Copper-mediated direct cross-coupling of 1,3,4-oxadiazoles and oxazoles with terminal alkynes

Article abstract of DOI:10.1002/chem.200902916

Copper-mediated direct cross-coupling of 1, 3, 4-oxadiazoles and oxazoles with terminal alkynes has been reported. The transition-metal-mediated direct functionalization reactions of C-H bonds have attracted significant interest in modern organic chemistry. The mixture was heated at 120°C, and the slow addition of 1 hour of a solution of phenylacetylene in DMAc (1.0 mL) as then started. The cross-coupling using CuCl₂/DMEDA system. Thus, the desired 2-alkynyl-5-aryloxazoles were obtained by performing the reaction in a higher ratio at 150°C with DMSO as solvent. The oxadiazoles with various substituents at the 2-position were also available for use. 4- Methyl-, 4-methoxy-, 4-trifluoromethyl-, and 4-chloro- phenyl-1, 3, 4-oxadiazoles coupled with 4-MeC₅H₄, to furnish compound.

Full text of DOI:10.1002/chem.200902916

DOI: 10.1002/chem.200902916
Copper-Mediated Direct Cross-Coupling of 1,3,4-Oxadiazoles and Oxazoles
with Terminal Alkynes
Masanori Kitahara,[a] Koji Hirano,[a] Hayato Tsurugi,[b] Tetsuya Satoh,[a] and
Masahiro Miura*[a]
The transition-metal-mediated direct functionalization re-
Kꢀndig[9a] and Cacchi[9b] succeeded in the synthesis of oxin-
doles and indoles, respectively, through an intramolecular
À
actions of C H bonds have attracted significant interest in
modern organic chemistry.[1] Among them, the direct oxida-
direct sp /sp C C coupling mediated by copper salts. Our
group also reported the copper-catalyzed oxidative coupling
of N,N-dimethylanilines with terminal alkynes under molec-
ular oxygen.[10] These promising examples encouraged us to
3
2
À
À
tive cross-coupling reactions utilizing two different C H
bonds, though difficult to achieve in a general way, are quite
attractive processes, since the preactivation, such as metala-
tion and halogenation of both coupling components, can be
eliminated so that they are economically and environmen-
tally advantageous (Scheme 1). Therefore, many researchers
develop a copper-based process directed toward the cou-
2
À
pling between heteroarene sp and alkyne sp C H bonds as
part of our study on the direct functionalization of heterocy-
clic compounds.[1b,d,e] We have found that useful heteroarene
cores, for example, 1,3,4-oxadiazole and oxazole, efficiently
undergo copper-mediated oxidative coupling with terminal
alkynes, which is reported herein. As certain 1,3,4-oxadia-
Scheme 1. Transition-metal (M) mediated direct oxidative coupling.
have recently explored this type of transformation, and a
Table 1. Optimization for the direct cross-coupling of 2-phenyl-1,3,4-oxa-
diazole (1a) with phenylacetylene (2a)[a]
2
2 [2]
À
number of C C bond formation reactions between sp /sp ,
[5]
sp3/sp2,[3] sp3/sp3,[4] and sp/sp C H bonds have become
À
available, in particular under palladium catalysis.[6] However,
the direct cross-coupling with sp and sp C H bonds, which
may be regarded formally as the direct version of Sonoga-
shira type reaction, still remains an important challenge.[7]
Recently, much attention has been focused on tractable
and less expensive copper salts or complexes in oxidative
2
À
Entry
Base
Solvent
Yield of 3aa [%][b]
[8–11]
1
2[c]
3[c]
4[c]
5
6
7
8
9
NaHCO3
NaHCO3
NaHCO3
NaHCO3
KHCO3
Na2CO3
K2CO3
Cs2CO3
K3PO4
Na2CO3
Na2CO3
Na2CO3
DMAc
DMF
NMP
45
45
49
48
44
55
55
51
À
direct C C coupling reactions.
Li described the utility of
3
À
a copper/peroxide catalyst system in the activation of sp C
H bonds adjacent to olefins, heteroatoms, and carbonyls.[8]
DMSO
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
[a] M. Kitahara, Dr. K. Hirano, Prof. Dr. T. Satoh, Prof. Dr. M. Miura
Department of Applied Chemistry
Faculty of Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7362
32
65 (64)
12
10[d]
11[d,e]
12[d,f]
E-mail: miura@chem.eng.osaka-u.ac.jp
0
[b] Dr. H. Tsurugi
[a] A solution of 2a (1.0 mmol) in DMAc (2.0 mL) was added over 1 h at
1208C under O2 to a mixture of CuCl2 (1.0 mmol), base (2.0 mmol), and
1a (2.5 mmol) in DMAc (1.0 mL), (see Experimental Section for the de-
tailed procedure). [b] GC yield. Isolated yield is in parentheses. [c] Slow
addition of 2a over 2 h. [d] 1a was added in two portions (1.25 mmolꢁ2)
at 0 and 0.5 h. [e] Under air. [f] Under N2.
Department of Chemistry
Faculty of Engineering Science, Osaka University
Toyonaka, Osaka 560-8531 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902916.
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Chem. Eur. J. 2010, 16, 1772 – 1775

COMMUNICATION
zole derivatives are known to act as ester and amide bioisos-
teres and p-conjugated systems involving the heterocycle
core may have good electron-transporting and hole-blocking
abilities, and alkynyl-substituted azoles work as selective
DNA cleavage agents, this transformation appears to be
also of interest in pharmaceutical and materials chemis-
try.[12,13]
In an initial attempt, treatment of 2-phenyl-1,3,4-oxadia-
zole (1a) with phenylacetylene (2a, slow addition) in the
presence of CuCl2 and NaHCO3 in N,N-dimethylacetamide
(DMAc) at 120 8C under O2 atmosphere (1 atm, balloon)
provided alkynyloxadiazole 3aa in 45% yield (Table 1,
entry 1).[14] Other solvents such as DMF, NMP, and DMSO
resulted in similar yields (entries 2–4). The effect of inorgan-
ic bases was also surveyed, with Na2CO3 and K2CO3 proving
to show higher efficiency (en-
could be used in the reaction and the product yield was ob-
served to increase with increasing the size of alkyl group on
alkyne terminus (3ai–ak). The oxadiazoles with various sub-
stituents at the 2-position were also available for use. 4-
Methyl-, 4-methoxy-, 4-trifluoromethyl-, and 4-chloro-
phenyl-1,3,4-oxadiazoles 1b–1e coupled with 2b to furnish
compounds 3bb–3eb. The reactions of 2-naphthyl- and 2-
phenethyl-substituted oxadiazoles 1 f and 1g with 2b gave
3 fb and 3 fg, respectively, in substantial yields.
Among other azole compounds, oxazoles were found to
be applicable to the present cross-coupling.[15,16] The opti-
mized conditions mentioned above, however, required some
adjustments when 5-aryloxazoles
4
were employed
(Table 3).[17] Thus, the desired 2-alkynyl-5-aryloxazoles 5
were obtained by performing the reaction in a higher 4/2
tries 5–9). We chose Na2CO3
Table 2. Copper-mediated direct cross-coupling of various 1,3,4-oxadiazoles 1 with terminal alkynes 2.[a]
and DMAc as the suitable base
and solvent, respectively, due to
their good generality and repro-
ducibility in the reaction. The
yield further increased by the
modification of the addition
method of 1a. Thus, by adding
the oxadiazole 1a in two por-
tions, the desired 3aa was ob-
tained in 64% isolated yield
(entry 10). Notably, the reaction
proceeded sluggishly or did not
occur under air or N2 (en-
tries 11 and 12)
Product
Yield [%][b]
66
Product
Yield [%][b]
61
We next examined the oxida-
tive coupling of various oxadia-
zoles 1 with various terminal al-
kynes 2 under the conditions
employed for entry 10 of
Table 1 (Table 2). Electron-rich
alkynes 2b and 2c effectively
coupled with 1a to furnish the
corresponding products 3ab
and 3ac, respectively. A slight
decrease of product yield was
observed in the reactions of 1a
with electron-deficient 2d and
2e leading to 3ad and 3ae, be-
cause of somewhat faster diyne
side-product formation (esti-
mated by GC-MS).[14] Alkynes
bearing a thiophene or naph-
thalene unit 2 f and 2g were in-
troduced to the oxadiazole core
without any difficulties (3af
and 3ag). The reaction of 1a
with conjugated enyne 2h gave
the coupling product 3ah with
the olefin moiety left intact.
47[c]
51
45[c]
62
64
70
65
74
50
52
60
63
47
43[d]
[a] Reaction conditions: see Table 1, entry 10. [b] Isolated yield. [c] With 1.5 mmol of Na2CO3. [d] Slow addi-
Moreover, aliphatic alkynes tion of 2b over 2 h.
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M. Miura et al.
Table 3. Copper-mediated direct cross-coupling of 5-aryloxazoles 4 with terminal alkynes 2.[a]
to lead to the product even in
the presence of a stoichiometric
amount of CuCl2 (Table 1,
entry 12).[19]
If the mechanism mentioned
above is reasonable, O2 would
work as an oxidant and render
the reaction catalytic in copper.
Indeed, a catalyst system of
CuCl2/N,N’-dimethylethylenedi-
amine (DMEDA) was found to
show its potential (Scheme 3).
Consequently, work is under-
way toward elucidating the de-
tailed reaction mechanism and
making this unique direct meth-
odology more efficient and
broad utility with other arene
systems.
Product
Yield [%][b]
51
Product
Yield [%][b]
58
50
46
46
42
[a] A solution of 2 (0.5 mmol) in DMSO (1.0 mL) was added over 1 h to a mixture of CuCl2 (0.5 mmol),
Na2CO3 (1.2 mmol), and 4 (2.5 mmol) in DMSO (0.5 mL) at 1508C under O2. 5-Aryloxazoles 4: R1 =Ph; 4a,
R1 =4-MeOC6H4; 4b, R1 =4-CF3C6H4; 4c, R1 =3,4-(MeO)2C6H3; 4d. [b] Isolated yield.
ratio at 1508C with DMSO as solvent. Despite somewhat
lower yields compared to the reaction with the correspond-
ing oxadiazoles, electronically diverse aryl groups at the 5-
position of oxazole were tolerable. In addition, a variety of
terminal alkynes bearing aryl, alkenyl, and alkyl substituents
were compatible under the reaction conditions.
Based on the Stahlꢃs report[11b] and our observations, we
are tempted to assume the reaction mechanism as follows
(Scheme 2). Initial ligand exchange between copper(II) spe-
Scheme 3. Cross-coupling using CuCl2/DMEDA system.
Experimental Section
Typical procedures—copper-mediated direct cross-coupling of 2-phenyl-
1,3,4-oxadiazole (1a) with phenylacetylene (2a)
Method A (Table 1, entry 10): Under an O2 atmosphere (1 atm, balloon),
CuCl2 (134 mg, 1.0 mmol), Na2CO3 (212 mg, 2.0 mmol), 1a (183 mg,
1.25 mmol), DMAc (1.0 mL), and 1-methylnaphthalene (ca. 70 mg, inter-
nal standard) were placed in a 20 mL two-necked reaction flask equipped
with a reflux condenser. The mixture was heated at 1208C, and the slow
addition (over 1 h) of a solution of 2a (102 mg, 1.0 mmol) in DMAc
(1.0 mL) was then started. After a half amount of 2a was added, an addi-
tional portion of 1a (183 mg, 1.25 mmol) in DMAc (1.0 mL) was added
in a single aliquot. After the addition of 2a was completed, the consump-
tion of 2a was checked by GC analysis (13% recovery of 1a). The result-
ing mixture was allowed to cool to room temperature and then quenched
with water. The mixture was extracted with ethyl acetate, and the organic
layer was dried over sodium sulfate. Concentration in vacuo followed by
silica gel column purification with hexane/ethyl acetate (100:0 to 96:4, v/
v) as an eluent gave 3aa (156 mg, 0.64 mmol) in 64% yield.
Scheme 2. Plausible mechanism.
cies 6 and a terminal alkyne followed by the cupration of an
oxadiazole or oxazole with the aid of Na2CO3 affords [CuII-
ACHTUNGTRENNUNG
(alkynyl)(heteroaryl)] (8).[18] Subsequent reductive elimina-
tion produces the corresponding product 3 or 5. The step of
going from [CuII
N
ACHTUNGTRENNUNG
Method B (Scheme 3): Under an O2 atmosphere (1 atm, balloon), CuCl2
(17 mg, 0.125 mmol), DMEDA (22 mg, 0.25 mmol), Na2CO3 (106 mg,
1.0 mmol), 1a (183 mg, 1.25 mmol), DMAc (1.0 mL), and 1-methylnaph-
thalene (ca. 50 mg, internal standard) were placed in a 20 mL two-necked
reaction flask equipped with a reflux condenser. The mixture was heated
at 1208C, and the slow addition (over 3 h) of a solution of 2a (51 mg,
0.5 mmol) in DMAc (2.0 mL) was then started. After the addition of 2a
was completed, the consumption of 2a was checked by GC analysis
(32% recovery of 1a). Product 3aa (68 mg, 0.28 mmol, 55%) was isolat-
ed similarly as above. For product data, see the Supporting Information.
cial for this cross-coupling reation. Thus, the beneficial
effect of the slow addition technique is consistent with the
mechanism. Namely, the activation of another alkyne mole-
cule with 7 gives [CuII
ACTHNUTRGENNUG{bisACHTUNGTREN(NGUN alkynyl)}] (9), en route to the un-
desired diyne.[14,15] Although further studies for clarifying
the exact role of O2 in the reaction are required, it may co-
ordinate to the copper center and enhance the productive
reductive elimination from 8 since the N2 atmosphere failed
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À
C H Activation
COMMUNICATION
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Acknowledgements
This work was partly supported by Grants-in-Aid from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
Keywords: alkynes · copper · direct coupling · heterocycles ·
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Received: October 22, 2009
Published online: January 11, 2010
Chem. Eur. J. 2010, 16, 1772 – 1775
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