Palladium-Catalyzed Direct C-H Functionalization of Indoles with the Insertion of Sulfur Dioxide: Synthesis of 2-Sulfonated Indoles

Article abstract of DOI:10.1021/acs.orglett.7b03365

A palladium-catalyzed direct C-H bond sulfonylation of indoles with the insertion of sulfur dioxide is achieved through a three-component reaction of 1-(pyridin-2-yl)indoles, DABCO·(SO₂)₂, and aryldiazonium tetrafluoroborates under mild conditions. Diverse 2-sulfonated indoles are generated by using 10 mol % of palladium(II) bromide as the catalyst at room temperature. This synthetic approach is efficient by merging palladium catalysis and insertion of sulfur dioxide via a radical process. 2-Pyrimidinyl can be used as the directing group well as for the C-H bond sulfonylation. Additionally, the directing group can be easily removed.

Full text of DOI:10.1021/acs.orglett.7b03365

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Palladium-Catalyzed Direct C−H Functionalization of Indoles with
the Insertion of Sulfur Dioxide: Synthesis of 2‑Sulfonated Indoles
†
†
,†,‡
Tong Liu, Wei Zhou, and Jie Wu*
†
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China
‡
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, China
*
S Supporting Information
ABSTRACT: A palladium-catalyzed direct C−H bond
sulfonylation of indoles with the insertion of sulfur dioxide is
achieved through a three-component reaction of 1-(pyridin-2-
yl)indoles, DABCO·(SO ) , and aryldiazonium tetrafluorobo-
2
2
rates under mild conditions. Diverse 2-sulfonated indoles are
generated by using 10 mol % of palladium(II) bromide as the
catalyst at room temperature. This synthetic approach is efficient by merging palladium catalysis and insertion of sulfur dioxide
via a radical process. 2-Pyrimidinyl can be used as the directing group well as for the C−H bond sulfonylation. Additionally, the
directing group can be easily removed.
o far, significant progress for the direct C−H bond
Scheme 1. Direct C−H Bond Sulfonylation of Indoles with
the Insertion of Sulfur Dioxide
1
S
functionalization in organic synthesis has been made.
Breakthroughs for C−H bond activation or functionalization
2
has been continuously observed in the past decade. The
important role of transition-metal catalysis in C−H bond
functionalizations with diverse directing groups has been
1
d
demonstrated. In the meantime, many efforts have been
given for the incorporation of a sulfonyl (−SO −) moiety into
2
3
small organic molecules due to the importance of sulfonyl
compounds in pharmaceuticals and materials. Among the
4
approaches developed, the insertion of sulfur dioxide into C−X
bonds and C−H bonds would be a direct pathway for the
generation of sulfonyl compounds. In the past few years,
methods for the preparation of sulfonyl compounds by using
DABCO·(SO ) or inorganic sulfites as the source of sulfur
2
2
5
,6
dioxide have emerged. Although approaches for the direct
functionalization of C−H bonds have been developed as
1
mentioned above, there are few reports for the insertion of
7
sulfur dioxide into C−H bonds. In 2014, aminosulfonylation
of arenes, sulfur dioxide, and hydrazines was described in the
presence of gold(III) chloride and palladium acetate as
Recently, we discovered that aryldiazonium tetrafluorobo-
rates would react with sulfur dioxide, providing arylsulfonyl
7
a
8
cocatalysts. In this transformation, the reaction scope for
the C−H bond functionalization of arenes was limited.
Recently, a copper-catalyzed para-selective C−H bond
functionalization of 1-isoquinoline carboxamides with sulfur
radicals under mild conditions. For example, vicinal
difunctionalization of alkenes through a four-component
reaction of aryldiazonium tetrafluoroborates, sulfur dioxide,
alkenes, and hydroxylamines could be accomplished by using
7
b
DABCO·(SO ) or potassium metabisulfite (K S O ) as the
dioxide was reported. A copper-chelated complex was
believed as the key intermediate, which triggered the reaction.
Prompted by these results, we conceived that the directing
groups in the substrates would facilitate the insertion of sulfur
dioxide assisted by transition metal catalysis. Thus, we focused
on 1-(pyridin-2-yl)indoles 1, with an expectation to introduce
sulfonyl group into 2-position of 1-(pyridin-2-yl)indoles via C−
H bond functionalization with the insertion of sulfur dioxide
2 2
2
2
5
8c
source of sulfur dioxide. Encouraged by these results, we
envisioned that aryldiazonium tetrafluoroborates could be
applied in the C−H bond functionalization with sulfur dioxide
as well. A plausible transformation via a three-component
reaction of 1-(pyridin-2-yl)indoles 1, aryldiazonium tetrafluor-
Received: October 28, 2017
(Scheme 1).
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03365
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
oborates 2, and DABCO·(SO ) under palladium catalysis is
proposed in Scheme 1. On the basis of previous reports, we
reasoned that palladium(II) catalyst would coordinate with 1-
indole 3a was obtained in 21% yield when the reaction was
performed in 1,2-dichloroethane (DCE) (Table 1, entry 2).
The structure of 1-(pyridin-2-yl)-2-sulfonated indole 3a was
identified by X-ray diffraction analysis. Reaction in 1,4-dioxane
afforded a trace amount of product (Table 1, entry 3). The
transformation was hampered when the reaction occurred in
DMF (Table 1, entry 4). The corresponding product 3a was
generated in 26% yield when the solvent was changed to
DMSO (Table 1, entry 5). A higher yield was produced when
acetonitrile was used as a replacement (Table 1, entry 6). We
further screened the reaction temperature, and found that the
reaction worked efficiently at 25 °C (Table 1, entries 7−9).
Several palladium catalysts were then examined, which revealed
that palladium bromide was the best choice (Table 1, entries
10−13). Since HBr would be released during the reaction
process, different bases (1.5 equiv) were added (Table 1,
entries 14−18). It was found that the yield could be improved
when potassium carbonate was used as the base (Table 1, entry
15). The yield was increased to 76% when 2.0 equiv of
potassium carbonate was added (Table 1, entry 19). The result
was inferior when the amount of base, 4-methylphenyldiazo-
nium tetrafluoroborate 2a, or palladium bromide was decreased
(Table 1, entries 20−22). A control experiment without the
addition of palladium bromide was performed. However, no
desired product was observed (Table 1, entry 23).
2
2
8
,9
(
pyridin-2-yl)indole 1, accompanied by the C−H bond
activation to produce a palladium-chelated complex A.
Meanwhile, arylsulfonyl radical and tertiary amine radical
cation would be generated from aryldiazonium tetrafluorobo-
8
rates 2 and DABCO·(SO ) . Arylsulfonyl radical would then
2
2
combine with complex A to form palladium(III) intermediate
B, which would be oxidized by tertiary amine radical cation
leading to intermediate C. Subsequently, 2-sulfonated indole 3
would be obtained through reductive elimination, with the
release of palladium(II) catalyst. It seemed that this synthetic
pathway was feasible. We therefore started to explore the
palladium-catalyzed direct C−H bond sulfonylation of indoles
with the insertion of sulfur dioxide.
We initiated our studies by exploring the optimal conditions
for the reaction of 1-(pyridin-2-yl)indole 1a, DABCO·(SO ) ,
2
2
and 4-methylphenyldiazonium tetrafluoroborate 2a in the
presence of palladium catalyst. At the outset, the reaction was
catalyzed by palladium acetate (10 mol %) in toluene at room
temperature (Table 1, entry 1). However, no product was
detected. Pleasingly, the desired 1-(pyridin-2-yl)-2-sulfonated
Table 1. Initial Studies for the Reaction of 1-(Pyridin-2-
yl)indole 1a, DABCO·(SO ) , and Phenyldiazonium
With the above optimized reaction conditions in hand, we
next explored the reaction scope of this C−H sulfonylation
with the insertion of sulfur dioxide starting from 1-(pyridin-2-
yl)indoles 1, DABCO·(SO ) , and aryldiazonium tetrafluor-
2
2
a
Tetrafluoroborate 2a
2
2
oborates 2. The result is presented in Scheme 2. All reactions
worked well to provide 2-sulfonated indoles in moderate to
good yields. Various aryldiazonium tetrafluoroborates 2 were
evaluated with 1-(pyridin-2-yl)indoles 1a and DABCO·(SO ) .
2
2
b
entry
[Pd]
base
solvent
toluene
temp (°C) yield (%)
Different functional groups including halo, trifluoromethyl,
methoxy, and tert-butyl were compatible under the conditions.
For example, the trifluoromethyl-substituted product 3h was
obtained in 56% yield. We subsequently examined the reactions
of diverse 1-(pyridin-2-yl)indoles 1 with DABCO·(SO ) , and
1
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
2
25
25
25
25
25
25
0
nd
21
DCE
1,4-dioxane
DMF
trace
nd
26
2
2
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
4-methylphenyldiazonium tetrafluoroborate 2a. As expected,
these transformations were efficient, giving rise to the desired
products. Interestingly, the aryl bromide remained during the
reaction process (compound 3o).
We further expanded the reaction of indole with another
directing group. 1-(Pyrimidin-2-yl)-1H-indole 4 reacted with
DABCO·(SO2)2 and p-tolyldiazonium tetrafluoroborate 2b
leading to the corresponding product 5 in 73% yield under
the standard conditions (Scheme 3).
31
trace
30
40
50
25
25
25
25
25
25
25
25
25
25
25
25
25
25
28
1
1
1
1
1
1
1
1
1
1
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
Pd (dba)3
30
PdCl2
25
PdBr2
35
Pd(TFA)2
13
c
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
Cs CO
2
21
3
Additionally, the directing group could be easily removed.
For instance, 1-(pyridin-2-yl)-2-sulfonated indole 3a was
c
K CO
2
60
3
c
Na CO
2
41
10
3
treated with MeOTf and NaOH, leading to 2-sulfonated
c
KHCO3
K PO
46
indole 6 in 71% yield (Scheme 4). As shown in Scheme 2, we
proposed that the reaction would be triggered by arylsulfonyl
radical, generated in situ from aryldiazonium tetrafluoroborate
c
57
3
4
d
e
K CO
76
2
3
3
3
3
3
K CO
2
51
2
and DABCO·(SO ) . To confirm the radical intermediate
2 2
d
d
d
,
,
f
K CO
2
62
during the reaction process, we thus added 2.0 equiv of 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) in the reaction of 1-
g
K CO
2
55
K CO
2
nd
(
pyridin-2-yl)indole 1a and DABCO·(SO ) with 4-methyl-
2 2
a
phenyldiazonium tetrafluoroborate 2a under the standard
conditions (Scheme 5, eq a). As expected, we did not detect
the formation of product 3a. In the meantime, ethene-1,1-
diyldibenzene was involved in the reaction of 1-(pyridin-2-
Reaction conditions: 1-(pyridin-2-yl)-1H-indole 1a (0.2 mmol),
DABCO·(SO ) (0.16 mmol), p-methylphenyldiazonium tetrafluor-
2
2
oborate 2a (0.4 mmol), solvent (2.0 mL), under N protection, 8 h.
2
b
c
Isolated yield based on 1-(pyridin-2-yl)-1H-indole 1a. Base (0.3
d
e
f
mmol). K CO (0.4 mmol). K CO (0.24 mmol). p-Methylphe-
nyldiazonium tetrafluoroborate 2a (0.3 mmol). PdBr (5 mol %).
yl)indole 1a, DABCO·(SO ) , and 4-methylphenyldiazonium
2 2
2
3
2
3
g
tetrafluoroborate 2a, with an expectation to trap the in situ
2
B
DOI: 10.1021/acs.orglett.7b03365
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Reaction of 1-(Pyridin-2-yl)indoles 1, DABCO·
Scheme 5. Investigation of the Mechanism
a,b
(
SO ) , and Aryldiazonium Tetrafluoroborates 2
2 2
2
.83, which showed that the C−H bond cleavage might be the
rate-limiting step (Scheme 5, eq c).
In summary, we have reported a palladium-catalyzed direct
C−H bond sulfonylation of indoles with the insertion of sulfur
dioxide through a three-component reaction of 1-(pyridin-2-
yl)indoles, DABCO·(SO ) , and aryldiazonium tetrafluorobo-
2
2
rates under mild conditions. Diverse 2-sulfonated indoles are
generated by using 10 mol % of palladium(II) bromide as the
catalyst at room temperature. 2-Pyrimidinyl can be used as the
directing group as well for the C−H bond sulfonylation.
Additionally, the directing group can be easily removed. This
synthetic approach for introducing a sulfonyl (−SO −) moiety
2
is efficient by merging palladium catalysis and arylsulfonyl
radical generated from DABCO·(SO2)2 and aryldiazonium
tetrafluoroborate. We believe that the insertion of sulfur dioxide
into C−H bond could provide a facile and promising route to
sulfonyl compounds.
a
b
Isolated yield based on 1-(pyridin-2-yl)indole 1. Reaction
conditions: 1-(pyridin-2-yl)-1H-indole 1 (0.2 mmol), DABCO·
(
SO ) (0.16 mmol), aryldiazonium tetrafluoroborate 2 (0.4 mmol),
2
2
K CO (0.4 mmol), PdBr (10 mol %), MeCN (2.0 mL), under N
protection, 25 °C, 8 h.
ASSOCIATED CONTENT
Supporting Information
2
3
2
2
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03365.
Scheme 3. Reaction of 1-(Pyrimidin-2-yl)-1H-indole 1a,
DABCO·(SO ) , and Phenyldiazonium Tetrafluoroborate 2a
2
2
1
Experimental procedures, characterization data, and H,
19
13
F, and C NMR of products (PDF)
Accession Codes
CCDC 1581331 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
Scheme 4. Transformation of Compound 3a
1
EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
formed 4-methylphenyl and 4-methylphenylsulfonyl radical.
Consequently, we did observe and isolate the corresponding
products in 13% and 25% yields, respectively (Scheme 5, eq b).
Furthermore, kinetic isotope experiments (KIE) of substrate 1a
and its deuterated analogue 1a-D1 were performed under the
standard conditions. The value of K /K was measured to be
*E-mail: jie_wu@fudan.edu.cn.
ORCID
Jie Wu: 0000-0002-0967-6360
Author Contributions
T.L. and W.Z. contributed equally.
H
D
C
DOI: 10.1021/acs.orglett.7b03365
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
Ikeda, M.; Saito, T.; Osada, H.; Shirasu, K. Front. Plant Sci. 2012, 3,
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45. (g) Crowley, P. J.; Fawcett, J.; Kariuki, B. M.; Moralee, A. C.;
Percy, J. M.; Salafia, V. Org. Lett. 2002, 4, 4125.
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The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
Chem. 2013, 11, 5393. (b) Deeming, A. S.; Emmett, E. J.; Richards-
Taylor, C. S.; Willis, M. C. Synthesis 2014, 46, 2701. (c) Liu, G.; Fan,
C.; Wu, J. Org. Biomol. Chem. 2015, 13, 1592. (d) Emmett, E. J.; Willis,
M. C. Asian J. Org. Chem. 2015, 4, 602. (e) Zheng, D.; Wu, J. Sulfur
Dioxide Insertion Reactions for Organic Synthesis; Nature Springer:
Berlin, 2017.
Financial support from National Natural Science Foundation of
China (Nos. 21672037 and 21532001) is gratefully acknowl-
edged.
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DOI: 10.1021/acs.orglett.7b03365
Org. Lett. XXXX, XXX, XXX−XXX