Palladium(II)-catalyzed oxidative C-H/C-H cross-coupling between two structurally similar azoles

Article abstract of DOI:10.1002/chem.201103813

Power of two: A widely functional-group tolerant, selective and rapid oxidative cross-coupling between two structurally similar azoles has been carried out by using a palladium/copper co-catalytic twofold C-H activation method (see scheme). Copyright

Full text of DOI:10.1002/chem.201103813

DOI: 10.1002/chem.201103813
À
À
Palladium(II)-Catalyzed Oxidative C H/C H Cross-Coupling
between Two Structurally Similar Azoles
Jiaxing Dong, Yumin Huang, Xurong Qin, Yangyang Cheng, Jing Hao, Danyang Wan,
Wei Li, Xingyan Liu, and Jingsong You*[a]
À
À
The heteroaryl–heteroaryl bond is a vital structural fea-
ture of molecules of medicinal interest, advanced materials,
and naturally occurring products.[1] From the point of view
of synthetic simplicity and atom economy, direct oxidative
cross-coupling between two heteroarenes through a twofold
oxidative C H/C H cross-coupling. Quite recently, Ofial
and co-workers reported an efficient palladium-catalyzed
dehydrogenative cross-coupling of a benzoazole (i.e., benzo-
thiazole and benzimidazole) with an azole at the C2 and C2’
positions in the presence of Pd
ACHTUNGRTEN(NUNG OAc)2 (5–10 mol%) and Cu-
À
C H activation would be the most ideal route to the bi-het-
eroaryl linkage, which obviates the complications associated
with prefunctionalization of both substrates in conventional
ACHTUNGTRENNUNG
AgNO3/KF (1.5+3 equiv) at 1208C for 24–48 h.[10] However,
the two heteroarenes show a clear difference in the proper-
ties of p-conjugated systems. Naturally, we wondered wheth-
er two non-benzofused azoles with more closely related
structures could selectively undergo the oxidative cross-cou-
pling (Scheme 1).[11] However, we may face a series of obsta-
À
À
Ar X/Ar M coupling reactions. However, this type of oxi-
dative cross-coupling is particularly challenging because the
reactivity/selectivity in the two metalation steps of the cata-
lytic cycle must be well-controlled to restrain unwanted ho-
mocoupling.[2] Over the past years, a few groups published
their pioneering reports on the oxidative cross-coupling re-
actions between a directing-group-bearing arene and an
arene,[3] between two simple arenes,[4] and between an arene
and a heteroarene.[5,6] Very recently, the use of palladium
À
À
catalysts allows the oxidative intermolecular C H/C H
cross-coupling of p-electron-rich thiophenes, furans, indoles
and pyrroles with various important classes of heteroarenes
such as xanthines, azoles, and indolizines, pyridine N-
oxides.[7–9] It is noteworthy that all these examples require
the presence of distinctly different p-electronic characteris-
tics between two heteroarenes, which may facilitate an in-
version in reactivity and selectivity.
Azoles are p-electron-excessive five-membered N-hetero-
2
À
arenes with a relatively acidic (sp ) C H bond and have
a wide range of homologous families. The azole motif is one
of the most abundant and relevant heterocycles in pharma-
ceuticals, important biologically active products, materials
sciences, and natural products. Thus, from both academic
and practical standpoints, interconnecting two structurally
similar azoles to form unsymmetrical bisazoles would offer
rich insights into the issues embedded in the metal-catalyzed
Scheme 1. Evolution of transition-metal-catalyzed oxidative (hetero)ary-
À
lation of heteroarenes via double C H activation.
cles: 1) Benzoazoles are more p-electron-excessive and ex-
hibit stronger acidity at the C2 hydrogen atom than the cor-
responding azoles;[12] 2) C2-unsubstituted azoles are particu-
larly prone to undergo oxidative homocoupling[13] and de-
composition under the transition-metal-catalyzed oxidative
conditions; and 3) unlike benzoazoles, azoles may deliver
more regioisomeric products (i.e., C4/C5 besides C2).
In the current work, we focused on evaluating the oxida-
tive coupling reactions of imidazoles, thiazoles, and oxazoles
that are among the most common azoles. In our initial in-
vestigation, we observed that 4,5-dimethylthiazole 2a and 5-
phenyloxazole 1a easily undergo homocoupling. Thus, two
such candidate substrates were chosen as model partners,
[a] J. Dong, Y. Huang, X. Qin, Y. Cheng, J. Hao, D. Wan, W. Li, X. Liu,
Prof. Dr. J. You
Key Laboratory of Green Chemistry
and Technology of Ministry of Education
College of Chemistry, State Key Laboratory of Biotherapy
West China Medical School, Sichuan University
29 Wangjiang Road, Chengdu 610064 (P.R China)
Fax : (+86)28-85412203
E-mail: jsyou@scu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103813.
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COMMUNICATION
Table 1. Oxidative cross-coupling of azoles.[a,b]
Azole1
Azole2
Heterocoupling
product
Yield
[%]
Azole1
Azole2
Heterocoupling
product
Yield
[%]
68, (4, 15),
86[c]
70, (<5,
1
2
12
13
trace), 75[c]
70, (trace,
14), 85[c]
71, (<5,
15), 81[c]
67, (trace,
17), 74[c]
73, (<5,
3
4
14
15
16), 82[c]
66, (15, 18),
75[c]
58, (<5,
trace), 70[c]
70, (11,
67,[c] (<5,
n.d)
5
6
16
17
trace), 90[c]
68, (9, 17),
77[c]
60,[c] (<5,
n.d)
68, (11,
68,[c] (<5,
n.d)
7
8
18
19
trace), 74[c]
66, (7, 15),
72[c]
66,[c] (<5,
n.d)
90, (trace,
5), 94[c]
64,[c] (trace,
n.d)
9
20
73, (<5,
71,[c] (trace,
n.d)
10
11
21
22
trace), 81[c]
78, (<5,
76, (trace,
14), 84[c]
trace), 86[c]
[a] Reaction conditions: azole1 (0.5 mmol), azole2 (0.5 mmol, 1.0 equiv), PdCl2 (5 mol%), PPh3 (10 mol%), CuCl (10 mol%), Cu
ACTHNUTRGNEU(NG OAc)2 (1.5 equiv) and
1,4-dioxane/DMSO (1.0/0.1 mL) at 1208C for 1 h under N2. [b] Isolated yields of 4. The yields in parentheses refer to the homocoupling of azole1 and
azole2, respectively. [c] Isolated product yields. Reaction conditions: azole1 (0.25 mmol), azole2 (0.375 mmol, 1.5 equiv), PdCl2 (5 mol%), PPh3 (10
mol%), CuCl (10 mol%), Cu
mation).
ACHTUNGTERN(NUNG OAc)2 (1.5 equiv) and 1,4-dioxane/DMSO (1.0/0.1 mL) at 1208C for 1–2 h under N2 (For details, see the Supporting Infor-
which gave us a great opportunity to better understand how
to restrain intractable homocoupling and how to efficiently
promote heterocoupling. After screening several parameters
(e.g., palladium source, oxidant, ligand, additive, solvent,
and time; see the Supporting Information, Table S1), the
heterocoupling product 4a was obtained in 68% yield when
PdCl2 (5 mol%) was used in combination with PPh3
(10 mol%), CuCl (10 mol%), and CuACTHNGUTERNNU(G OAc)2 (1.5 equiv) in
1,4-dioxane/DMSO at 1208C for 20 h (see the Supporting
Information, Table S1, entry 14). To our surprise, this trans-
formation could be carried out even in just one hour (see
the Supporting Information, Table S1, entry 17; Table 1,
entry 1), whereas most of the known examples require a re-
action time of one to two days.
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J. You et al.
With the optimized conditions in hand, we first inspected
the scope of this protocol with respect to oxazoles and thia-
zoles. It was gratifying to find that a variety of oxazoles
could couple with diverse thiazoles with the C2/C2’-selectivi-
ty in satisfactory yields (Table 1, entries 1–15). The C4 and/
or C5 unsubstituted thiazoles were amenable to the oxida-
tive coupling reactions at the C2 position of thiazoles in ac-
ceptable yields (Table 1, entries 8, 13, and 15). In addition to
the dehydrogenative couplings between oxazoles and thia-
zoles, the current catalytic system was suitable for the reac-
tions of oxazoles or thiazoles with imidazoles that are often
more sluggish in reactivity than oxazoles and thiazoles
(Table 1, entries 16–21). Worthy of note was that all reac-
tions of imidazoles described herein did not give rise to un-
wanted bisimidazoles. The catalyst system could also
smoothly accelerate the cross-coupling between two thia-
zoles (Table 1, entry 22). More importantly, these reaction
conditions were compatible with the presence of some trou-
blesome functional groups such as ester, aldehyde, terminal
vinyl, nitro, and even free hydroxyl groups to afford highly
functionalized bisazoles, which could then be subjected to
further synthetic transformations. This method could be ap-
plied to azoles with the chloro substituent on the aromatic
ring, which provided a complementary platform for further
functionalizations through traditional transition-metal-cata-
lyzed coupling reactions (Table 1, entry 3). The transforma-
tion was regioselective at the C2 position of azole, and other
regioisomeric products were not yet observed. An X-ray
Scheme 2. Evolution of palladium-catalyzed oxidative couplings of 1e
À
and 2a through double C H activation.
mocoupling to give 2aa and 1ee in 99 and 90% yields, re-
spectively (Scheme 2). However, when these two azoles re-
acted with each other in the ratio of 1: 1, the cross-coupling
reaction occurred almost exclusively to deliver 4i in 90%
yield. In addition, the synthesis of 4i was performed without
problems on a gram scale, which would represent a bench-
scale preparation.
À
À
The catalyst system could also be applied to the C H/C
H cross-coupling of a series of benzofused azoles with vari-
ous azoles. Considering Ofial and co-workers reported the
dehydrogenative cross-couplings of benzothiazole and benzi-
midazoles with azoles,[10] herein we only give one example
of benzoxazole shown in Scheme 3; the detailed results of
these couplings will be published in due course. It is note-
worthy that 93% yield of 4w was obtained even when the
reaction time was reduced to 10 min.
À
analysis of single crystals 4q confirmed that the direct C H/
À
C H cross-coupling took place between the C2 and C2’ po-
sitions on the two azole rings (Figure 1).[14]
The previously reported examples of the oxidative cross-
couplings of (hetero)arenes with arenes have often used up
to 40–100 equivalents of one of the coupling components.[7b]
In this work, the reactions could give satisfactory chemose-
lectivity even when the ratio of two partners is decreased to
1:1. To further demonstrate the overall chemoselectivity,
two competition experiments were performed (Scheme 4).
Oxazole 1e and thiazoles 2a and 2c were added in one pot
in a ratio of 2:1:1 under the standard reaction conditions;
both the yields of 4i and 4k were much better than that of
4x. It demonstrated that if Pd reacted with the oxazole first,
then the resulting oxazole–Pd complex reacted unselectively
Figure 1. ORTEP diagram of 4q. Thermal ellipsoids are shown at the
50% probability level.
It is important to stress that
the thiazoles and oxazoles in-
vestigated herein were suscepti-
ble to homocoupling to afford
a symmetric bisazole under the
optimized conditions (For de-
tails, see the Supporting Infor-
mation, Table S2). For example,
4,5-dimethylthiazole and ethyl
4-methyloxazole-5-carboxylate
Scheme 3. Direct C2/C2’ cross-coupling of benzoxazole and 4,5-dimethythiazole. [a] 4,5-Dimethylthiazole
alone could rapidly undergo ho- (1.5 equiv) was used.
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À
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Palladium(II)-Catalyzed Oxidative C H/C H Cross-Coupling
COMMUNICATION
enes. Considering that the CuI
species might be formed from
CuII salts in the transforma-
tion,[15] Cu
ACTHNUTRGNE(UNG OAc)2 was replaced
by Ag2CO3 as the oxidant to ex-
clude the interference of CuII
sources. The control experi-
ments demonstrated that the
absence of CuCl would greatly
lower a conversion rate of 1a
from 85 to 22%, which clearly
clarified the pivotal role of CuI
salts (see the Supporting Infor-
mation, Table S1, entries 21 and
22).
Although the mechanism is
not well understood at this
stage, on the basis of the above
observations, we proposed that
a plausible catalytic cycle could
À
consist of 1) C H metalation of
azole to form the arylpalladium
species (Azole1)–PdLn through
a carboxylate-assisted concert-
Scheme 4. Competition reactions of oxidative couplings among azoles. [a] Isolated product yields based on 2a.
[b] Isolated product yields based on 2c (For details, see the Supporting Information).
ed
metalation-deprotonation
(CMD) process, 2) reversible
À
C H cupration of the other
with 2a and 2c (or their copper complexes). Alternatively, if
Pd reacted with either of the thiazoles first, the resulting
thiazole–Pd complex reacted selectively with the oxazole (or
its copper complex) over another electronically similar thia-
zole. As compared with the coupling between thiazole 2d
with an electron-withdrawing formyl substituent and the
thiazole 2h with two inductively electron-donating alkyl
groups (Table 1, entry 22), the thiazole 2a reacted with the
electronically quite similar thiazole 2c with a ratio of 1:1 to
give rise to an approximate statistical distribution of prod-
ucts, showing a fairly poor chemoselectivity (Scheme 4). The
above observations implied that a difference in electronic
characteristics between two partners might facilitate a better
selectivity of heterocoupling.
azole,[5g] and subsequent transmetalation with the (Azole1)–
PdLn species to form the key heterocoupling intermediate
(Azole1)–Pd–(Azole2) complex, and 3) productive reductive
elimination. In addition, triphenylphosphine was supposed
to stabilize the arylmetal intermediate, accelerating the
transformation (see the Supporting Information, Table S1,
entries 23–27).
In conclusion, we have discovered a palladium/copper co-
catalytic twofold C H activation that allows, for the first
À
time, the chemo- and regioselective oxidative cross-coupling
between two non-benzofused azoles at the C2 and C2’ posi-
tions. The catalytic system has the following features, includ-
ing: 1) rapidity; 2) wide functional group tolerance; 3) high
selectivity; and 4) an easily available and inexpensive cata-
lytic system. The findings have provided us an inspiration
that the difference in electronic characteristics between the
two structurally similar heteroarenes would facilitate oxida-
To get some mechanistic insights, the following experi-
ments were carried out. Addition of 2,2,6,6-tetramethyl-1-pi-
peridinyloxy (TEMPO, 20 mol%) as a radical scavenger had
a negligible effect on the reaction between 1a and 2a, which
ruled out a radical pathway (see the Supporting Informa-
À
À
tive C H/C H cross-coupling.
tion, Table S1, entry 18). When CuACTHNUTRGNENUG(OAc)2 was replaced by
CuCl2 and CuBr2, only a trace amount of heterocoupling
product was obtained (see the Supporting Information,
Table S1, entries 10 and 11). When Ag2CO3 was used as the
Acknowledgements
This work was supported by grants from the National NSF of China (Nos
21025205, 20972102, and 21021001), Doctoral Foundation of Education
Ministry of China (20090181110045), PCSIRT (No IRT0846) and the Na-
tional Basic Research Program of China (973 Program, 2011CB808600).
We thank the Centre of Testing & Analysis, Sichuan University for NMR
and X-ray measurements.
oxidant, replacement of PdCl2 with PdACHTNUGTRNEUNG(OAc)2 improved the
conversion rate of 1a from trace to 22% (see the Support-
ing Information, Table S1, entries 7 and 21). These observa-
tions hinted that the acetate anion played a critical role in
the transformation.
It is well known that CuI salts have been used as catalyst
À
or activator in direct C H functionalization of N-heteroar-
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J. You et al.
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Keywords: azole · bimetallic catalysis · copper · cross-
coupling · heteroarene · palladium
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Received: December 5, 2011
Revised: February 29, 2012
Published online: April 4, 2012
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Chem. Eur. J. 2012, 18, 6158 – 6162