Palladium-catalyzed direct and site-selective desulfitative arylation of indoles with sodium sulfinates

Article abstract of DOI:10.1002/adsc.201100603

An efficient method was developed for the desulfitative arylation of indoles with sodium sulfinates using palladium as catalyst and copper chloride dihydrate as oxidant. The direct arylation occurred exclusively in the C-2 position of indoles and proceeded well for a range of different substrates. Copyright

Full text of DOI:10.1002/adsc.201100603

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DOI: 10.1002/adsc.201100603
Palladium-Catalyzed Direct and Site-Selective Desulfitative
Arylation of Indoles with Sodium Sulfinates
Mingyue Wu,a Jiaying Luo,a Fuhong Xiao,a Shufeng Zhang,a Guo-Jun Deng,a,*
and He-An Luoa,b,*
a
Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan 411105, Peopleꢀs Republic of China
Fax : (+86)-731-5829-2251; e-mail: gjdeng@xtu.edu.cn
College of Chemical Industry Engineering, Xiangtan University, Xiangtan 411105, Peopleꢀs Republic of China
b
E-mail: hluo@xtu.edu.cn
Received: July 30, 2011; Published online: January 20, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100603.
Abstract: An efficient method was developed for
the desulfitative arylation of indoles with sodium
sulfinates using palladium as catalyst and copper
chloride dihydrate as oxidant. The direct arylation
occurred exclusively in the C-2 position of indoles
and proceeded well for a range of different sub-
strates.
functionalization of indoles and thus affords a short-
cut for the arylation of indoles. Apart from aryl hal-
ides,[9–11] a variety of coupling species, including orga-
noboranes,[12] hypervalent iodine arylating agents,[13]
]15]
arylsiloxanes,[14] and even arene C H bonds, have
À
been explored in the direct arylation reaction of in-
doles. Recently, the widely commercially available ar-
omatic carboxylic acids have been successfully used as
the aryl sources in the direct arylation of various
À
Keywords: arylation; C H activation; desulfination;
indoles; palladium
À
arene C H bonds via decarboxylative coupling reac-
tions.[16,17] This strategy has been also successfully ap-
plied for the direct arylation of indoles, and various
benzoic acids are selectively coupled with indoles in
the C-2 or C-3 position using palladium salts as the
The direct conversion of C H bonds into C C bonds catalysts and silver salts as the oxidants.[18] However,
can potentially lead to more efficient synthesis with a this approach is generally limited to the ortho-substi-
reduced number of synthetic operations and thus has tuted aromatic benzoic acids.
À
À
attracted great interest recently. Since the seminal
Compared with the widely studied carboxylic acid
work reported by Murai et al.[1] and Fujiwara and co- partners, aromatic sulfinic acid sodium salts are rarely
workers,[2] remarkable progress has been made in the used as aryl sources via desulfination. Sulfinic acid
transition metal-catalyzed activation and subsequent sodium salts are relatively stable, easy to handle, and
[3]
À
reaction of C H bonds. In the past few years the when necessary, are accessible preparatively from
2
À
direct arylation of sp C H bonds has emerged as a their corresponding sulfonyl chlorides. Sulfinic acid
À
very powerful synthetic method for rapid C C bond sodium salts have the potential to serve as the ideal
forming reactions.[4] Specifically, palladium has been aryl sources for C C bond forming reactions via re-
À
reported predominantly as a versatile catalyst for con- leasing SO2 under relatively mild conditions.[19] How-
[5]
À
À
structing C C bonds through C H activation.
ever, sulfinic acids (or sodium salts) are mainly used
Indoles and their derivatives exist widely in nature. as sulfonylation reagents and rarely used as aryl sour-
Since the first preparation of indole by Baeyer in ces via desulfitative reactions.[20] We have recently re-
1886,[6] the synthesis of these compounds and their ported a palladium-catalyzed desulfitative Heck-type
functionalization has attracted much attention in both reaction of aromatic sulfinic acid sodium salts with
industrial and academic research because of their bio- various olefins with oxygen as the terminal oxidant
logical and pharmaceutical properties.[7] In particular, under mild conditions.[21] The desulfination proceeded
the direct arylation of indoles at C-2 and/or C-3 via smoothly at 858C, which is much lower than the tem-
À
C H activation has received considerable attention perature required by most of the decarboxylative re-
over the past decade.[8] This method avoids the pre- actions. We also developed an efficient method for
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Mingyue Wu et al.
Table 1. Optimization of the reaction conditions.[a]
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Scheme 1. General reaction.
the synthesis of aryl ketones by palladium-catalyzed
desulfitative addition of aromatic sodium sulfinates to
various nitriles under mild conditions.[22] In continua-
tion of our interest in using aromatic sulfinic acids
(salts) as the aryl sources, herein we describe the first
palladium-catalyzed desulfitative C-2 arylation of in-
À
doles with sodium sulfinates via C H activation
(Scheme 1).
We began our study by examining the reaction of 1-
methylindole (1a) with p-toluenesulfinic acid sodium
salt (2a) in toluene/dioxane by using PdACTHNURGTNEUNG(OAc)2 as cat-
alyst and oxygen as oxidant. No desired product was
detected as determined by GC-MS and 1H NMR
methods (Table 1, entry 1). Other oxidants such as
TBP (tert-butyl peroxide), FeCl3, CuACTHNUTRGNEN(UG OAc)2·H2O and
CuBr2 were inefficient for this kind of transformation
(entries 2–5). To our delight, the desired product (3a)
was obtained in 84% yield when 2 equivalents of
CuCl2·2H2O were used as the oxidant (entry 6).
Other palladium salts were also investigated and
moderate to good yield was obtained (entries 7–13).
Among them, PdACHTUNGTRENU(NG COD)Cl2 showed the best reactivi-
ty, and the desired product 3a was obtained in 88%
yield (isolated yield 73%, entry 13). The solvent had a
significant impact on the reaction yield. The reaction
yield decreased when the ratio of toluene to dioxane
was changed (entries 14 and 15). A much lower yield
was obtained when the reaction was carried out in di-
oxane and DMF, and the desired product was ob-
tained in 23% and 32% yield, respectively (entries 17
and 18). Decreasing the catalyst loading decreased
the reaction yield (entry 19).
With the optimized reaction conditions in hand, we
then explored the scope and generality of this trans-
formation. Various indoles bearing electron-withdraw-
ing and electron-donating substituents were investi-
[a]
Conditions: 1a (0.2 mmol), 2a (0.4 mmol), catalyst
(5 mol%), oxidant (2 equiv.), 1108C, 24 h under argon
unless otherwise noted.
GC-MS yield based on 1a.
2.5 mol% catalyst were used.
[b]
[c]
gated (Table 2). The reaction temperature was in- served when the more bulky substrate 1-pivaloylin-
creased slightly to 1208C and Pd(OAc)2 was used as dole was used.
the catalyst to afford higher reaction yields for indoles The reaction results of various arylsulfinic acid
AHCTUNGTRENNUNG
bearing substituents. Indoles with a methyl group in sodium salts with 1-methylindole (1a) are presented
C-3, C-6 and C-7 all smoothly coupled with 2a and in Table 3. A series of functional groups including
gave the desired products in 80%, 66% and 81% tert-butyl, methoxy, fluoro, bromo and trifluoromethyl
yields, respectively (table 2, entries 2–4). Notably, the was tolerated under the optimal reaction conditions,
methoxy, bromo, fluoro, nitro and even ester moieties and the desired products were obtained in moderate
on indoles were all well tolerated under these reac- to good yields (entries 2–6). Only a trace of product
tion conditions and the desired products were ob- was observed when (4-nitrophenyl)sulfinic acid
tained in good yields (entries 5–10). Under similar sodium salt (2h) reacted with 1a (entry 7). More
conditions, 1-ethylindole (1k) and 1-acetylindole (1l) bulky substrates such as 2-naphthylsulfinic acid
also could couple with 2a to give the desired products sodium salt (2i) also efficiently reacted with 1a and
in 70% and 65% yields, respectively (entries 11 and gave the product in 80% yield (entry 8). In most
12). However, only a trace amount of product was ob- cases, the desulfitative arylation occurred exclusively
336
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Adv. Synth. Catal. 2012, 354, 335 – 340

Palladium-Catalyzed Direct and Site-Selective Desulfitative Arylation of Indoles with Sodium Sulfinates
Table 2. Desulfitative arylation of 2a with various indoles.[a]
[a]
Conditions: 1 (0.2 mmol), 2a (0.4 mmol), PdACHTNUTRGENN(UG COD)Cl2 for entry 1 and PdACHUTNGTREN(NUGN OAc)2 for
entries 2–12 (5 mol%), CuCl2·2H2O (2 equiv.), 1108C, 24 h under argon unless other-
wise noted.
Isolated yield.
1208C.
[b]
[c]
in the C-2 position of indoles as determined by
Although the exact mechanism of this coupling is
1H NMR methods.[23] No desired product was ob- still not clear, on the basis of the results reported by
served when free NH-indole was used as substrate us and others,[24] a plausible mechanism is proposed
under the same conditions.
and shown in Scheme 2. First, the Pd(II) catalyst
reacts with the indole 1 in the C-2 position to form an
Adv. Synth. Catal. 2012, 354, 335 – 340
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337

Mingyue Wu et al.
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Table 3. Desulfitative arylation of 1a with various sodium sulfinates.[a]
[a]
[b]
Conditions: 1a (0.2 mmol),
(2 equiv.), 1108C, 24 h in argon unless otherwise noted.
Isolated yield.
2 (0.4 mmol), PdCAHTUNTGERNU(NG COD)Cl2 (5 mol%), CuCl2·2H2O
Ar-Pd(II)-X intermediate A (X=OAc), which is sub-
sequently displaced by sulfinic acid 2 to form inter-
mediate B. This intermediate species undergoes desul-
fination to generate the aryl-palladium complex C. A
reductive elimination of C affords the desired product
3 and the Pd(0) catalyst is reoxidized to Pd(II) by the
CuCl2, thus closing the catalytic cycle.
In summary, we have demonstrated a novel palladi-
um-catalyzed desulfitative direct arylation of indoles
in the presence of an oxidant. Various aromatic sulfin-
ic acid sodium salts with or without substituents selec-
tively coupled with indoles and exclusively afforded
the C-2 arylated indole adducts. Unlike the decarbox-
ylativie coupling reaction, no electron-withdrawing or
electron-donating group ortho to the sulfinic acid
group was necessary to ensure the desulfitative cou-
pling. Functional groups such as methoxy, bromo,
fluoro, nitro and even ester moieties on the indoles
were all well tolerated under these reaction condi-
Scheme 2. Proposed mechanism.
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Palladium-Catalyzed Direct and Site-Selective Desulfitative Arylation of Indoles with Sodium Sulfinates
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Typical Procedure
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21172185, 20902076), the Hunan Pro-
vincial Natural Science Foundation of China (11JJ1003), the
“One Hundred Talent Project” of Hunan Province
(11QDZ28) and the Undergraduate Investigated-Study and
Innovated-Experiment Plan of Hunan Province.
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340
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 335 – 340