Rhodium(III)-Catalyzed Cross Coupling of Sulfoxonium Ylides and 1,3-Diynes to Produce Naphthol-Indole Derivatives: An Arene ortho C?H Activation/Annulation Cascade

Article abstract of DOI:10.1002/cctc.202001315

Synthesis of naphthol-indoles starting from easily available 1,3-diynes and sulfoxonium ylides via Rh^III-catalyzed C?H activation/hydroamination cascade process in a one pot manner has been developed. The reaction proceeds through a Rh^III-catalyzed arene ortho C?H activation and silver-catalyzed hydroamination cascade to afford a variety of C2 position functionalized indoles in good yields with broad substrate scope and good functional group tolerance.

Full text of DOI:10.1002/cctc.202001315

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Rhodium(III)-Catalyzed Cross Coupling of Sulfoxonium Ylides
and 1,3-Diynes to Produce Naphthol-Indole Derivatives: An
Arene ortho C–H Activation/Annulation Cascade
Authors: Min Shi, Ruixing Liu, and Yin Wei
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To be cited as: ChemCatChem 10.1002/cctc.202001315
Link to VoR: https://doi.org/10.1002/cctc.202001315
01/2020

10.1002/cctc.202001315
ChemCatChem
COMMUNICATION
Rhodium(III)-Catalyzed Cross Coupling of Sulfoxonium Ylides
and 1,3-Diynes to Produce Naphthol-Indole Derivatives: An Arene
ortho C–H Activation/Annulation Cascade
Ruixing Liu,[a] Yin Wei,[a] Min Shi*[a,b]
Abstract: Synthesis of naphthol-indoles starting from easily
available 1,3-diynes and sulfoxonium ylides via Rh(III)-catalyzed C-H
activation/hydroamination cascade process in a one pot manner has
been developed. The reaction proceeds through a Rh(III)-catalyzed
arene ortho C-H activation and silver-catalyzed hydroamination
cascade to afford a variety of C2 position functionalized indoles in
good yields with broad substrate scope and good functional group
tolerance.
Indoles can be efficiently synthesized using ethynylaniline
derivatives through intramolecular hydroamination upon metal or
Lewis acid catalysis.[5] Therefore, we designed and synthesized
1,3-diynes tethering an amino group at the proximal position,
rendering that the functionalized diynes can distinguish the two
alkyne units to solve chemo- and regioselectivity in the next
cascade reaction.
Biaryl scaffold plays important roles in numerous areas of
organic chemistry.[1] For example, a variety of 2, 2-bipyridine
derivatives are often used as ligands in transition metal-
catalyzed chemical transformations. The synthetic methods on
the construction of biaryl compounds basically utilize
cross/homo coupling strategy, in which functionalized arenes are
frequently employed as the coupling reaction partners (Scheme
1A).[2] Recently, the direct C-H functionalization of arenes has
made this synthetic protocol more straightforward, obviating the
need for prefunctionalization of one arene in the coupling
reaction.[3] Besides the coupling of two aromatics, there is
another strategy to construct these biaryl scaffolds, starting from
diyne molecules. 1,3-Diynes have been thoroughly investigated
since the discovery of the Glaser-Hay Reaction in 1869. They
are easily available by a number of synthetic methods and can
be widely used in the synthesis of arenes and heteroarenes. For
example, an elegant C-H activation/1,3-diyne cascade strategy
for the synthesis of bisheterocycles has been reported by
Glorius and Volla, respectively (Scheme 1B).[4] Thus, the
introduction of 1,3-diyne moiety can avoid the use of coupling
strategy to construct C-C bond connecting two aryls, which
could largely simplify the synthetic procedures.
Scheme 1. Synthesis of adjacent biaromatics.
Sulfoxonium ylides have recently demonstrated a wide utility
in C−H activation, because of their high flexibility, stability, and
diverse reactivity.[6] Employment of sulfoxonium ylide as an
arene substrate bearing a built-in directing group provides an
efficient method to synthesize aromatic derivatives. In 2017, Li
and co-workers reported [4+2] annulation of sulfoxonium ylides
with alkynes for the rapid construction of substituted naphthols,
in which sulfoxonium ylide functioned as a carbene-like directing
group (DG), affording functionalized aromatic compounds.[7]
Given the rapid assembly of otherwise hard-to-access skeletons,
operational simplicity, and controllable selectivity, sulfoxonium
ylides provide good sources in the synthesis of related fused
carbocyclic scaffolds. Based on its unique property, Prabhu
reported an example to synthesize furanone-fused naphthol
derivatives by domino Rh(III)-catalyzed C−H activation,
regioselective annulations, and lactonization.[6l]
However, utilizing 1,3-diynes in C-H activation instead of
monoalkynes
would
face
several
challenges:
(a)
chemoselectivity between the two alkyne units (Scheme 1C); (b)
regioselectivity of the migratory insertion process; and (c)
selectivity between mono- and bifunctionalization. One of the
solutions on above problems is to make the two alkyne units
having significant discrepancy on the cyclization reactivity.
Herein, we wish to report our findings on rhodium-catalyzed
synthesis of naphthol-indoles through annulation of sulfoxonium
ylides and diynes. Furthermore, by taking advantage of
intramolecular hydroamination, we will also indicate that the
functionalized 1,3-diynes have good chemo- and regioselectivity
in this C-H activation/1,3-diyne/hydroamination cascade reaction
(Scheme 1, this work).
The initial optimization of the reaction conditions was
conducted with sulfoxonium ylide 1a and diyne 2a as substrates,
and all of the results are summarized in Table 1. When the
reaction of 1a with 2a was carried out in the presence of
[a]
R. Liu, Dr. Y. Wei, Prof. Dr. M. Shi
State Key Laboratory of Organometallic Chemistry, Center for
Excellence in Molecular Synthesis, University of Chinese Academy
of Sciences, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 345 Linglin Lu, Shanghai 200032, China.
Email: mshi@mail.sioc.ac.cn. Fax: 86-21-64166128
Prof. M. Shi,
[b]
Shenzhen Grubbs Institute, Southern University of Science and
Technology, Shenzhen, 518000 Guangdong, China
Supporting information for this article is given via a link at the end of
the document
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10.1002/cctc.202001315
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[Cp*RhCl2]2 (4 mol%), AgSbF6 (20 mol%), PivOH (2.0 equiv),
and Zn(OAc)2 (2.0 equiv) in DCE at 70 °C for 12 h, we were
pleased to observe the formation of the corresponding naphthol-
indole 3aa in 67% yield (entry 1). Since the use of zinc salt was
crucial in the reaction, we next examined several other zinc salts
in the reaction (for the initial examination of the reaction, see
Table S1 in the Supporting Information). On the examination of
various zinc salts such as Zn(OAc)2, Zn(OTf)2, Zn(NTf)2, it was
found that the addition of Zn(OTf)2 could afford 3aa in 78% yield
(entries 1-3). However, using Zn(OTf)2 as an additive, the
obtained product contained some impurities that were difficult to
be separated from the desired product 3aa (entry 3). The use of
other zinc salts such as zinc oxide and chloride could not afford
the corresponding product 3aa, suggesting the crucial role of
counterion in zinc salt (entries 4-5). In addition, zinc glycinate
was proved to have similar effect as that of zinc acetate (entry 6),
presumably due to that its counterion has the similar role as that
of acetate. Other additives such as Cu(OAc)2, Cu(OTf)2, AgOAc
AgOTf and NaOAc, which have been widely used in the
previously reported C−H activation reactions, were inactive in
this transformation, highlighting the crucial role of zinc acetate
as the additive (entry 7). Moreover, Lewis acids as ln(OTf)3 and
Yb(OTf)3 are also totally invalid for this transformation (entry 7).
This observation may be due to the highly coordinating ability
between zinc acetate and oxygen atom in this transformation.
The exploration of solvent effects revealed that the use of
ethanol as solvent could give 3aa in 95% yield, which was better
than others including DCE, MeCN and TFE (entries 8-10).
sulfoxonium ylides containing either electron-donating or
t
electron-withdrawing groups such as F, Me, Bu, OMe and CF3
could be all tolerated, giving the corresponding indole products
3ba-3ga in good yields ranging from 63% to 85%. The other ring
fused sulfoxonium ylides were also compatible under the
standard reaction conditions, affording the desired indole
derivatives 3ha and 3ia in 84% and 77% yields, respectively.
Ph
OH
O
O
S
[RhCp*Cl2]2, AgSbF6, PivOH
Zn(OAc)2, EtOH, 70 o
R
+
C
R
NHTs
Ph TsN
3aa - 3ia
1a - 1i
2a
OH
OH
OH
F
Me
Ph TsN
3aa (95%)
Ph TsN
Ph TsN
3ba (79%)
3ca (82%)
OH
OH
OH
Me
tBu
MeO
Me Ph TsN
Ph TsN
Ph TsN
3fa (67%)
3da (74%)
3ea (85%)
OH
OH
OH
F3C
O
O
Ph TsN
Ph TsN
Ph TsN
3ia (77%)
3ga (63%)
3ha (84%)
a
Reaction condition: 1 (0.10 mmol), 2a (0.15 mmol), [Cp*RhCl2]2 (4 mol%),
AgSbF6 (20 mol%), PivOH (0.20 mmol), Zn(OAc)2 (0.20 mmol). All the
reactions were carried out in a 0.10 mmol scale in EtOH (1.0 mL) at 70 oC for
12 h.
Scheme 2. Substrate scope for indoles..
Table 1. Optimization of the reaction conditions for the synthesis of 3aa.
Ph
OH
Next, we investigated the substrate scope of 1,3-diyne units in
this reaction. The R3 group on the 1,3-diyne terminal position
could be extended to para-chlorobenzene, TIPS, cyclopropane
and butyl groups, smoothly furnishing the corresponding
products 3ab-3ae in good yields ranging from 62% to 85%. The
structure of 3ae has been determined by X-ray diffraction. When
R3 is a TMS group, the silyl group was removed after the
reaction, affording the corresponding product 3af in 34% yield.
We also examined the R2 protecting groups on the amino moiety
and found that a variety of sulfonyl groups likewise para-
methoxybenzenesulfonyl group (3ag), benzylsulfonyl group
(3ah), benzenesulfonyl group (3ai), and methanesulfonyl group
(3aj) are compatible in this reaction, delivering the desired
products in good yields. The diversified R1 group such as methyl
group, chlorine and fluorine atoms, and CF3 group could be
introduced into the aromatic ring of 1,3-diyne units, leading to
the synthesis of functionalized indole derivatives in good yields
with a simple operation. Unprotected aniline such as 2o was not
compatible for the reaction under the standard conditions (for
product 3ao).
O
O
S
[RhCp*Cl2]2, AgSbF6, PivOH
R
+
additives, Solvent, 70 o
C
NHTs
2a
3aa Ph TsN
yield (%)c
67
1a
entrya
additive
Zn(OAc)2
Zn(OTf)2
Zn(NTf)2
ZnCl2
solvent
DCE
DCE
DCE
DCE
DCE
DCE
DCE
TFE
1
2
78
55
3
4
n.r.
n.r.
69
5
ZnO
6
Zn(glycine)2
-/otherb
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
7
n.r.
n.r.
95
8
9
EtOH
MeCN
10
41
a
Substrate 1a (0.10 mmol), 2a (0.15 mmol), PivOH (0.20 mmol),
[Cp*RhCl2]2 (4 mol%), AgSbF6 (20 mol%), additives (2.0 equiv) were
added. All the reactions were carried out in a 0.10 mmol scale in solvent
(1.0 mL) at 70 oC for 12 h. b The other additives are Cu(OAc)2, Cu(OTf)2,
AgOAc, AgOTf, NaOAc, ln(OTf)3 and Yb(OTf)3. c Isolated yields.
After determining the optimal reaction conditions, we
subsequently investigated the generality of this protocol by using
different sulfoxonium ylides to react with 2a and the results are
shown in Scheme 2. To our delight, a range of substituted
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10.1002/cctc.202001315
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Scheme 4. Control Experiments
On the basis of the previous literature[7] and the above
obtained experimental results, a plausible mechanistic pathway
is proposed in Scheme 5. First, 1,3-diyne 2a undergoes
intramolecular hydroamination to generate 2-alkynyl indole
intermediate 4 in the presence of AgSbF6.[10] Meanwhile, the
cationic Cp*Rh(III) complex is generated upon treatment of
[Cp*RhCl2]2 with AgSbF6, which then coordinates with
sulfoxonium ylide 1a and subsequently undergoes an ortho
arene C−H activation to generate rhodacyclic intermediate A.
The coordination of rhodium complex
A with 4 gives
intermediate B, which goes through a migratory insertion to
afford intermediate C. Then, another Rh(III) intermediate D can
be formed through an intramolecular rearrangement, which in
turn gives the reactive carbenoid species E by α-elimination of
DMSO with the assistance of Zn(OAc)2. Next, the migratory
insertion of Rh−alkenyl bond into the resulting carbenoid moiety
in intermediate E produces a Rh(III) alkyl species F, which
undergoes protonolysis of Rh−C bond by HX to release the final
product along with the regeneration of the active catalyst. The
a Reaction condition: 1a (0.10 mmol), 2 (0.15 mmol), [Cp*RhCl2]2 (4 mol%),
AgSbF6 (20 mol%), PivOH (0.20 mmol), Zn(OAc)2 (0.20 mmol). All the
reactions were carried out in 0.10 mmol scale in EtOH (1.0 mL) at 70 oC for 12
h.
Scheme 3. Substrate scope of 1,3-diynes.
regioselectivity
was
determined
on
the
step
of
To get more mechanistic insights of this cascade annulation, a
series of control experiments were carried out to investigate the
reaction sequence. As shown in Scheme 4, using 1,3-diyne 2a
as substrate in the presence of [Cp*RhCl2]2, AgSbF6, or
Zn(OAc)2 or with cationic Cp*Rh(III) complex as a single catalyst,
we found that only AgSbF6 can catalyze the hydroamination of
2a to form 2-alkynyl indole species 4 in 99% yield. The others all
failed to afford the hydroamination product.[8] Finally, we also
identified that using 4 instead of 2a to react with 1a under the
standard conditions could give the desired product 3aa in 95%
yield. These experimental results suggested that the first step of
this cascade transformation is a Ag-catalyzed cyclization of 2a to
produce the corresponding 2-alkynyl indole intermediate.
Moreover, Zinc acetate may play a role on assisting the
cyclization process in the second step of rhodium-catalyzed
process.[6l, 9]
coordination/insertion of intermediate 4. The different steric
hindrance of two groups on acetylene moiety is the key factors
on this selectivity.
1a
3aa
HX
[Rh]
Ar
O
[Cp*Rh]III
S
O
TsHN
O
TsN
Ph
O
Rh
Cp*
AgSbF6
F
A
[Rh]
Ar
Ph
Ph
E
2a
.
4
S
DMSO Zn
Zn
O
O
O
O
S
[Rh]
Ar
[Rh]
S
Ph
O
Ph
Ar
B
D
O
[Rh]
Ph
Ar
C
Scheme 5. Mechanistic paradigm
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Scheme 6. Synthetic Application
In conclusion, we have developed a novel synthetic method
for the rapid construction of naphthol-indole derivatives in good
yields from easily prepared 1,3-diynes and sulfoxonium ylides
via a Rh(III)-catalyzed C-H activation and silver-catalyzed
hydroamination cascade process in a one pot manner. This
synthetic protocol can provide a variety of C2 functionalized
indole derivatives with a good functional group tolerance under
mild reaction conditions. Further investigations on expanding the
applications of this synthetic method are ongoing in our
laboratory.
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Acknowledgements
We are grateful for the financial support from the Strategic
Priority Research Program of the Chinese Academy of Sciences
(Grant No. XDB20000000), the National Natural Science
Foundation of China (21372250, 21121062, 21302203,
20732008, 21772037, 21772226, 21861132014 and 91956115)
Keywords: rhodium • silver • C-H activation • sulfoxonium
ylides • naphthol-indole
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Synthesis of naphthol-
Ruixing Liu, Yin Wei,
Min Shi*
indoles
starting
from
easily available 1,3-diynes
and sulfoxonium ylides via
Page No. – Page No.
Rh(III)-catalyzed
C-H
Rhodium(III)-Catalyzed
Cross Coupling of
activation/hydroamination
cascade process in a one
pot manner has been
developed.
Sulfoxonium
Ylides
and
1,3-Diynes to
Produce
Naphthol-
Indole Derivatives: An
Arene ortho C–H
Activation/Annulation
Cascade
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