Micellar system in copper-catalysed hydroxylation of arylboronic acids: Facile access to phenols

Article abstract of DOI:10.1039/c1cc14974a

Copper-catalysed oxidative hydroxylation of arylboronic acids was accomplished in water containing an amphiphilic surfactant, providing facile access to phenol derivatives.

Full text of DOI:10.1039/c1cc14974a

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COMMUNICATION
Micellar system in copper-catalysed hydroxylation of arylboronic acids:
facile access to phenolsw
Kiyofumi Inamoto,* Kanako Nozawa, Misato Yonemoto and Yoshinori Kondo*
Received 11th August 2011, Accepted 21st September 2011
DOI: 10.1039/c1cc14974a
Copper-catalysed oxidative hydroxylation of arylboronic acids
was accomplished in water containing an amphiphilic surfactant,
providing facile access to phenol derivatives.
Use of water as a reaction medium is highly advantageous
from the viewpoint of cost, safety, availability and environment-
1
friendliness. Although its use in organic synthesis is some-
times hampered by the insolubility of the polar organic
molecules, the addition of an amphiphilic surfactant is known
to improve the reaction performance in many cases, including
transition metal-catalysed processes, by hydrophobic and
Fig. 1 Synthesis of phenols via transition metal-catalyzed processes.
concentration effects in the microheterogeneous two-phase
2
system.
,3
8
palladium catalysts. During the course of our present work,
Phenols frequently occur in numerous biologically active
4
compounds, ranging from natural products to pharmaceuticals.
Hu, Wang and co-workers reported the copper-catalysed,
They also serve as important building blocks in material
industries as well as versatile synthetic intermediates in
base-mediated transformation of arylboronic acids for the
,10
9
preparation of phenols.
The process employs 10 mol% of
5
constructing O-heterocycles. The development of methods
a copper catalyst and 20 mol% of a phenanthroline ligand
along with 3 equiv. of KOH as a base, affording variously
substituted phenols. Herein, we describe our independent
finding that various arylboronic acids are efficiently converted
into the corresponding phenols in the presence of a copper
catalyst. The reactions efficiently proceed under mild (room
temperature to 60 1C), ligand- and base-free conditions in
water containing an amphiphilic surfactant. The wide
functional group tolerance (e.g. alkoxycarbonyl, acetyl, cyano,
halogen atoms etc.) is accomplished by using a non-alkaline
condition. A preliminary study to disclose the precise reaction
mechanism is also presented.
to access such a structural motif, therefore, remains an area
of intensive research. Classical methods to prepare phenols
include nucleophilic aromatic substitution of aryl halides
possessing electron-withdrawing substituents and hydrolysis
of arene diazonium salts. These methods, however, often suffer
from several drawbacks such as poor functional group
compatibility, narrow substrate scope and difficulties in
obtaining starting materials, thus limiting their usefulness
and applicability. Several procedures which employ transition
metal catalysts have recently been introduced, providing
milder, more efficient access to phenols (Fig. 1). Iridium-
catalysed aromatic C–H borylation followed by oxidation
has been reported by Smith et al., delivering a series of non-
In our copper-catalysed phenol synthesis, 4-methoxy-
phenylboronic acid 1a was initially employed as a substrate
(Table 1). Although the reaction in the presence of 5 mol% of
6
ortho-substituted phenols. After the first report of Buchwald’s
7
group, several catalytic systems based on a palladium–
a
CuCl under an O atmosphere did not proceed at all in pure
2 2
phosphine combination, which effect direct hydroxylation of
7
aryl halides, have also been developed. More recently, copper
water even after 24 h (entry 1), the addition of a certain
surfactant has turned out to be effective for the process.
Among several kinds of surfactants tested (entries 2–7), the
best yield was obtained when Brij S-100 was employed,
affording 4-methoxyphenol 2a in high yield (entries 7 and 8).
It is noteworthy that the addition of a strong base and a
ligand, that was crucial for high conversion in a previous
also proved to be an efficient catalyst for the same process,
providing a less expensive and less toxic route compared to the
Graduate School of Pharmaceutical Sciences, Tohoku University,
6-3, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan.
E-mail: inamoto@mail.pharm.tohoku.ac.jp,
ykondo@mail.pharm.tohoku.ac.jp; Fax: +81-22-795-3921;
Tel: +81-22-795-6804
9
example, is not necessary, providing a simple, more applic-
able route to phenols. A following screening revealed that
2
CuCl was the most effective catalyst among the copper sources
w Electronic supplementary information (ESI) available: Experimental
procedures and spectral/analytical data. See DOI: 10.1039/c1cc14974a
tested (entries 7 and 8 vs. entries 9–16). Essentially, no product
Chem. Commun., 2011, 47, 11775–11777 11775
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a
a,b
Table 1 Effect of reaction parameters
Table 2 Oxidative hydroxylation of arylboronic acids
b
x (mol%) Time/h Yield (%)
Entry Cu
CuCl
Surfactant
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
None
SDS
PTS
0
15
3
24
24
24
24
24
24
24
6
24
24
24
24
24
24
24
24
24
6
0
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
42
68
56
70
58
95
95
54
25
0
TPGS
Triton X-100 10
10
c
Brij S-30
10
10
10
10
10
10
10
10
10
10
10
10
10
10
c
c
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
Brij S-100
CuBr
CuF
Cu(OTf)
CuSO
Cu(OAc)
Cu(OH)
Cu
2
0
1
2
3
4
5
6
7
8
9
2
2
4
0
2
36
15
10
50
0
2
2
O
CuCl
None
d
e
CuCl
CuCl
2
41
0
2
6
a
c
b
Reactions were carried out on a 0.30 mmol scale. Isolated yield.
d
0 mol% of Brij S-100 was used. NaCl (3.0 M) was added.
2
Scheme 1 Preliminary mechanistic study.
a
b
Reactions were carried out on a 0.30 mmol scale. Determined by
1
the isotopically labeled water (H2 O) was performed.
8
1
c
H-NMR using 1,1,2-trichloroethane as an internal standard. Isolated
Namely, when the reaction of boronic acid 1a was carried
18
d
e
yield. Run under an air atmosphere. Run under an Ar atmosphere.
out employing H2 O as a solvent under the optimal reaction
1
8
conditions, incorporation of O into the phenol was observed
(Scheme 1), implying that the oxygen source for the phenol
was obtained from the reaction in the absence of a copper
1
3
catalyst (entry 17). Use of an air atmosphere instead of O
2
gave
formation is water as previously hypothesized by Evans et al.
a considerably lower yield (entry 18). The reaction carried out
under an Ar atmosphere resulted in no conversion, suggesting
This is also consistent with the result previously reported by
1
Lam et al., in which similar incorporation of O was observed
8
11
14,15
that the molecular oxygen is crucial for the process (entry 19).
in the mechanistic studies of copper-mediated N-arylation.
Under our optimized conditions, an array of oxidative
hydroxylation of arylboronic acids 1b–l can be achieved with
good yields (Table 2). It should be mentioned that functional
groups such as alkoxycarbonyl (1j) and acetyl (1k) are well
tolerated, both of which seem difficult to remain intact under
In summary, an efficient copper-based catalytic system was
developed which successfully effect the transformation of
arylboronic acids to the corresponding phenols. The process
employs water as a reaction medium along with an amphi-
philic surfactant, realizing a mild, ligand- and base-free
approach to synthesizing functionalised phenols. Investigation
to unveil the reaction mechanism as well as to broaden the
substrate scope is currently underway. We are also applying
this environmentally benign micellar system to other transition
metal-catalyzed processes.
9
previously reported basic conditions. It is also particularly
noteworthy that the whole range of halogen atoms, including
iodine, can be incorporated because such compounds can be
further functionalised via certain synthetic methods (e.g.,
transition metal-catalyzed cross-coupling). For the reactions
of boronic acids 1f and 1g, the addition of NaCl had a positive
This work was partly supported by a Grant-in-Aid for
Scientific Research (B) (No.23390002), a Grant-in-Aid for
Challenging Exploratory Research (No. 23659001) and a
Grant-in-Aid for Young Scientists (B) (No. 23790002) from
Japan Society for the Promotion of Science.
1
2
effect, as reported by Lipshutz, resulting in obtaining
phenols in improved yields.
Although the precise reaction mechanism of the process
remains to be elucidated, a preliminary mechanistic study with
1
1776 Chem. Commun., 2011, 47, 11775–11777
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Notes and references
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4
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11775–11777 11777