Au-promoted Pd-catalyzed arylative cyclization of N,N-dimethyl-o-alkynylaniline with aryl iodides: Access to 2,3-diaryl indoles and mechanistic insight

Article abstract of DOI:10.1016/j.tetlet.2020.152766

We have developed a Au-promoted Pd-catalyzed cyclization/cross-coupling of N,N-dimethyl-o-alkynylaniline with aryl iododes to synthesize 2,3-diarylindoles under mild and base-free conditions. A related vinyl-Au species has been isolated through Au-promoted cyclization of N,N-dimethyl-o-alkynylaniline and structurally characterized. Further study on its reactivity suggests the vinyl-Au species might be out of catalytic cycle, and PhPd(OTf)(PPh₃)₂ is probably the reaction intermediate.

Full text of DOI:10.1016/j.tetlet.2020.152766

Tetrahedron Letters 65 (2021) 152766
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Au-promoted Pd-catalyzed arylative cyclization of N,N-dimethyl-o-
alkynylaniline with aryl iodides: Access to 2,3-diaryl indoles and
mechanistic insight
⇑
Kemeng Yuan, Jiwei Wang, Feijun Wang, Jun Zhang
Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry, School of Chemistry and Molecular Engineering, East China University
of Science and Technology, 130 Meilong Road, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We have developed a Au-promoted Pd-catalyzed cyclization/cross-coupling of N,N-dimethyl-o-alkyny-
laniline with aryl iododes to synthesize 2,3-diarylindoles under mild and base-free conditions. A related
vinyl-Au species has been isolated through Au-promoted cyclization of N,N-dimethyl-o-alkynylaniline
and structurally characterized. Further study on its reactivity suggests the vinyl-Au species might be
out of catalytic cycle, and PhPd(OTf)(PPh3)2 is probably the reaction intermediate.
Ó 2020 Published by Elsevier Ltd.
Received 21 October 2020
Revised 5 December 2020
Accepted 14 December 2020
Available online 5 January 2021
Keywords:
Indole
Gold catalysis
Palladium catalysis
Heterocycle synthesis
Arylative cyclization
Introduction
a) Pd-catalyzed arylative cyclization of alkynes with aryl iodides
Ph
Ar
ArI, [Pd(phen)2](PF4)2 (5 mol%)
Cs2CO3 (1.1 equiv)
DMA, 150 oC, 20 h
Transition metal-catalyzed cross-coupling reactions have been
well-established as powerful tools for the formation of C A C bonds
in modern synthetic chemistry. [1] Tremendous efforts have been
made to develop methods for the access to 2,3-disubstituted
indoles, due to the increasing interests in successful biomedical
applications of substituted indoles. [2] 2-Substituted 3-arylindoles
could be synthesized by Pd-catalyzed cyclization/cross-coupling of
either N,N-dimethyl-o-alkynylaniline [3a] or o-alkynyltrifluoro-
acetanilides [3b] with aryliodides (Scheme 1a). In both cases, stoi-
chiometric quantities of base are required, and in the former case, a
high temperature of 150 °C is also needed. In such process, Pd cat-
alytic cycle starts with oxidative addition of ArI into PdLn species to
form ArPdXL2 intermediate, which is supposed to promote the
intramolecular nucleophilic cyclization of alkynes. [4] Because Ag
(I) salts are often used as halide abstractor in Pd catalysis, [5] we
envision the introduction of Ag(I) salts might accelerate the reac-
tion rate in such transformation. On the other hand, bimetallic
catalysis has recently emerged as a promising approach in C A C
coupling reactions and provides complementary applications, due
to its unique reactivity and selectivity. [6] Among them, Pd/Au
Ph
N
Me
NMe2
Ar
R
ArI, Pd(PPh3)4 (5 mol%)
K2CO3 (1.5 qequiv)
R
MeCN, 80 oC, 3 h
N
NHCOCF3
H
b) This work: Au-promoted Pd-catalyzed arylative cyclization of
N,N-dimethyl-o-alkynylaniline with aryl iodides
Ar2I
AuL
Ar1
Ar2
Pd(PPh3)4 (10 mol%)
PPh3AuOTf (10 mol%)
Ar1
Ar1
N
CH3CN, 50 o
C
N
A
NMe2
Reaction Intermediate?
Me
Scheme 1. Pd-catalyzed arylative cyclization of o-alkynylaniline, and Au-promoted
Pd-catalyzed arylative cyclization of o-alkynylaniline for the synthesis of 2-
substituted 3-arylindoles.
bimetallic catalysis is of particular interest and has found to be
an efficient catalytic system in several C A C coupling reactions,
such as Sonogashira reaction, [7] Stille reaction, [8] and other cou-
⇑
Corresponding author.
E-mail address: zhangj@ecust.edu.cn (J. Zhang).
https://doi.org/10.1016/j.tetlet.2020.152766
0040-4039/Ó 2020 Published by Elsevier Ltd.

K. Yuan, J. Wang, F. Wang et al.
Tetrahedron Letters 65 (2021) 152766
pling reactions. [9] One of the most successful examples of Pd/Au
bimetallic catalysis is Au/Pd co-catalyzed cyclization/cross-cou-
pling reaction, which combines the well-established ability of gold
to activate C A C multiple bonds for cyclization and the efficient
redox turnover of palladium. The process involves a key transmet-
alation of vinylgold, with RPd+ intermediate and subsequent reduc-
tive elimination at Pd center. [10] Therefore, we speculated Pd
catalyst in combination with coinage salt, such as Ag(I) or Au(I)
salts, would be an ideal system for the catalytic cyclization/cross-
coupling of o-alkynylaniline. Herein, we develop a Au-promoted
Pd-catalyzed cyclization/cross-coupling of N,N-dimethyl-o-alkyny-
laniline with aryl iodides to efficiently synthesize 2,3-diarylindoles
under mild and base-free conditions (Scheme 1b). A related vinyl-
Au species A has been isolated through the Au-promoted cycliza-
tion of N,N-dimethyl-o-alkynylaniline, which was structurally
characterized by single crystal X-ray diffraction. The study on its
reactivity towards aryl iodide under Pd catalysis suggests that
the vinyl Au species A might be out of catalytic cycle, and PhPd
(OTf)(PPh3)2 is probably the reaction intermediate.
after 4 h, and it took 12 h to achieve a 99% (NMR yield) yield of
3a. Given these results, we decided to introduce a second metal
such as Au complex, which is expected to be highly efficient in
the intramolecular cyclization reaction of alkynes. [11] Delight-
fully, the addition of 10 mol% of PPh3AuOTf increased considerably
the reaction rate. With the assistance of Au(I) it only required 4 h
to obtain 99% NMR yield. Using AgOTf as an additive only improved
slightly the reaction yield.
Optimization of various palladium catalysts in different sol-
vents (entries 1 ~ 8, Table 1) was studied. The best result was
achieved for 3a (99% yield; entry 4, Table 1), when using Pd
(PPh3)4 as catalyst in MeCN. Decreasing the amount of substrate
2a to 1.5 equivalent didn’t result in yield loss (entries 9 and 10,
Table 1).
We further tested the transformation of 1a with different aryl
iodides (2) under the standard reaction conditions (Scheme 3).
All of the reactions worked well, giving the desired products
3a ~ 3 g in 90 ~ 99% yields. When using different para-halogenated
and para-methyl phenyl iodides, a long reaction time are required
to achieve high yields (89 ~ 94% for 3 h ~ 3j). For 2-iodonaph-
thalene, longer reaction time is also required (3 k). The reaction
was also tolerated with heteroaryl halide (2-iodothiophene), giving
the corresponding indole 3 l in 88% yield. The reactions with differ-
ent N,N-dimethyl-o-alkynylanilines were also examined. The
desired products 3 m ~ 3q, and 3 s ~ 3 t were obtained in
80 ~ 93% yields. It should be noted that for the reactions which fur-
nish the 2,3-diarylindole products bearing an orth- groups (methyl
or methoxyl) either at 2-aryl or at 3-aryl group, longer reaction
time are required (3 m, 3n, and 3r). When using 2-iodoanisole, a
poor yield of 57% was observed for 3r.
Considering the unique ability of gold to activate alkynes
towards nucleophilic attack, we assume that a vinyl gold interme-
diate might be involved in the catalytic cycle. We have recently
reported the isolation of several vinyl Au intermediates through
intramolecular nucleophilic attack on Au-activated alkynes. [12]
To our delight, the treatment of 1a with one equivalent of PPh3-
AuOTf resulted in the formation of a vinyl gold complex 4 with a
high yield (94%) in 5 min (Eqn 1, Scheme 4). The structure of 4
was determined by single crystal X-ray diffraction, which exhibits
the expected linear coordination geometry (Fig. 1). The Au-C bond
distances [Au-C7, 2.046(3Å] in 4 is similar with that of a C(sp2)-Au
Results and discussion
First, we test the Pd(PPh3)4-catalyzed cyclization/cross-cou-
pling of N,N-dimethyl-o-alkynylaniline 1a with iodobenzene 2a.
(Scheme 2). The reaction proceeded sluggishly in CH3CN at 50 °C,
as only a low yield of product 3a (36%, NMR yield) was detected
Pd(PPh3)4 (10 mol%)
MOTf (10 mol%)
(w/o)
CH3CN, 50 oC, time
Ph
Ph
+
PhI
Ph
Me
N
N
Me
1a
Me
3a
2a
(5 equiv)
Without MOTf: 36 %a (4 h), 99 %a (12 h)
With PPh3AuOTf: 99 %a (4 h)
With AgOTf: 47 %a (4 h)
a NMR yield
Scheme 2. Pd-catalyzed arylative cyclization of N,N-dimethyl-o-alkynylaniline 1a
with PhI.
Table 1
Optimization of Reaction Conditionsa.
Ph
Ph
[Pd] (10 mol%)
PPh3AuOTf (10 mol%)
Ph
+
PhI
Me
solvent, 50 oC
N
Me
N
2a
(x equiv)
Me
1a
3a
entrya
Pd
2a (x equiv)
sovent
yieldb(%)
1
2
3
4
5
6
7
8
9
Pd(PPh3)Cl2
Pd(OAc)2
PdCl2
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
5
5
5
5
5
5
5
5
CH3CN
CH3CN
CH3CN
CH3CN
CH3OH
DMF
toluene
THF
CH3CN
CH3CN
trace
14
11
99
96
25
7
trace
99
99
3
1.5
10
aUnless otherwise stated, reactions were performed with 0.45 mmol of 1a, 10 mol% palladium catalyst and 10 mol% PPh3AuOTf in 4 mL of solvent under a nitrogen
atmosphere for 4 h. bNMR yields.
2

K. Yuan, J. Wang, F. Wang et al.
Tetrahedron Letters 65 (2021) 152766
X
N
N
N
3a
3b
, 96%
, 99%
3c
X = OMe, , 95%
3d
X = CF3, , 90%
3e
X = NO2, , 97%
S
3f
X = CN, , 95%
3g
X = COCH3, , 93%
c
3h
X = CH3, , 94%
X = Cl, , 89%
X = Br, , 90%
N
c
3i
N
c
3j
3l
, 88%
d
3k
, 83%
N
N
N
c
3o
, 80%
c
3m
, 87%
CF3
3n
, 83%
Fig. 1. Molecular structure of 4. The counter anion and hydrogen atoms have been
omitted for clarity. Selected bond distances (Å) and angles (deg): Au(1)-C(7) 2.046
(3), Au(1)-P(1) 2.2920(7), C(6)-C(7) 1.469(4), C(7)-C(8) 1.331(4), C(1)-C(6) 1.385(4),
N(1)-C(1) 1.481(4), N(1)-C(8) 1.523(3), C(7)-Au(1)-P(1) 172.01(8), C(8)-C(7)-Au(1)
122.6(2), C(6)-C(7)-Au(1) 129.8(2), C(8)-C(7)-C(6) 107.4(2).
MeO
N
N
N
d
3r
, 57%
3q
, 93%
3p
, 80%
COCH3
Ph
AuPPh3
Ph
PPh3AuOTf (1 equiv)
CH3CN, rt, 5 min
(1)
Me
N
N
Me
OMe
Me
OMe
Me
N
1a
OTf
N
4
, 94%
3s
, 89%
3t
, 83%
AuPPh3
Ph
Ph
Pd(PPh3)4 (0.1 equiv)
PhI (5 equiv)
CH3CN, 50 oC,12 h
Scheme 3. Scope of cyclization reactions.a,b Unless otherwise stated, reactions
were performed with 0.45 mmol of 1, 1.5 equiv 2 and 10 mmol% Pd(PPh3)4,
10 mmol% PPh3AuOTf in 5 mL of CH3CN at 50 °C under a nitrogen atmosphere for
4 h. bIsolated yields. c8 h. d24 h.
a
Ph
(2)
(3)
N
Me
4
N
Me
, 85%
OTf
Me
3a
Ph
Ph
Pd(PPh3)4
(1 equiv)
PhI (5 equiv)
Ph
4
+
single bond (e.g., 2.045(6) Å). [13] Under catalysis of Pd(PPh3)4,
vinyl gold 4 could react with excess PhI in MeCN at 50 °C to
give arylation product 3a in 85% yield (Eqn 2, Scheme 4). How-
ever, the transformation is sluggish, requiring 12 h to obtain
complete conversion, while in catalytic reaction, it only takes
4 h to give desired product 3a in 99%. Interestingly, when treat-
ment of vinyl gold 4 in the presence of Pd(PPh3)4 and PhI at the
room temperature, 1a, ring-opening product of 4 was observed
after 2 h, and arylation product 3a was finally obtained in
99% yield. This finding suggests the formation of the vinyl gold
4 is reversible.
Me
N
CH3CN, rt
N
Me
Me
3a
, 70%; , 0%
1a
1a
2 ha:
3a
3a
a
1a
24 h : , 0%;
, 99%
a: NMR yield
Scheme 4. Isolation of key Au species 4 and its reactivity with PhI under catalysis of
Pd.
Based on the observations, we propose a plausible mechanism
(Scheme 6). Oxidative addition of Pd(PPh3)4 with ArI forms PdArI
(PPh3)2, which reacts with PPh3AuOTf to form ArPd(OTf)(PPh3)2
(I). Pd species I promotes the cyclization of 1 to form vinyl Pd(II)
complex II. Pd(II) complex II further undergoes reductive elimina-
tion to give the indolium intermediate III, and release Pd0. III fur-
ther undergoes the anion exchange with Ar(PPh3)2PdI or (PPh3)AuI
to give III’, which undergoes elimination of MeI to give the final
product 3.
Next, we examined the reaction between PPh3AuOTf and trans-
[PdPhI(PPh3)2] (5) formed by oxidative addition of Pd(PPh3)4 with
PhI. A new major species appearing at 21.54 ppm was observed
by 31P NMR (Scheme 5). We assigned it as PhPd(OTf)(PPh3)2 (6)
by comparison with the 31P NMR data of an authentic complex,
C6F5Pd(OTf)(PPh3)2 which appears at 23.50 ppm [14]. The observa-
tion suggests Au salt (PPh3AuOTf) probably acts as an iodide
abstractor, and PhPd(OTf)(PPh3)2 might be the reaction
intermediate.
3

K. Yuan, J. Wang, F. Wang et al.
Tetrahedron Letters 65 (2021) 152766
PPh3AuOTf
(1 equiv)
PPh3
Ph Pd
PPh3
Ph Pd
I
Pd(PPh3)4 + PhI
OTf
CH3CN
RT, 30 min
- 2PPh3
PPh3
5
PPh3
6
Scheme 5. The reaction of PPh3AuOTf and trans-[PdPhI(PPh3)2] (5).
Declaration of Competing Interest
Ph
AuL
+ LAuOTf
Ph
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
N
- LAuOTf
N
Ar
2 PdII
OTf
1
4
L
OTf
I
LAuI
LAuOTf
Acknowledgment
Ar
Ar
Financial support from the National Natural Science Foundation
of China (21171056 and 21671066) is greatly acknowledged.
2PdII
L
2 PdII
-
L
OTf
I
Ph
Ar
N
+
N
II
Appendix A. Supplementary data
Ph
3
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152766.
ArI
L4Pd0
L = PPh3
- MeI
Ar
Ar
-
L2PdArI
or LAuI
-
I
X
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iodides to prepare 2-substituted 3-arylindoles. A related vinyl-Au
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4

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Tetrahedron Letters 65 (2021) 152766
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