Copper-catalyzed arylsulfonylation of N-arylsulfonyl-acrylamides with arylsulfonohydrazides: Synthesis of sulfonated oxindoles

Article abstract of DOI:10.1039/c4ob01231c

A copper-catalyzed arylsulfonylation of N-arylsulfonyl-acrylamides with sulfonylhydrazides through a tandem radical process was developed. This methodology provided an alternative strategy for the synthesis of sulfonated oxindoles by forming C-S, C-N and C-C bonds in a single operation. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob01231c

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Copper-catalyzed arylsulfonylation of
N-arylsulfonyl-acrylamides with
arylsulfonohydrazides: synthesis of sulfonated
oxindoles†
Qingshan Tian,a Ping Hea and Chunxiang Kuang*a,b
Cite this: Org. Biomol. Chem., 2014,
12, 6349
Received 14th June 2014,
Accepted 8th July 2014
DOI: 10.1039/c4ob01231c
www.rsc.org/obc
A copper-catalyzed arylsulfonylation of N-arylsulfonyl-acrylamides bioactivities.10 Recently, difunctionalizaton of alkenes pro-
with sulfonylhydrazides through a tandem radical process was vided a powerful strategy for the synthesis of various organic
developed. This methodology provided an alternative strategy for compounds.11 In particular, transition metal catalyzed difunc-
the synthesis of sulfonated oxindoles by forming C–S, C–N and tionalization of alkenes provides some versatile strategies for
C–C bonds in a single operation.
the synthesis of various functionalized oxindoles. For example,
Fe,12 Ag,13 Cu,14 and Pd15 catalyzed oxidative tandem difunc-
tionalization/cyclization reactions of alkenes in arylacryl-
amides have been independently developed. However,
examples of arylsulfonylation of activated alkenes via the
radical pathway to prepare sulfonated oxindoles are quite rare.
Jiao’s and Li’s groups have independently developed oxidative
arylsulfonylation of activated alkenes to synthesize a variety of
Organosulfones play very important roles in organic chemistry.
They can be found in a wide range of agrochemicals and phar-
maceuticals.1 In addition, the sulfone group is regarded as a
highly valuable class of precursors for various chemical trans-
formations in organic synthesis.2 In general, the main syn-
thetic route for the preparation of organosulfones is the
oxidation of the corresponding sulfides.3 The reaction of
organomagnesium halide compounds with sulfonate esters
has also been described.4 Moreover, transition-metal-catalyzed
synthesis of organosulfones from aryl halides or triflates has
also been developed.5 However, these methods suffer from
harsh reaction conditions, isomeric products and poor toler-
ance of functional groups. Therefore, the development of a
more concise and efficient method for organosulfone synthesis
is highly desirable. Recently, direct addition of a sulfonyl
radical to C–C multiple bonds provided a particularly useful
contribution to the synthesis of organosulfones.6 For example,
Lei et al.’s group reported the aerobic sulfonylation of alkenes
and alkynes using aryl sulfinic acids as sulfonyl radical precur-
sors in the presence of pyridine and O2.7 Recently, Wang et al.
also reported a copper-catalyzed oxysulfonylation of alkenes
with dioxygen and sulfonylhydrazides, leading to β-ketosul-
fones in good yields.8 Furthermore, the addition of a sulfonyl
radical to C–C double bonds has been extensively studied.9
Oxindoles and their derivatives represent a large class of
N-heterocycles, which exhibit advanced pharmaceutical and
sulfonated oxindoles through
a carbon radical process;
however, these methods could not synthesize 6-substituted oxi-
ndoles efficiently (Scheme 1a).16 Recently, Nevado’s group
developed a new tandem radical strategy for the construction
of functionalized oxindoles through an amidyl radical pathway
(Scheme 1b).17 Based on these studies, we report an alternative
strategy for the synthesis of sulfonated oxindoles; especially,
the synthesis of various 6-substituted sulfonated oxindoles was
more efficient than previous methods. In this reaction, the sul-
aDepartment of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092,
P. R. China. E-mail: kuangcx@tongji.edu.cn
bKey Laboratory of Yangtze River Water Environment, Ministry of Education,
Siping Road 1239, Shanghai 200092, P. R. China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ob01231c
Scheme 1 Difunctionalization of alkenes.
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fonyl radical first formed with the catalysis of Cu(OTf)2 from sulfate. No improvement was achieved by increasing catalyst
arylhydrazides using potassium persulfate as the terminal loading to 40 mol% (Table 1, entry 13). In contrast, when cata-
oxidant, and then the sulfonyl radical triggers the tandem lyst loading decreased to 10 mol%, the yield was reduced to
radical process to construct sulfonated oxindoles through an 61% (Table 1, entry 14). The screening results of oxidants,
amidyl radical intermediate (Scheme 1c).
such as DTBP, DCP, and TBHP, indicated that K2S2O8 was the
We initiated our studies by examining N-methyl-N-phenyl- more efficient oxidant in this reaction (Table 1, entries 15–18).
sulfonylmethacrylamide (1a) with tosyl hydrazide (TsNHNH2) The control experiment showed that the presence of Cu(OTf)2
in the presence of a catalytic amount of copper(I) chloride and is essential in this reaction (Table 1, entry 19).
K2S2O8 (3 equiv.) as the oxidant at 80 °C for 16 hours (Table 1).
With the optimized reaction conditions in hand, we next
Gratifyingly, the desired product (3a) was isolated in 38% yield investigated the scope of sulfonohydrazides in this reaction
(Table 1, entry 1). Inspired by this result, we investigated system (Table 2). In most cases, arylsulfonohydrazides were
various copper salts in this reaction, such as CuBr, CuI, smoothly converted to sulfonated oxindoles in moderate to
Cu(OAc)2, CuSO4, and Cu(OTf)2 (Table 1, entries 2–6). The good yields. Arylsulfonohydrazides with electron-donating
results showed that Cu(OTf)2 was more efficient than the other groups reacted well and a majority of the products (3a–3c)
copper salts, and the desired product (3a) was isolated in 81% were obtained in good yields. In addition, arylsulfonohydr-
yield (Table 1, entry 6). In addition, decreasing the tempera- azides with electron-withdrawing groups also produced the
ture caused the yield to descend (Table 1, entry 7). No evident desired compounds (3i–3k) in moderate yields. Gratifyingly,
improvement in yield was obtained when the reaction was con- arylsulfonohydrazides with halogen atoms (F, Cl, Br) were well
ducted at an elevated temperature (100 °C) (Table 1, entry 8). tolerated and provided the corresponding products in good
Various solvents were also examined (CH3CN, H2O, DMF and yields (3d–3h). When the benzene ring of the substrates was
DMSO) and the result showed that water as a co-solvent is ben- changed to a naphthalene ring, the reaction successfully pro-
eficial for this reaction (Table 1, entries 9–12), presumably vided the desired product 3l in good yield. However, the
owing to its ability to improve the solubility of potassium per-
Table 2 Copper-catalyzed reaction of 1a with arylsulfonohydrazidesa
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst (mol%)
Oxidant
Yieldb (%)
1
2
3
4
CuCl (20)
CuBr (20)
CuI (20)
Cu(OAc)2 (20)
CuSO4 (20)
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
38
41
58
20
35
81
49
83
21
37
55
68
80
61
Trace
50
44
53
15
5
6
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (40)
Cu(OTf)2 (10)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
Cu(OTf)2 (20)
7c
8d
9e
10f
11g
12h
13
14
15
16
17
18
19
DTBP
DCP
TBHP
K2S2O8
a Reaction conditions: 1a (0.2 mmol), PhSO2NHNH2 (0.4 mmol),
copper catalyst (0.04 mmol) and oxidant (0.6 mmol) in CH3CN–H2O
(2/1, 1 mL), 80 °C under Ar for 16 h. b Isolated yields. c Reaction
temperature: 60 °C. d Reaction temperature: 100 °C. e CH3CN used as a
solvent. f H2O used as a solvent. g DMF used as a solvent. h DMSO used a Reaction conditions: 1a (0.2 mmol),
2 (0.4 mmol), Cu(OTf)2
as a solvent. TBHP = tert-butyl hydrogen peroxide (70% in water), (0.04 mmol) and K2S2O8 (0.6 mmol) in CH3CN–H2O (2/1, 1.0 mL),
DCP = dicumyl peroxide, DTBP = di-tert-butyl peroxide.
stirred at 80 °C for 18 h. Isolated yields are given.
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desired products were not obtained from the reaction of 1a including butyl, cyclohexyl and benzyl groups, showed good
with alkyl-substituted sulfonohydrazides, such as ethyl tolerance under the optimized reaction conditions, providing
sulfonohydrazide.
the corresponding oxindoles (4h–4j) in moderate yields.
We next explored the scope of N-arylsulfonylacrylamides in Notably, N-acetyl and N-free substrates were not reactive at all
the reaction system (Table 3). In general, the reaction tolerated (4k and 4l). Meanwhile, no reaction occurred in the case of the
a wide variety of substrates with both electron-donating and monosubstituted alkene in the substrate (R3 = H) (4m).
electron-withdrawing substituents to produce the desired pro-
Several copper-catalyzed difunctionalizations of alkenes are
ducts (4a–4m) in moderate to good yields. Substrates bearing well known to proceed through a radical pathway;8,14 thus, our
electron-donating groups afforded the desired products in method for the arylsulfonylation of N-arylsulfonylacrylamides
high yields (4a–4c). It was worth noting that halogen atoms may also proceed through a radical pathway. In order to gain
(F, Br) on substrates were also tolerated in this reaction (4d, more insights into the reaction mechanism, we used the well-
4e); thus these halogen atoms enabled further elaboration of known radical-trapping reagents TEMPO (2,2,6,6-tetramethyl-
the products into more complex molecules. In addition, sub- piperidine-N-oxyl) and BHT (2,6-di-tert-butyl-4-methylphenol)
strates containing electron-withdrawing substituents were also to elucidate the mechanism. As illustrated in Scheme 2, the
amenable to the reaction conditions, offering the corres- addition of 2.5 equiv. of TEMPO and BHT suppressed the aryl-
ponding products (4d, 4f) in moderate yields. Highly electron- sulfonylation process, thus suggesting that this transformation
deficient substituents (NO2 and CN) on the aryl moiety did not might involve a radical process. We also explored the regio-
result in any desired products. When the benzene ring of the selectivity of this radical reaction. Substrate (5) with a benzene
substrate was changed to a naphthalene ring, the reaction suc- ring on the nitrogen atom of N-arylsulfonylacrylamides offered
cessfully provided the desired product 4g in good yield. Fur- three sites for cyclization (a, b and 5-ipso sites); sites a and b
thermore, different protecting groups on the nitrogen atom, would produce an oxindole derivate (6b) and a 6-member ring
product (6c) respectively. Interestingly, the reaction proceeded
on the 5-ipso site with high site control and a single product
Table 3 Copper-catalyzed reaction of tosylhydrazide with 1a
6a was isolated in 64% yield. Presumably, an amidyl radical in
this reaction can undergo hydrogen abstraction from the
medium to give amide 6a.18
Based on the experimental results and previous studies, we
tentatively proposed
a reaction mechanism (Scheme 3).
Scheme 2 Mechanism consideration and regioselectivity experiment.
a Reaction conditions: 1 (0.2 mmol), TsNHNH2 (0.4 mmol), Cu(OTf)2
(0.04 mmol) and K2S2O8 (0.4 mmol) in CH3CN–H2O (2/1, 1.0 mL),
stirred at 80 °C for 18 h. Isolated yields are given.
Scheme 3 A plausible reaction mechanism.
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Initially, the sulfonyl radical 7 was generated from 2 via single
electron transfer and the deprotonation process. Then, the sul-
fonyl radical 7 was added to substrate 1a, leading to a new C–S
bond formation and a radical intermediate 8. A 5-ipso cycliza-
tion then took place on the aromatic ring, generating a C–C
bond and aryl radical 9, which would undergo rapid desulfony-
lation to form the key amidyl radical 10 with the extrusion of
sulfur dioxide. The amidyl radical 10 would then undergo
cyclization onto the aromatic ring to produce oxindole 3. In
this transformation, the oxidant was required in stoichiometric
amounts to complete the final re-aromatisation of the inter-
mediate of 11.
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