Catalyst-controlled selectivity in C-S bond formation: Highly efficient synthesis of C2- and C3-sulfonylindoles

Article abstract of DOI:10.1002/cctc.201500917

Exploring a potential catalyst system for catalyst-controlled selectivity in C-S bond formation is a fascinating challenge. Herein, we described two novel and highly efficient methods for the selective synthesis of C2- and C3-sulfonylindoles showing good biological activities by employing iodide and copper catalysts, respectively. Mechanistic studies point to the crucial role of the electronic properties of the sulfonylated intermediates. I see two and I see three: Two novel and highly efficient methods for the selective synthesis of C2- and C3-sulfonylindoles by employing iodide and copper catalysts, respectively, are described. Mechanistic studies point to the crucial role of the electronic properties of the sulfonylated intermediates. phen=1,10-phenantroline, TBHP=tert-butyl hydroperoxide.

Full text of DOI:10.1002/cctc.201500917

DOI: 10.1002/cctc.201500917
Communications
Catalyst-Controlled Selectivity in CÀS Bond Formation:
Highly Efficient Synthesis of C2- and C3-Sulfonylindoles
Yong Yang, Wanmei Li, Chengcai Xia, Beibei Ying, Chao Shen, and Pengfei Zhang*[a]
Exploring a potential catalyst system for catalyst-controlled se-
lectivity in CÀS bond formation is a fascinating challenge.
Herein, we described two novel and highly efficient methods
for the selective synthesis of C2- and C3-sulfonylindoles show-
ing good biological activities by employing iodide and copper
catalysts, respectively. Mechanistic studies point to the crucial
role of the electronic properties of the sulfonylated intermedi-
ates.
control the site selectivity of indoles and the sulfur-containing
segment is currently a serious concern.
Over the years, copper catalysts have been frequently used
for synthetic organic chemistry;[5] the use of copper salt cata-
lysts has been shown to be successful, because copper pos-
sesses the ability to alter single- and double-electron quantities
through transformation of its four oxidation states: Cu0, CuI,
CuII, and CuIII. Therefore, copper catalysts possess the ability to
associate different functional groups through Lewis acid inter-
actions or p coordination; this has been proven by many sci-
entific experiments.[5a]
The development of suitable catalysts and conditions to pro-
mote a desired CÀS bond-forming reaction is one of the funda-
mental challenges of catalysis.[1] Recently, indole and its deriva-
tives have aroused intense interest as privileged structural
motifs for the pharmaceutical and agrochemical industries;[2]
CÀS bonds attached to indoles have been recognized, because
indole skeletons containing a sulfur moiety, in particular at the
C2, C3, and N positions, are present in many biologically active
molecules (Figure 1).[3] Thus, the synthesis of sulfonylindoles
Consequently, herein the copper-catalyzed 3-sulfonylation of
indoles was successfully implemented through p coordination
of 2C/3C-indole. Over the past few years, iodine-catalyzed sys-
tems have been proven to be powerful tools to form CÀN,
CÀO, and CÀS bonds;[6] these systems have been employed in
radical and ion reactions. In sharp contrast, iodide catalysts are
less explored over the entire field of chemistry.[7] The action of
iodide in this reaction system is similar to that of iodine, but
iodide is much gentler than iodine. Therefore, in terms of
safety and efficiency of the catalyst system, the iodide-cata-
lyzed reactions are significant for synthetic organic chemistry.
In recent years, major advances have been achieved in the
CÀH bond functionalization of indoles;[3] unfortunately, the re-
gioselective formation of CÀS bonds at the C2 and/or C3 sites
of indoles has not been explored much. For this purpose, we
wanted to investigate the area immediately. Currently, sulfur
compounds exist widely in nature, and sources of various sul-
fides for direct sulfonylation include organic and inorganic sul-
fides. Simultaneously, sulfide sources are frequently reported
to have unstable, toxic, and odorous properties, and incorpora-
tion of these functional groups requires multiple synthetic
steps with the generation of waste from reagents, solvents,
and purification; furthermore, undesired byproducts are also
produced. Therefore, we wanted to explore a way to attain the
desirable requirement of atom economy[8] and relative safety.
To solve these problems and on the basis of our previous work
on CÀS couplings,[9] we chose sodium sulfinates as a sulfonyl
source. Sodium sulfinates are stable to air and moisture and
are safe to humans; notably, the reaction generates environ-
mentally benign byproducts, as we expected. During the
course of our phased studies, we found that iodide and
copper salt catalysts allow the direct formation of C2- and C3-
sulfonylindoles, respectively, through CÀH functionalization of
indoles. Above all, the two methods, which operate in air and
in the presence of moisture, produce the products in excellent
yields without the use of an excess amount of the sodium sul-
finate and without a noble metal salt.
Figure 1. Selected drugs containing the sulfonylindole moiety.
through either construction or modification of indole rings by
innovative methods has triggered attention in medicinal and
synthetic chemistry. Although many useful methods of sulfony-
lation at the C2 and/or C3 position(s) of indoles have been re-
ported that indirectly address these problems,[4] direct synthet-
ic methods for the regioselective formation of C2- and/or C3-
sulfonylindoles remain an elusive and unmet goal in synthetic
chemistry. On the other hand, the elaboration of a strategy to
[a] Y. Yang, Dr. W. Li, Dr. C. Xia, B. Ying, Dr. C. Shen, Prof. P. Zhang
College of Material, Chemistry and Chemical Engineering
Hangzhou Normal University
Hangzhou 310036 (P.R. China)
E-mail: chxyzpf@hotmail.com
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500917.
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Initially, indole (1a) and sodium 4-tolylsulfinate (2a) were
used as the standard substrates under different conditions
(Table 1). Originally, C3-sulfonylated indoles were thought to
the use of CuCl (20 mol%), 1,10-phenantroline (10 mol%), KI
(1 equiv.), and DMF (2 mL) at 110 8C in air for 4 h (Table 1,
entry 14). During the course of screening the solvents, it is no-
table and surprising that an analogous organic compound was
observed in 25% yield upon using CuI as the catalyst and
AcOH as the solvent. Next, we did some trials to synthesize
C2-sulfonylated indoles (Table 2). We surveyed various cata-
lysts, including CuI, NaI, NH4I, and KI, in addition to no catalyst,
and we found that NH4I catalyzed the reaction most efficiently
and increased the yield of corresponding product 4a up to
65%. We also screened various oxidants, and the target com-
Table 1. Optimization of the reaction conditions with the use of the
copper salt catalyst.[a]
Entry
Catalyst
[mol%]
Additive
[equiv.]
Ligand
[mol%]
Solvent
Yield
[%][b]
Table 2. Optimization of the reaction conditions with the use of the
iodide catalyst.[a]
1
2
CuCl
CuI
KI
KI
–
–
DMF
DMF
63
44
3
4
5
6
7
8
9
10
11
12
13
14
15[c]
16
17
18
19
20
21
22
CuCl2
CuBr
NiCl2
Pd(OAc)2
–
KI
KI
KI
KI
–
–
–
–
–
–
–
–
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
42
53
trace
20
trace
trace
trace
trace
trace
46
KI
Entry
Catalyst
[mol%]
Additive
[equiv.]
Solvent
Yield
[%][b]
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
PivOH
KBr
K2S2O8
K2CO3
KI
KI
KI
KI
KI
KI
KI
KI
KI
KI
KI
1
2
3
4
5
6
7
8
CuI
NaI
NH4I
KI
–
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
–
–
–
–
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
25
37
65
40
trace
99
90
88
60
67
–
2,2-dipyridyl
PPh3
72
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
86 (44[d]
trace
45
)
–
DMF
TBHP
DTBP
H2O2
AcOOH
TBPB
–
DMSO
NMP
diglyme
EtOH
toluene
anisole
DMF
trace
15
15
trace
trace
86
9
10
11[c]
12[d]
8
–
trace
[a] Conditions: 1a (1 mmol), 2a (1 mmol), catalyst (10 mol%), additive
(1 equiv.), solvent (2 mL), 608C, air, 2 h; DTBP=di-t-butyl peroxide,
TBPB=tert-butyl perbenzoate. [b] Yield of isolated product. [c] Performed
under a N2 atmosphere. [d] 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO,
1 equiv.) was added.
[a] Conditions: 1a (1 mmol), 2a (1 mmol), catalyst (20 mol%), ligand
(10 mol%), additive (1 equiv.), solvent (2 mL), 110 8C, air, 4 h; PivOH=piv-
alic acid. [b] Yield of isolated product. [c] Performed under a N2 atmos-
phere. [d] 808C.
be synthesized. Assuming that a copper catalyst was needed,
the first reaction was performed in the presence of CuCl
(20 mol%) by using 1a (1 mmol) and 2a (1 mmol) as sub-
strates, KI (1 equiv.) as an additive, and DMF (2 mL) as the sol-
vent (Table 1, entry 1); fortunately, the desired product was ac-
quired in 63% yield. Then, other catalysts including CuI, CuCl2,
CuBr, NiCl2, and Pd(OAc)2 were examined, in addition to some
noncatalysts, but only moderate yields of the products were
provided. Subsequently, adding ligands and various additives
were examined. It was disappointing that some ligands and
various additives generated poor yields. For comparison, if the
additive was KI and the ligand was 1,10-phenantroline (1,10-
phen), the desired product was obtained in 86% yield (Table 1,
entry 14). Subsequently, we performed the reaction under a ni-
trogen atmosphere with extraction of air (Table 1, entry 15),
but the product was not detected. The effect of solvent was
also investigated (Table 1, entries 16–21); the use of solvents
such as DMSO, N-methylpyrrolidone (NMP), diglyme, EtOH, tol-
uene, and anisole instead of DMF was less effective for this
transformation. Therefore, the optimal conditions comprised
pound was obtained in 99% yield with the aid of tert-butyl hy-
droperoxide (TBHP, 1 equiv.) and the NH4I (10 mol%) catalyst.
According to our optimal conditions, the scope of the
above-mentioned two systems for a variety of substrates was
investigated. Indoles possessing both electron-donating sub-
stituents (e.g., ÀCH3, ÀOH, and ÀOCH2Ph) and electron-with-
drawing substituents (e.g., ÀBr, ÀCN, ÀCHO, ÀCOOCH3, and
ÀCl) in the benzene ring of the indole tolerated the reaction
conditions, and surprisingly, regardless of whether the sub-
stituent was installed in the ortho, meta, or para position of
the indole, all substrates smoothly coupled to provide the
products in good yields (Table 3). On the other hand, the influ-
ence of the electronic properties was subtle. Disappointingly, if
iodide was used as the catalyst with indoles possessing an
electron-withdrawing substituent (e.g., ÀCN, ÀCOOCH3) in the
benzene ring, the corresponding products were not detected.
For copper salt and iodide catalysts, the reactive points for the
sulfonylation reaction are subtly different. The iodide and
copper salt catalysts promote C2 and C3 sulfonylation if the C2
and C3 positions of the indole are free, respectively. Neverthe-
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Table 3. Scope of the indoles.
Entry
R1
Ar
Yield [%][a] (product)
3[b]
4[c]
1
2
3
4
5
6
7
8
H
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
4-H3CC6H4
C6H5
82 (3a)
80 (3b)
81 (3c)
88 (3d)
89 (3e)
78 (3 f)
82 (3g)
76 (3h)
65 (3i)
72 (3j)
80 (3k)
88 (3l)
75 (3m)
n.r.
93 (4a)
92 (4b)
93 (4c)
96 (4d)
97 (4e)
66 (4 f)
70 (4g)
82 (4h)
trace
4-CH3
5-CH3
6-CH3
7-CH3
5-Br
5-CHO
5-OH
5-CN
5-COOCH3
5-OCH2Ph
6-Cl
2-CH3
3-CH3
H
H
H
Scheme 1. Control experiments.
9
10
11
12
13
14
15
16
17
18
trace
90 (4i)
91 (4j)
90 (3m)
92 (4k)
99 (4l)
98 (4m)
90 (4n)
93 (4o)
86 (3n)
70 (3o)
67 (3p)
60 (3q)
4-ClC6H4
4-H3COC6H4
4-AcNHC6H4
H
19
H
H
58 (3r)
86 (4p)
20
56 (3s)
80 (4q)
21
H
70 (3t)
77 (4r)
[a] Yield of isolated product. [b] Conditions: 1a (1 mmol), 2a (1 mmol),
CuCl (20 mol%), 1,10-phenantroline (10 mol%), KI (1 equiv.), DMF (2 mL),
110 8C, air, 4 h; n.r.=no reaction. [c] Conditions: 1a (1 mmol), 2a
(1 mmol), NH4I (10 mol%), TBHP (1 equiv.), AcOH (2 mL), 608C, air, 2 h.
Scheme 2. Proposed mechanism with the copper catalyst.
A molecule of HX is lost molecule though electrophilic addi-
tion to generate copper complex [II]. Then, interchange of the
anion between copper complex [II] and the sodium sulfinate
occurs to form copper complex [III]. Finally, the reaction cycle
is closed with the aid of oxygen.
less, if the C2 position is occupied by a substituent, the iodide
and copper salt catalysts both promote reaction at the C3 po-
sition of the indole ring. In contrast, if the C3 position of the
indole is occupied, regretfully, only the KI catalyst can perform
sulfonylation at the C2 position of the indole. However, the
result offers powerful testimony as to which reaction site has
priority under iodide and copper salt catalysis. We also evaluat-
ed substituent effects of the sodium sulfinates; explicitly,
4-ClC6H4, 4-CH3OC6H4, 4-AcNHC6H4, 2-naphthyl, 8-quinolinyl, 4-
(3,5-dimethyl-1,2-oxazole), and unsubstituted sodium sulfinates
were utilized for the coupling reaction, and all underwent
smooth coupling with the indole to give the products in mod-
erate to excellent yields with extremely high regioselectivity.
Subsequently, we also investigated the reactions mechanism.
On the basis of our results (Scheme 1) and literature reports,[10]
two plausible mechanisms are proposed. Initially, the outcome
of the Cu-catalyzed mechanism is illustrated (Scheme 2). From
observation of the experiment results, oxygen is necessary for
the copper-catalyzed reaction. With the aid of oxygen, CuI is
transformed into CuII and gains the ability to associate with
the indole through p coordination to form copper complex [I].
The iodide-catalyzed mechanism was also investigated
(Scheme 3). Initially, HI is formed by reaction of the iodide with
acid. It is speculated that HI decomposes at a suitable temper-
ature, and on the basis of this conjecture, hydrogen and an
iodide radical emerge, because if the reaction is performed
without TBHP, the target product will be deoxidized and the
reaction mixture turns purple, which might suggest the forma-
tion of hydrogen and I2. On the basis of literature reports by
Lei[10g] and co-workers and our control experiments, the sulfi-
nate anion is activated by O2 to form an oxidative radical mole-
cule; this was proved by our trial experiments (Scheme 3).
Then, the sulfonate radical associates with the iodide radical to
form RSO2I. Finally, nucleophilic attack of RSO2I at the C2 posi-
tion of the indole is accompanied by loss of HI, which leads to
the formed target product.
In summary, we presented an unprecedented regioselective
sulfonylation reaction of indoles with sodium sulfinates
through electrophilic addition to copper and nucleophilic
attack of p-toluenesulfonyl iodides. The two facile and efficient
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Scheme 3. Proposed mechanism with the iodide catalyst.
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Experimental Section
General procedure for the synthesis of 3-sulfonylindoles 3
A mixture of the (un)substituted indole (1 mmol), (un)substituted
sodium sulfinate (1 mmol), CuCl (20 mol%), 1,10-phen (10 mol%),
and KI (1 equiv.) in DMF (2 mL) was stirred in air at 110 8C for 6 h.
Then, the mixture was extracted with CH2Cl2 (10 mL) and water
(20 mL), the organic phase was separated, and the aqueous phase
was extracted with CH2Cl2 (5 mL). The combined organic layer was
dried with anhydrous Na2SO4, and the solution was then concen-
trated under reduced pressure. The product was purified by flash
column chromatography (200–300 mesh silica gel, petroleum
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General procedure for the synthesis of 2-sulfonylindoles 4
A mixture of the (un)substituted indole (1 mmol), (un)substituted
sodium sulfinate (1 mmol), NH4I (10 mol%), and TBHP (1 equiv.) in
AcOH (2 mL) was stirred in air at 608C for 2 h. Then, the mixture
was extracted with CH2Cl2 (10 mL) and water (20 mL), the organic
phase was separated, and the aqueous phase was extracted with
CH2Cl2 (5 mL). The combined organic layer was dried with anhy-
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Acknowledgements
This work was financially supported by the National Natural Sci-
ence Foundation of China (Nos.21376058, 21302171), the Zhe-
jiang Provincial Natural Science Foundation of China (No.
LZ13B020001), and Key-Sci-Tech Innovation Team of Zhejiang
Province (No. 2010R50017).
Keywords: CÀH activation
heterocycles · iodine
·
copper
·
cross-coupling
·
Received: August 16, 2015
Published online on November 5, 2015
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