Re-Catalyzed Annulations of Weakly Coordinating N-Carbamoyl Indoles/Indolines with Alkynes via C?H/C?N Bond Cleavage

Article abstract of DOI:10.1002/chem.201901518

Described herein are rhenium-catalyzed [3+2] annulations of N-carbamoyl indoles with alkynes via C?H/C?N bond cleavage, which provide rapid access to fused-ring pyrroloindolone derivatives. For the first time, the weakly coordinating O-directing group was successfully employed in rhenium-catalyzed C?H activation reactions, enabled by the unique catalytic trio of Re₂(CO)₁₀, Me₂Zn and ZnCl₂. Mechanistic studies revealed that aminozinc species plays an important role in the reaction. Based on the mechanistic understanding, a more powerful catalytic trio of Re₂(CO)₁₀, [MeZnNPh₂]₂ and Zn(OTf)₂ was devised and applied successfully in the [4+2] annulations of indolines and alkynes affording pyrroloquinolinone derivatives.

Full text of DOI:10.1002/chem.201901518

A Journal of
Accepted Article
Title: Re-Catalyzed Annulations of Weakly Coordinating N-Carbamoyl
Indoles/Indolines with Alkynes via C-H/C-N Bond Cleavage
Authors: Yunhui Yang and Congyang Wang
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Re-Catalyzed Annulations of Weakly Coordinating N-Carbamoyl
Indoles/Indolines with Alkynes via C-H/C-N Bond Cleavage
Yunhui Yang,[a,b,c] and Congyang Wang*[a,b,c]
Abstract: Described herein are rhenium-catalyzed [3+2] annulations
of N-carbamoyl indoles with alkynes via C-H/C-N bond cleavage,
which provided rapid accesses to fused-ring pyrroloindolone
derivatives. For the first time, the weakly coordinating O-directing
group was successfully employed in rhenium-catalyzed C-H
rhenium-catalyzed C-H activation reactions. Herein, we describe
the first use of weakly coordinating carbonyl of N-carbamoyl
functionality as an efficient directing group for rhenium-catalyzed
C-H activation reactions by resorting to an unique catalytic trio of
Re2(CO)10/Me2Zn/ZnCl2 (Scheme 1b). Thus, [3+2] annulations of
activation reactions, enabled by the unique catalytic trio of Re2(CO)10, indoles with alkynes have been achieved through both C-H and
Me2Zn and ZnCl2. Mechanistic studies revealed that aminozinc
species played important roles in the reaction. Based on the
C-N bond cleavage.[5] Furthermore, mechanistic studies
revealed the key roles of aminozinc species, based on which a
new catalytic trio of Re2(CO)10/[MeZnNPh2]2/Zn(OTf)2 was
devised and successfully implemented in [4+2] annulations of
indolines with alkynes through C-H/C-N bond cleavage.[6]
mechanistic understanding,
a more powerful catalytic trio of
Re2(CO)10, [MeZnNPh2]2 and Zn(OTf)2 was devised and applied
successfully in the [4+2] annulations of indolines and alkynes
affording pyrroloquinolinone derivatives.
Over the past few decades, transition-metal-catalyzed inert C-H
bond activation has been considered as one of the most direct
methods for constructing new chemical bonds in synthetic
organic chemistry due to its intrinsic atom- and step-economic
characteristics.[1,2] Compared with widely used late transition
metals such as Ru, Rh, Pd, and Ir, the activation of inert C-H
bonds by rhenium has been less studied despite its unique
properties
and
promising
reactivity.[3]
Since
2005,
Kuninobu/Takai as well as our group have disclosed a series of
rhenium-catalyzed aromatic C-H annulation reactions with polar
or unpolar unsaturated molecules such as alkynes, allenes, α,β-
unsaturated esters, isocyanates, aldehydes, diisocyanates, and
so on (Scheme 1a).[3,4] It should be pointed out that strongly
coordinating N-atom directing groups, such as aldimine,
ketimine, imidate, diazo, and other functionalities, were
inevitably used in the arene substrates. Though Kuninobu/Takai
has elegantly demonstrated rhenium-catalyzed C-H annulations
of aromatic ketones and aldehydes, a catalytic amount of aniline
derivatives was required to form aldimine/ketimines in situ as
effective directing groups for the C-H activation step.[4d,f] As far
as we are aware, there has been no example reported using
weakly coordinating O-atom directing groups such as carbonyl in
Scheme 1. Rhenium-catalyzed C-H annulation reactions.
Recently, we have developed a series of manganese-
catalyzed C-H addition reactions to (un)polarized unsaturated
molecules by using Mn(CO)5Br, Me2Zn, and ZnBr2 (or CuBr2)
system.[7] This powerful combination enabled the C-H activation
of weakly coordinating ketone substrates and the thereafter
addition of the C-Mn bond to simple aldehydes and imines,
which were otherwise difficult to achieve via other transition
metal catalysis. Though successful, challenges still remain, such
as more difficult but synthetically useful directing groups for the
C-H substrates and less electrophilic reaction partners for the C-
Mn bond. In this context, the carbamoyl group is worthy of
investigations due to its values in organic synthesis, weak
coordination ability, and low electrophilicity of the carbonyl group.
As such, we commenced to probe the possible C-H annulations
of N-carbamoyl indoles with alkynes. Initially, the system of
manganese catalysts together with Me2Zn/ZnBr2 was tested and
unfortunately no product was detected. To our delight, the [3+2]
annulation product through C-H/C-N cleavage, pyrroloindolone
3aa (Scheme 2) was generated from N-carbamoyl indole 1a and
1,2-diphenylethyne 2a in 42% NMR yield by using a catalytic trio
of Re2(CO)10, Me2Zn, and ZnCl2.[8] After an extensive
investigation on reaction parameters, 3aa was obtained in 88%
[a]
Y. Yang, Prof. Dr. C. Wang
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS
key Laboratory of Molecular Recognition and Function, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
E-mail: wangcy@iccas.ac.cn
Homepage: http://wangcy.iccas.ac.cn/
Y. Yang, Prof. Dr. C. Wang
University of Chinese Academy of Sciences
Beijing 100049 (China)
[b]
[c]
Y. Yang, Prof. Dr. C. Wang
Physical Science Laboratory, Huairou National Comprehensive
Science Center
Beijing 101400 (China)
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/chem.201901518
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isolated yield with 5 mol% Re2(CO)10, 30 mol% Me2Zn, and 30
mol% ZnCl2.[8]
understanding of this system (vide infra). 4-Octyne was also
compatible to this reaction, albeit giving a low yield of product
(3at).
Scheme 2. Substrate scope for [3+2] annulations. Reaction conditions: 1 (0.5
mmol), 2 (1.25 mmol), Re2(CO)10 (0.025 mmol), Me2Zn (0.15 mmol), ZnCl2
(0.15 mmol), toluene (2.5 mL), 150 oC, 48 h. Isolated yields of 3 are shown. [a]
24 h. [b] Re2(CO)10 (0.05 mmol). [c] Ratio of regioisomers. [d] [MeZnNPh2]2
were used instead of Me2Zn. [e] 72 h.
Scheme 3. Substrate scope for [4+2] annulations. Reaction conditions: 4 (0.5
mmol), 2 (1.25 mmol), Re2(CO)10 (0.025 mmol), [MeZnNPh2]2 (0.075 mmol),
Zn(OTf)2 (0.15 mmol), toluene (0.625 mL), 150 oC, 48 h. Isolated yields of 5
are shown. [a] Re2(CO)10 (0.05 mmol). [b] Ratio of regioisomers. [c] 72 h.
With the optimized conditions established, the scope of
indoles was first examined (Scheme 2). N-carbamoyl indoles
with electron-donating and -withdrawing groups reacted readily
to give pyrroloindolones (3aa-3ga). Other substitution patterns
on the indole skeleton were well tolerated except the 7-position
presumably due to the steric hindrance. Remarkably, N-
carbamoyl pyrroles underwent the [3+2] cyclization smoothly
affording the desired products in good yields (3ka-ma). The
scope of alkynes was further explored using 1a as the reaction
partner. The presence of para-, meta-, and ortho-substituents on
the benzene moiety of alkynes, whether electron-rich or
electron-poor, had no obvious influence on the reaction outcome
(3ab-o). It is noteworthy that fluoro, chloro and bromo groups
were well tolerated in the reaction, which provides handles for
further synthetic transformations. Unsymmetrical aryl alkyl
alkynes participated in the reaction smoothly to generate 3ap-
3as as major regioisomers. Of note, obviously improved yields of
products (3aq, 3as) were obtained by using catalytic
[MeZnNPh2]2 instead of Me2Zn based on our mechanistic
Encouraged by the above results, we further examined this
catalytic trio for [4+2] annulations of N-carbamoyl indolines 4
and alkynes via C-H/C-N cleavage. After screening the reaction
parameters, delightedly, the annulations took place efficiently
with C-H activation at 7-position of the indoline skeleton leading
to pyrroloquinolinone 5aa in 84% isolated yield by using the
catalytic trio of Re2(CO)10, [MeZnNPh2]2 and Zn(OTf)2 (Scheme
3). The scope of indolines was first tested and a series of
electronically varied functional groups were compatible to this
protocol giving the expected products in high yields (5ba-ga). N-
carbamoyl indolines bearing substitutions at different positions
all underwent the desired annulations well affording tricyclic and
tetracyclic products smoothly (5ha-na). Diaryl-substituted
alkynes containing various alkyl, alkoxy, trifluoromethyl and
halogen groups on the aromatic rings were nicely applicable to
the reaction (5ab-h). Again, ortho- and meta-substituents as well
as extended conjugation on diaryl alkynes had no obvious
influence on the reaction outcome (5ai-p). Aryl alkyl alkynes
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were also amenable to the reaction conditions leading to the
corresponding products (5aq-t) in good regioselectivity. Dialkyl
alkyne showed lower reactivity compared to that of diaryl ones.
Of note, no expected reaction occurred when 2-methyl N-
carbamoyl indole was used as a substrate under the same
reaction conditions.
catalytic cycle such as activation of carbonyl in the substrate
and/or the rhenium catalyst.
To shed light on the reaction mechanism, a series of
experiments was carried out. First, five-membered rhenacycles
C were detected from the reaction of Re2(CO)10 with 1a in the
presence of Me2Zn (Scheme 4a). Second, diphenylamine was
obtained in almost equal amount of product 3aa under standard
reaction conditions.[8] We proposed it might exist as amino-zinc
species in the reaction. Therefore, the amino-zinc species,
[MeZnNPh2]2 was prepared and tested as a co-catalyst in the
reaction (Scheme 4b). The desired product 3aa was obtained in
60% yield, even better than that of Me2Zn. Furthermore,
[EtZnNPh2]2 and [Zn(NPh2)2]2 were synthesized and examined,
which turned out to work as well as [MeZnNPh2]2. These results
suggested amino-zinc species being the possible catalytic
species in the reaction. In addition, zinc chloride enhanced the
reaction efficiency in all cases. Third, kinetic isotope effect (KIE)
experiments were conducted through two parallel reactions and
the KIE value was measured to be 2.1 (Scheme 4c), indicating
that the C-H bond cleavage might be involved in the turnover-
limiting step.[9]
Scheme 5. Proposed Mechanism.
In conclusion, rhenium-catalyzed [3+2] annulations of indoles
and [4+2] annulations of indolines with alkynes via C-H/C-N
bond cleavage have been developed to expediently access to
fused-ring pyrroloindolone and pyrroloquinolinone derivatives.
Weakly coordinating and low electrophilic carbamoyl group was
used as an efficient directing group and reaction partner with C-
Re bond in rhenium catalysis for the first time. Remarkably, the
herein devised unique catalytic trios of Re2(CO)10/Me2Zn/ZnCl2
and Re2(CO)10/[MeZnNPh2]2/Zn(OTf)2 overcame the limitation of
stoichiometric use of Me2Zn/ZnBr2 in our previous manganese
system,[7] which represented a substantial step towards more
powerful group-7 metal catalyst systems. Further explorations
on the applications of these catalytic trios in synthetic chemistry
are underway in our lab.
a) Detection of rhenacycle intermediate
H
O
Re(CO)n
O
ZnMe2 (1.0 equiv.)
N
N
Re2(CO)10
(1.0 equiv.)
+
toluene (0.2 M)
150 oC, 48 h
NPh2
1a
(1.0 equiv.)
NPh2
C (n = 3 or 4)
Detected by 1H NMR
13C NMR, HRMS
b) Key role of amino-zinc species
5 mol% Re2(CO)10
Ph
cat. [Zn]
N
+
1a
+
2a
Ph2NH
toluene (0.2 M)
150 oC, 24 h
Ph
(1.0 equiv.)
(2.5 equiv.)
3aa
O
[MeZnNPh2]2
[Zn(NPh2)2]2
(15 mol%)
ZnMe2
(30 mol%)
[EtZnNPh2]2
(15 mol%)
[Zn] =
(15 mol%)
without ZnCl2
30%
90%
60%
91%
53%
91%
62%
90%
30 mol% ZnCl2
c) KIE determined from two parallel reactions
conditions
Acknowledgements
1a
+
+
2a
3aa
D
O
KH
(1.0 equiv.)
(2.5 equiv.)
N
KH/KD = 2.3
conditions
Financial support from the National Natural Science Foundation
of China (21772202, 21831008) and Beijing Municipal Science &
Technology Commission (project No. Z181100004218004) are
gratefully acknowledged.
NPh2
1a-d
2a
3aa
KD
1a-d
(1.0 equiv.)
(2.5 equiv.)
Scheme 4. Mechanistic experiments.
Keywords: Rhenium Catalysis • Annulations • Indoles •
Based on these results and previous clues,[3a,b,4] a plausible
catalytic cycle is proposed in Scheme 5. Rhenium carbonyl
precursor reacts with aminozinc species to give aminorhenium
species A, which undergoes coordination with 1a followed by C-
H activation forming rhenacycle C. The ensuing insertion of
Indolines • C-H Activation
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Yunhui Yang, Congyang Wang*
Page No. – Page No.
Re-Catalyzed Annulations of Weakly
Coordinating N-Carbamoyl
Indoles/Indolines with Alkynes via C-
H/C-N Bond Cleavage
Re-catalyzed [3+2] annulations of N-carbamoyl indoles with alkynes via C-H/C-N
bond cleavage leading to pyrroloindolones were achieved by using an unique
catalytic trio of Re2(CO)10, Me2Zn and ZnCl2. Inspired by mechanistic
understanding, a more powerful trio of Re2(CO)10, [MeZnNPh2]2 and Zn(OTf)2 was
devised and applied in [4+2] annulations of indolines and alkynes affording
pyrroloquinolinones.
This article is protected by copyright. All rights reserved.