Palladium-catalyzed direct arylation of N-heteroarenes with arylsulfonyl hydrazides

Article abstract of DOI:10.1002/chem.201201450

Hydrazides applied: A palladium-catalyzed direct Ci-H arylation of heteroarenes, with arylsulfonyl hydrazides as the arylating reagents, has been developed (see scheme). The reaction is chemoselective, in that arylsulfonyl hydrazides containing halogen substituents can be employed without participation of the halogen substituent in the reaction. The method offers a straightforward approach to a variety of aryl-heteroaryl compounds. Copyright

Full text of DOI:10.1002/chem.201201450

COMMUNICATION
DOI: 10.1002/chem.201201450
Palladium-Catalyzed Direct Arylation of N-Heteroarenes with Arylsulfonyl
Hydrazides
Bo Liu, Jian Li, Feijie Song, and Jingsong You*[a]
Aryl–heteroaryl substructures are ubiquitous in natural
products, bioactive molecules, functional materials, and
pharmaceuticals. In the past decades, the transition-metal-
Table 1. Optimization of the cross-coupling reaction of caffeine 1 with 4-
methylbenzenesulfonohydrazide 2a.[a]
À
catalyzed direct C H arylation of heteroarenes with aryl
(pseudo)halides for the construction of aryl–heteroaryl
motifs has attracted much attention because it obviates the
use of organometallic reagents, which are generally difficult
to synthesize and are, in some cases, unstable. Therefore,
this method offers a more economical and straightforward
Entry
Oxidant
Additive
Solvent
Yield (%)[b]
1
2
3
4
5
6
7
8
–
O2
BQ
K2S2O8
Ag2CO3
AgOAc
–
–
–
–
–
–
–
–
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
dioxane/DMSO[c]
DMSO
NMP
toluene
dioxane
dioxane
dioxane
dioxane
n.r.
n.r.
n.r.
n.r.
trace
trace
38
approach to preparing compounds containing biACHTNUTRGNE(NUG hetero)aryl
moieties.[1–6] Although significant progress has been made in
this area, the development of an air-stable, readily available,
and inexpensive arylating reagents is still in high demand.
In our continuing interest in the discovery of new arylat-
ing reagents based on arylsulfonyl chlorides,[7] we envisioned
that arylsulfonyl hydrazides could be employed as novel aryl
sources, which would undergo loss of N2 and SO2 gas in situ.
Arylsulfonyl hydrazides are stable in air and can be pre-
pared in one step from readily available arylsulfonyl chlor-
ides and hydrazine hydrates. In the process of this work,
Tian and co-workers reported the palladium-catalyzed Heck
reaction of alkenes with arylsulfonyl hydrazides.[8] However,
Cu
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
A
N
n.r.[d]
44
43
48
60
28
34
56
62
9
G
CuCl
TBAB
PivOH
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
10
11
12
13
14
15
16
17[e]
18[e,f]
19[e,g]
A
N
R
N
N
E
N
G
71
82
94
À
to the best of our knowledge, the direct C H arylation of
G
A
heteroaromatic compounds with arylsulfonyl hydrazides has
not been reported. Compared with the reactions of olefins,
the functionalization of heteroarenes is more challenging be-
cause of the susceptibility of heteroarenes to undergo homo-
coupling and decomposition under the oxidative reaction
conditions. In line with our ongoing endeavors in the func-
tionalization of heterocycles,[9] we herein report our results
[a] Reactions were carried out using PdACHTNUTRGNEUNG(OAc)2 (0.0125 mmol), oxidant
(0.5 mmol), additive (0.025 mmol), caffeine (0.25 mmol), and 4-methyl-
benzenesulfonohydrazide 2a (0.5 mmol) in 1.0 mL solvent at 1208C for
24 h. [b] Yields of isolated product. [c] The ratio of dioxane and DMSO
was 9:1. [d] No PdACHTNUTRGNEUNG(OAc)2 was employed. [e] The reaction temperature
was 1008C. [f] 1.5 equiv of 2a was used. [g] 1.2 equiv of 2a was used.
BQ=benzoquinone, DMSO=dimethyl sulfoxide, NMP=N-methyl-2-
pyrrolidone, n.r.=no reaction, PivOH=pivalic acid, TBAB=tetrabutyl
ammonium bromide.
À
on the palladium-catalyzed direct C H arylation of N-
hetero
reagents.
To test the feasibility of arylsulfonyl hydrazides as aryl
arenes using arylsulfonyl hydrazides as the arylating
sources for the arylation of N-heteroarenes, the coupling re-
action of 4-methylbenzenesulfonohydrazide 2a with caffeine
1 was investigated (Table 1). The nature of the catalyst, oxi-
dant, solvent, and additive were all crucial for a successful
reaction. No reaction occurred in the absence of either cata-
lyst or oxidant (Table 1, entries 1 and 8). Among the various
[a] B. Liu, J. Li, F. Song, Prof. Dr. J. You
Key Laboratory of Green Chemistry and
Technology of Ministry of Education, College of Chemistry, and
State Key Laboratory of Biotherapy, West China Medical School
Sichuan University
oxidants examined, CuACTHNUTRGNE(UGN OAc)2 gave the best results (38%
yield) when the reaction was conducted in a mixture of di-
oxane and DMSO (9:1) as solvent at 1208C for 24 h
(Table 1, entry 7). Other oxidants such as O2, benzoquinone,
K2S2O8, Ag2CO3, and AgOAc were all ineffective (Table 1,
entries 2–6). When the reaction was conducted in the pres-
ence of 10 mol% of PPh3, the product yield was improved
to 60% (Table 1, entry 12). We rationalized that triphenyl-
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.201201450.
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[2]
À
phosphine might stabilize the arylmetal intermediate, thus
promoting the transformation. A number of solvents were
then screened and dioxane was found to be slightly superior
to a dioxane/DMSO mixture (Table 1, entries 12–16). We
found that lowering the reaction temperature to 1008C re-
sulted in better yields (71%; Table 1, entry 17). Notably, re-
ducing the amount of 2a from 2.0 to 1.2 equivalents further
increased the yield up to 94% (Table 1, entries 18 and 19).
Thus, the best results were obtained by stirring the mixture
of N-heteroarene (1.0 equivalent), arylsulfonyl hydrazide
direct C H arylation of N-heteroaromatic compounds.
However, herein the reaction of halogen-substituted arylsul-
fonyl hydrazides exhibited excellent chemoselectivity. Only
the coupling reaction involving the sulfonylhydrazide group
occurred and the halogen group did not participate in the
reaction; this chemoselectivity enables the halogen group to
be used in further transformations (Scheme 1, 3j and 3k).
Xanthines, including caffeines, theobromines, and theo-
phyllines, are imidazole-containing biologically relevant het-
erocycles. For example, 8-arylated xanthines are selective
and potent antagonists that target human A2B adenosine re-
ceptors.[10] They are also used as fluorescent molecules for
cell imaging.[9b,11] To our delight, 8-arylsubstituted xanthines
could also be synthesized in excellent yields by using our de-
veloped method (Scheme 2, 4a–c). Unfortunately, when pu-
(1.2 equivalent), Pd
ACHTUNGTRENNUNG(OAc)2 (5 mol%), PPh3 (10 mol%), and
Cu(OAc)2 (2.0 equivalents) in dioxane at 1008C for 24 h.
ACHTUNGTRENNUNG
Only a trace amount of homocoupling products derived
from the heteroaromatic compounds and the arylsulfonyl
hydrazides were observed under these reaction conditions.
The scope of the reaction with respect to the arylsulfonyl
hydrazide was subsequently examined in the coupling reac-
tion with caffeine under the optimized reaction conditions
(Scheme 1). We found that a variety of arylsulfonyl hydra-
Scheme 2. Palladium-catalyzed cross-coupling reactions of N-heteroar-
enes with arylsulfonyl hydrazides. Reactions were carried out using Pd-
ACHUNTGRENUN(G OAc)2 (0.0125 mmol), CuHCAUTNGTREN(NUGN OAc)2 (0.5 mmol), PPh3 (0.025 mmol), hetero-
arene (0.25 mmol), and arylsulfonyl hydrazide (0.3 mmol) in dioxane at
1008C for 24 h. The yields of the isolated products are given in parenthe-
ses. [a] Reactions were carried out using Pd
AHCTUNGTRENNUNG
ACHTUNGRTEN(NUNG OAc)2 (0.0125 mmol), Cu-
Scheme 1. Palladium-catalyzed cross-coupling reactions of caffeine with
a variety of arylsulfonyl hydrazides. Reactions were carried out using Pd-
(0.3 mmol) in dioxane/DMSO at 1208C for 24 h. For some products, the
ACHTUNGTRENNUNG(OAc)2 (0.0125 mmol), CuCAHUTGNTRENN(GUN OAc)2 (0.5 mmol), PPh3 (0.025 mmol), caf-
feine (0.25 mmol), and arylsulfonyl hydrazide (0.3 mmol) in dioxane at
À
newly formed C C bond is indicated using a bold bond to avoid ambigui-
ty as to which aryl group has been incorporated in the reaction.
1008C for 24 h. The yields of the isolated products are given in parenthe-
ses.
rines and azoles were subjected to the same reaction condi-
tions, most of the starting materials were recovered. Howev-
er, changing the solvent from dioxane to the mixture of di-
oxane/DMSO (9:1) and increasing the reaction temperature
to 1208C significantly improved the reaction, thus affording
the products in 64–87% yield (Scheme 2, 4d–n). Notably,
zides could couple with caffeine, thus afforded the desired
products in good to excellent yields, no matter if the arylsul-
fonyl hydrazides were substituted with electron-donating,
electron-withdrawing, or sterically hindered functional
groups. The presence of halogen, nitro, and trifluoromethyl
groups were compatible with the reaction conditions. Aryl
halides are widely used as the arylating reagents for the
À
both the C2 and the C5 position of azoles participate in C
H arylation reactions.[2d] However, when using the reaction
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Direct Arylation of N-Heteroarenes
COMMUNICATION
zide (0.30 mmol), CuACHTUNRGTNEUNG(OAc)2 (90 mg, 0.5 mmol), PPh3 (6.6 mg,
conditions described herein, the 2,5-unsubstituted azoles se-
lectively coupled with the sulfonyl hydrazides at the C2 po-
sition to give the desired product in 87% yield; the C5 aryl-
ACHTUNGTRENNUNGation reaction did not occur (Scheme 2, 4i). Besides the N-
heteroarenes mentioned above, quinoxaline N-oxide could
0.025 mmol), and solvent (1.0 mL). The tube was then capped with
a rubber septum, and evacuated and backfilled with N2 (the latter process
was repeated). The reaction mixture was stirred at room temperature for
5 min and then heated at the indicated temperature for 24 h. The result-
ing solution was cooled to ambient temperature, diluted with 20 mL of
CH2Cl2, filtered through a Celite pad, and washed with 10–20 mL of
CH2Cl2. The combined organic phase was evaporated and the residue
was purified by column chromatography on silica gel to provide the de-
sired product.
À
also undergo the C H arylation reaction, although the yield
of product 4o was relatively low (Scheme 2, 42%).
Although the mechanism of the C arylation of N-hetero-
arenes with arylsulfonyl hydrazides is not very clear at pres-
ent, we propose a catalytic cycle based on the experimental
results and proposed mechanisms reported in the literature
(Scheme 3). The reaction of PdACHTNUGRTENUNG(OAc)2 with arylsulfonyl hy-
Acknowledgements
This work was supported by grants from the NSF of China (Nos
21025205, 20972102, 21021001, and J1103315 J0104), and the National
Basic Research Program of China (973 Program, 2011CB808600). We
thank members of the Centre of Testing & Analysis, Sichuan University
for NMR measurements.
À
Keywords: arylsulfonyl hydrazides · C H arylation · cross-
coupling · nitrogen heterocycles · palladium
À
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Scheme 3. Proposed mechanism for the palladium-catalyzed C H aryla-
tion of N-heteroaromatic compounds with arylsulfonyl hydrazides.
drazide gives an ArPdII intermediate (IM1), which is formed
through successive deprotonation of the arylsulfonyl hydra-
zide, b-hydride elimination, and liberation of N2 and SO2
gas.[8] Subsequent transmetallation with the heteroarene–
copper(I) species IM2 and reductive elimination of the re-
sulting intermediate IM3 gives the desired cross-coupling
product.[12] PdII species are then regenerated by the oxida-
tion of the Pd0 species with Cu
organocopper(I) species IM2 in the catalytic cycle is sup-
ported by the fact that oxidants other than Cu(OAc)2 are in-
ACHTUNGRTEN(NUNG OAc)2. The involvement of
AHCTUNGTRENNUNG
effective in the coupling reactions (Table 1). We suspect that
a CuI species is generated from either the in situ reduction
or disproportionation of CuACHTNUGRTNEUNG
(OAc)2;[13] the CuI species then
reacts with azoles to give copper complex IM2. The CuI-cat-
À
alyzed C H functionalization of nitrogen-containing hetero-
cycles has also been well documented.[14]
In conclusion, we have demonstrated that readily avail-
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can be used as arylating reagents for the direct palladium-
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Experimental Section
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Received: April 27, 2012
Revised: June 21, 2012
Published online: && &&, 0000
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Direct Arylation of N-Heteroarenes
COMMUNICATION
Cross-Coupling Reactions
B. Liu, J. Li, F. Song,
J. You* ......................... &&&& — &&&&
Palladium-Catalyzed Direct Arylation
of N-Heteroarenes with Arylsulfonyl
Hydrazides
Hydrazides applied: A palladium-cata-
hydrazides containing halogen sub-
stituents can be employed without par-
ticipation of the halogen substituent in
the reaction. The method offers
a straightforward approach to a variety
of aryl–heteroaryl compounds.
À
lyzed direct C H arylation of heteroar-
enes, with arylsulfonyl hydrazides as
the arylating reagents, has been devel-
oped (see scheme). The reaction is
chemoselective, in that arylsulfonyl
Chem. Eur. J. 2012, 00, 0 – 0
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