Copper promoted Chan-Lam type O-arylation of oximes with arylboronic acids at room temperature

Article abstract of DOI:org/10.1016/j.tetlet.2012.09.003

An efficient copper promoted protocol for the synthesis of aryloxime ethers using aryloximes and arylboronic acids has been developed under mild reaction conditions. This method offers an efficient alternative to the existing protocols.

Full text of DOI:org/10.1016/j.tetlet.2012.09.003

Tetrahedron Letters 53 (2012) 6219–6222
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Copper promoted Chan–Lam type O-arylation of oximes with arylboronic
acids at room temperature
⇑
Manoj Mondal, Gayatri Sarmah, Kongkona Gogoi, Utpal Bora
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2012
Revised 30 August 2012
Accepted 1 September 2012
Available online 8 September 2012
An efficient copper promoted protocol for the synthesis of aryloxime ethers using aryloximes and arylbo-
ronic acids has been developed under mild reaction conditions. This method offers an efficient alternative
to the existing protocols.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Chan–Lam
Oxime ether
Phenylboronic acid
Copper acetate
Cesium carbonate
Oximes undergo special types of C–O cross coupling reactions to
yield O-aryl or O-alkyl oxime ethers that are structural components
of numerous natural products, agrochemicals, and pharmaceuti-
cals.1,2 Moreover O-aryl or O-alkyl oxime ethers are used for the syn-
thesis of asymmetric primary amines,3 substituted pyrroles,4
trisubstituted isoxazoles,5 amino acids,6,3b cis-1, 2-amino alco-
hols,7,3b fluorenones,8 heterocyclic oximes,9 four-membered cyclic
nitrones,10 pyrimidine,11 aldehyde,12 nitriles,12 etc. Conventional
method for the synthesis of O-aryl or O-alkyl oxime ether involves
the condensation of O-arylhydroxylamines with carbonyl com-
pounds in the presence of base,13 or PTC.14 Other methods involving
their synthesis are the reaction of oximes with alkyl or aryl halides,15
fluorobenzene derivatives,16 alcohols,17 aryl nitrates,18 activated ole-
fins,19 allylic sp3 C–H bonds20 or esters,21 methyl sulfate,22 alkenes23
etc. Among these reported methods, the most interesting and conve-
niently used method is the copper catalyzed Chan–Lam C–O cross-
coupling strategy involving aryl oxime and aryl substrates.15 In the
last two decades copper (II) salts have emerged as the most versatile
reagents for the C–O bond formation reactions.24,25 The pioneering
work on copper catalyzed cross-coupling reaction of aryl oxime
and aryl halide for the synthesis of oxime ether was reported by Mai-
tra and co-workers15a Arylboronic acids are the most versatile orga-
nometallic species which are frequently used in many well known
cross coupling reactions26 because of the greater stability, structural
diversity, and lower toxicity over aryl halides. Recently Meyer and
co-workers27 and Huang and co-workers28 have reported the use
of arylboronic acid instead of aryliodide as coupling partner for the
synthesis of oxime ethers. Similarly supported copper catalysts were
also used for the coupling of oxime and arylboronic acid by Cai and
co-workers29 But most of the methods reported till today for the
synthesis of oxime ether use expensive aryl or alky halides (mostly
iodide), chlorinated solvents (like DCM,27 DCE,28 etc.), high concen-
tration of base,15a,27,28 high temperature,15a long reaction time,27–29
ligands,15a oxidants,29 moisture free atmosphere,27,29 and stoichiom-
etric amount of the metal source. Moreover, most of the existing
copper promoted protocols for oxime ether synthesis involving
arylboronic acids require long reaction time which resulted in the
formation of many side products such as biaryl, arene, phenol, diaryl
ether, acetyl-aryl ether etc., and consequently lower the yield of the
desired products.26f Thus, the feasibility of using copper in the
synthesis of O-aryloxime ether by cross coupling of aryl boronic acids
and oximes deserves serious attention. In this communication we
wish to report the effect of different bases, solvents, and copper
loadings on the Chan–Lam C–O cross coupling methodology for the
synthesis of O-aryloxime ether at room temperature using aryl
oxime and aryl boronic acids as coupling partners.
To investigate the effectiveness of different bases in Chan–Lam
type O-arylation, the reaction of acetophenone oxime (1a) with
phenylboronic acid (2a) was chosen as a model reaction using
DMSO as a solvent, Cs2CO3 as a base, and the reactions were per-
formed in aerobic condition at room temperature in the presence
of Cu(OAc)2.30 The results are summarized in the Table 1. It has been
seen from Table 1 that the reaction proceeded with 82% isolated
yield within 5 hours (Table 1, entry 1). This is a significant result
as most of the reported procedures of oxime ether synthesis require
⇑
Corresponding author. Tel.: +91 373 2370210; fax: +91 373 2370323.
E-mail address: utbora@yahoo.co.in (U. Bora).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.003

6220
M. Mondal et al. / Tetrahedron Letters 53 (2012) 6219–6222
Table 1
Optimization of reaction conditions for synthesis of O-aryloxime ethera
OH
N
O
N
B(OH)2
Cu(OAc)2
base, solvent, rt
+
1a
2a
3a
Entry
Catalyst (equiv)
Solvent (4 ml)
Base (1 equiv)
Time (h)
Yield of 3ab (%)
1
2
3
4
5
6
7
8
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.2
0.4
0.6
0.8
1.0
0.5
0.5
0.5
0.5
0.5
0.5
1
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
CH3OH
iPrOH
DCM
Cs2CO3
KOH
Na3PO4Á12H2O
Na2CO3
K2CO3
5
82
24
24
7
7.5
24
6
24
20
36
8
5
4.5
3
16
6.5
6
10
4
Trace
Trace
64
71
40
NaHCO3
TEA
58
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
—
43c
55d
45
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
50
72
73
70
51
61
59
30
CH3CN
Toluene
H2O
H2O
H2O/DMSO
DMSO
53
48
48
4
Trace
41e
47
1
0.5
48
No reaction
a
b
c
Reaction conditions: acetophenone oxime (1 mmol), phenylboronic acid (2 mmol), 28 °C, in air.
Isolated yield.
0.5 equiv of base.
2 equiv of base.
0.2 equiv of TBAB used.
d
e
very long reaction time. Interestingly we found a dramatic effect of
bases in the progress of the reaction as the use of KOH and Na3-
PO4Á12H2O gave only a trace amount of product (Table 1, entries 2
and 3). However Na2CO3 and K2CO3 gave good yields of product (Ta-
ble 1, entries 4 and 5). The yield of the product was significantly de-
creased when we used NaHCO3 and we could isolate 40% yield of
product (Table 1, entry 6). We examined the effect of organic base
in our reaction conditions by using TEA which gave 58% isolated
yield within 6 h (Table 1, entry 7). During this study Cs2CO3 was
found to be the most effective base, with 1 equiv sufficient for opti-
mal isolated yields of 3a (82%) (Table 1, entries 1, 8 and 9). Several
test reactions were carried out using different amounts of copper
acetate and we found that the presence of 0.5 equiv of Cu(OAc)2
gave better yield of products (Table 1, entries 1 and 10–14). To study
the effect of solvent in our system we carried out the reaction in the
presence of various solvents and the reaction was found to proceed
in both protic and aprotic solvents, although significant variations in
yields were noticed (Table 1, entries 15–20). The best result was ob-
tained when DMSO was used as solvent. However no product for-
mation was observed in water (Table 1, entry 1 and 20). Although
the use of a phase transfer catalyst like TBAB improved the yield
to 41% in 48 h in water (Table 1, entry 21). Use of co-solvents like
water-DMSO could not increase the yield of the product and gave
47 % yield (Table 1, entry 22). However the reaction did not proceed
in the absence of base (Table 1, entry 23). Usually strong bases in-
duced side reactions thereby decreasing the yield and too weak
bases were unable to activate boronic acid. Advantages of using
Cs2CO3 over other bases are cleaner reaction, higher yields, shorter
reaction time, smaller amount of reagent required, milder reaction
condition and easier work-up procedure.
To demonstrate the scope and limitations of the above optimal
conditions, a series of acetophenone oxime derivatives (1a–c) and
electronically diverse arylboronic acids (2a–h) were used as sub-
strates for the oxime ether synthesis (Table 2). As shown in Table 2,
the reaction was compatible with a wide range of aryloximes and
arylboronic acids although electronic properties of the substituents
in oximes and arylboronic acids influence the yield of the product.
As discussed earlier, 1a and 2a efficiently reacted to give 3a in very
good yield (Table 2, entry 1). Likewise, 1a reacts with 4-meth-
oxyphenylboronic acid (2b) and 4-methylphenylboronic acid (2d)
to yield 3b and 3d in good yields (Table 2, entries 2, 4). When 4-
chlorophenylboronic acid (2c) was treated with 1a the product
3c formed in higher yield (Table 2, entry 3). However when 4-ace-
tylphenylboronic acid (2e) was used as the coupling partner, both
1a and 1b gave 65% and 55% yields respectively (Table 2, entries 5
and 11). Cross-coupling of 3-methyphenylboronic acid (3g) with
1a gave a higher yield of product (74%) although a longer reaction
time is required for this reaction (Table 2, entry 7). However 2-
methyphenylboronic acid (2f) did not efficiently undergo cross-
coupling with 1a and resulted in lower yield of product (Table 2,
entry 6), which may be due to steric hindrance. In another case
1b gave 60% and 70% isolated yields when coupled with 2a and
2d respectively (Table 2, entries 9 and 10). Moreover 4-methylace-
tophenone oxime (1c) was found to be a suitable substrate for
oxime ether synthesis under current reaction conditions (Table 2,
entry 12). However 2-thiopheneboronic acid (2h) was not compat-
ible under these reaction conditions (Table 2, entry 8).
To the best of our knowledge, the mechanism involving inor-
ganic bases for the formation of O-aryloxime ether from oximes
in the presence of Cu(OAc)2 has never been reported. Lam et al.

M. Mondal et al. / Tetrahedron Letters 53 (2012) 6219–6222
6221
Table 2
Cross-coupling of acetophenone oxime derivative with arylboronic acidsa
OH
O
B(OH)2
N
N
R2
Cu(OAc)2
Cs2CO3, DMSO, rt
+
R1
R1
R2
Entry R1
R2
Product
Time (h) [Yieldb
(%)]
Entry R1
R2
Product
Time (h) [Yieldb
(%)]
O
O
N
N
N
4-H
1a
3-Me
2g
CH3
1
2
3
4-H 2a
5 [82]
8 [70]
16 [80]
7
8
9
4-H 1a
4-H 1a
36 [74]
3a
3g
S
O
OCH
Cl
O
3
N
N
4-H
1a
4-OMe
2b
2h
24 [Trace]
3b
3h
O
O
N
4-H
1a
4-OH
1b
4-Cl 2c
4-H 2a
24 [60]
HO
3c
3i
O
CH3
OAc
O
CH3
N
N
N
4-H
1a
4-OH
1b
4-Me
2d
4
5
4-Me 2d
4 [79]
10
11
6 [70]
HO
HO
H3C
3j
3d
O
O
OAc
N
4-H
1a
4-OAc
2e
4-OH
1b
4-OAc
2e
22 [65]
30 [55]
3e
H3C
3k
O
N
O
N
4-H
1a
4-Me
1c
6
2-Me 2f
48 [18]
12
4-H 2a
6 [65]
3l
3f
a
Reaction conditions: Aryl Oximes (1 mmol), Arylboronic acid (2 mmol), Cu(OAc)2 (0.5 mmol), Cs2CO3 (1 mmol), DMSO (4 ml).
Isolated yield: All compounds were characterized by 1H NMR, 13C NMR, FT-IR and Mass spectroscopy.
b
Ph
reported that when amine based substrates are added to insoluble
mixture of Cu(OAc)2 in methylene chloride, Cu(OAc)2 gets
instantaneously dissolved and gives a deep blue copper-complex
responsible for the formation of the cross coupling product.31
However Cu(OAc)2 is soluble in DMSO and we found very good
yield of products in DMSO (Table 1, entry 1).
It is believed that during the course of reaction, oximes gets
coordinated to Cu(OAc)2 to generate in situ Cu(II) complex A. The
complex A upon transmetallation with arylboronic acid formed
copper (II) complex B. Reductive elimination of complex B yields
the corresponding oxime ether. The plausible steps are shown in
Scheme 1. To prove the involvement of the Cu (II) complex A, we
carried out a reaction between 1a and Cu(OAc)2. The IR spectro-
scopic studies of the product of the above reaction showed the
presence of a band in the region of 3271–3477 cmÀ1, which could
be assigned to OH group. The presence of OH group in the metal
complex implies that the coordination of metal occurs through
HO
OAc
(II)
N
Cu(OAc)2
Cs2CO3
HO
AcO
N
Cu
Ph
N
OH
Ph
A
PhO
N
PhB(OH)2
Ph
Ph
Ph
N
- Cu0
HO
Ph
(II)
N
Cu
OH
Ph
B
Scheme 1.
the nitrogen atom of the oxime group. This assumption was sup-
ported by the observation that (C@N) of the free oxime ligand
m

6222
M. Mondal et al. / Tetrahedron Letters 53 (2012) 6219–6222
at 1639 cmÀ1 is considerably shifted to lower frequency32 at
1610 cmÀ1 after complexation. Formation of A was further sup-
ported by the observation of a new band in the region of
9. Ohno, H.; Aso, A.; Kadoh, Y.; Fujii, N.; Tanaka, T. Angew. Chem., Int. Ed. 2007, 46,
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451 cmÀ1, assigned to
m
(Cu–N) stretching vibration.33 Moreover
antisymmetric and symmetric COO stretching bands at 1582 and
1425 cmÀ1 are found to be shifted to lower and higher frequencies
1562 and 1444 cmÀ1 respectively upon coordination indicating the
presence of COO group in the complex.32a Additionally mass spec-
troscopic studies also support the formation of A. The same Cu (II)
complex A was formed in the presence or absence of the base indi-
cating that the base did not play any role in the complex formation.
It is important to mention here that when we stir the mixture of
1a, 1b, and Cs2CO3 in the presence of Cu(OAc)2, a deep blue colored
solution was formed. This may be due to the formation of a copper
oxime complex.31 Interestingly we found that 0.5 equiv of
Cu(OAc)2 is enough for the formation of the O-aryloxime ether,
which indirectly supports the involvement of copper complex A
in the reaction. Moreover the reaction of preformed Cu(II) complex
A and 2a gave the same product in good yield which supports the
involvement of in situ formed Cu(II) complex A in the reaction.
In summary we have developed an efficient protocol for the
synthesis of oxime ethers under mild reaction conditions. The reac-
tion was promoted by Cu(OAc)2 in the presence of Cs2CO3 as base.
The reaction was carried out in DMSO which is a usable solvent.34
This method offers a mild and efficient alternative to the existing
protocols since the reaction proceeded at room temperature within
shorter reaction time in the presence of inorganic base.
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The authors acknowledge the Department of Science and Tech-
nology, New Delhi for financial support for this work (NO. SR/FT/
CS-098/2009). The authors are also thankful to the UGC New Delhi
for Special Assistance Programme (UGC-SAP) and the Department
of Science and Technology for financial assistance under the DST-
FIST programme to the Department of Chemistry, Dibrugarh
University.
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Cu(OAc)2 (100 mg, 0.5 mmol), Cs2CO3 (325 mg, 1 mmol), aryl boronic acid (2a,
242 mg, 2 mmol) and 4 ml of DMSO was added in open atmosphere in a 50 ml
round bottom flask. The mixture was stirred at room temperature and the
progress was monitored by TLC. After completion of the reaction, the reaction
mixture was quenched with dil. NH4Cl-H2O solution and extracted with ethyl
acetate (3 Â 20 ml). Then the extract was washed with brine (2 Â 20 ml) and
dried over Na2SO4 and evaporated on reduced pressure. Residue was purified
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