coupling using a flow reactor packed with a
polymer monolith containing an immobilized Pd catalyst.
First, the solubility of lithium arylborates was tested be-
1
fore integrating the borylation of aryl halides (Ar X) and
2
Suzuki–Miyaura coupling of aryl halides (Ar X) using a mono-
lith reactor (Fig. 1). The halogen–lithium exchange of bromo-
1
benzene (Ar X) in THF with n-BuLi in hexane followed by the
reaction with BIJOMe) in THF was selected as a model reac-
3
tion. After the borylation reactions, the solubility of the
resulting arylboronic ester in a coupling solvent was checked.
Various solvents including THF, methanol, and ethanol were
added, and it turned out that a clear solution was obtained
when methanol was added.
Next, we examined the reaction integration of the
1
borylation of aryl halides (Ar X) and Suzuki–Miyaura coupling
2
with aryl halides (Ar X). A flow reactor packed with a polymer
monolith containing immobilized Pd was used.
The polymer monolith containing an immobilized Pd cata-
lyst was prepared by the following method (Fig. 2). 1,3-BisIJN,
N-diglycidylaminomethyl)cyclohexane was added to a solution
of polyIJethylene glycol) (PEG, molecular mass = 200), 4,4′-
diaminodicyclohexylmethane,
and
6-(phenylamino)-1,3,5-
Fig. 3 (a) SEM image of the polymer monolith before treatment with
triazine-2,4-dithiol and the mixture was stirred at room tem-
perature for 30 min. The resultant homogeneous solution
was poured into a cylindrical stainless steel column (an
empty HPLC column, 4.6 mmID × 150 mm length) and the
column was annealed at 100 °C. A THF solution containing
palladium acetate (0.5 wt%) was injected into the column at
2
PdIJOAc) , (b) SEM image of the polymer monolith after treatment with
PdIJOAc)2 and reduction with NaBH , (c) TEM image of the cross
4
section of the skeletons with Pd nanoparticles in the polymer
monolith, and (d) a picture of a flow reactor (size: 4.6 mmID × 150 mm
length).
−
1
0
.05 ml min . The column adsorbed with palladium acetate
R2
was annealed in PEG (molecular mass = 300). Then, aqueous
solution of sodium borohydride (0.5 wt%) was injected into
out in M2 (ϕ = 500 μm) and R2 (ϕ = 1000 μm, L = 50 cm (t
=
1
2.0 s)) at the same temperature (T = 0 °C). The resulting so-
lution was collected in a vessel. Then, a solution of
p-iodobenzonitrile (0.033 M in MeOH) was added and the
−
1
the column at 0.05 ml min to reduce PdIJII) adsorbed on the
surface of the monoliths. SEM images of the monolith before
and after Pd-immobilization are shown in Fig. 3a and b, re-
spectively. As shown in Fig. 3c, Pd nanoparticles are
immobilized on the surface of the polymer.
2
mixture was passed through the monolith reactor at T (°C)
A typical procedure for the integration of lithiation,
borylation, and Suzuki–Miyaura coupling is as follows:
−
1
bromobenzene (0.10 M in THF) (flow rate: 6.0 mL min ) and
a solution of n-BuLi (0.60 M in hexane) (flow rate: 1.0 mL
−
1
min ) were introduced into M1 (ϕ = 500 μm) and R1 (ϕ =
R1
1
1
000 μm, L = 25 cm (t = 1.7 s)) at T = 0 °C by using syringe
pumps to give phenyllithium. The reaction with trimethoxy-
−
1
borane (0.12 M in THF) (flow rate: 5.0 mL min ) was carried
2
R
Fig.
4 Temperature (T )–the residence time (t ) map for the
crosscoupling of bromobenzene and p-bromobenzonitrile using the
monolith reactor. Contour plots with scattered overlay of the yields of
biphenyl-4-carbonitrile (%), which are indicated by small circles.
Fig. 2 Immobilization of Pd on the polymer monolith in a flow
reactor.
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Table 1 Crosscoupling of Ar X and Ar X using the flow microreactor
Conditions of lithiation
and borylation
a
Yield (%)
1
R1
1
2
Ar X
t
(s)
T
(°C)
Ar X
Product
A
B
1.7
0
96
100
0
3
76
87
6
1
8
41
29
92
91
83
6
0.059
0
68
1
1
3
7
2
4
91
84
97
0
0
.059
.059
0
63
1
5
87
24
63
2
98
52
b
0
1
.059
.7
0
0
—
54
94
83
7
7
8
1
87
86
a
b
Determined by GC. Isolated yield.
R
using a plunger pump. The reaction was carried out with var-
yield increased with an increase in t because of the progress
R
ious residence times (t ) in the monolith reactor, and at vari-
of the crosscoupling reaction. The coupling product was
obtained in a good yield (>93%) with a t longer than 4.7
2
R
ous temperatures (T ).
As profiled in Fig. 4, the yield of biphenyl-4-carbonitrile
significantly depends upon both T and t . At 100 °C, the
min. The reaction at 120 °C resulted in a slightly better yield
(t = 9.4 min, quantitative yield). Notably, the crosscoupling
2
R
R
reactions were completed within a few minutes. It is interest-
ing that the Suzuki–Miyaura coupling proceeds without any
additional base. Hereafter, we carried out the coupling reac-
R
tions under conditions A (100 °C, t = 4.7 min) and B (120
R
°
C, t = 9.4 min).
The present flow microreactor method was successfully
applied to the crosscoupling of various functional aryl and
heteroaryl iodides as coupling partners. In contrast, the use
of aryl bromides resulted in much lower yields, because the
coupling reaction was much slower. Notably, a cyano group
1
9
Fig. 5 Synthesis of adapalene.
tolerated the optimized conditions,
although such
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functional groups easily undergo decomposition in conven-
tional batch reactions. Therefore, biaryls bearing electrophilic
functional groups on both aromatic rings were synthesized in
flow. Furthermore, a triaryl compound having one bromine
atom on one of the aromatic rings was also synthesized via
the lithiation of 4,4′-dibromobiphenyl, although such a
transformation is very difficult to achieve using conventional
batch reactors because of the formation of a significant
amount of dilithiated species (Table 1).
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2
0
Lastly, we applied the presented method to the synthesis
2
1
of adapalene, which is used in the treatment of acne, psori-
asis, and photoaging. The coupling of lithium [3-(1-
adamantyl)-4-methoxyphenyl]trimethoxyborate and methyl
6
-iodo-2-naphthoate was carried out in the monolith reactor
and the desired product was produced in 86% yield (Fig. 5).
A scaled-up synthesis was also achieved by simply extending
the operation time to 21 h. The desired product was obtained
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propanediol gave the corresponding adapalene in 89% yield.
3
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