A Copper-Catalyzed Decarboxylative Amination/Hydroamination Sequence: Switchable Synthesis of Functionalized Indoles

Article abstract of DOI:10.1002/anie.201605900

A copper-catalyzed decarboxylative amination/hydroamination sequence of propargylic carbamates with various nucleophiles is described for the first time. It features an earth-abundant metal catalyst, mild reaction conditions, and high efficiency. Further treatments of the resultant key intermediates using an acid or a base in one pot enable the controllable and divergent synthesis of two types of functionalized indoles. Moreover, experiments to demonstrate the synthetic potential of this methodology are performed.

Full text of DOI:10.1002/anie.201605900

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International Edition: DOI: 10.1002/anie.201605900
German Edition: DOI: 10.1002/ange.201605900
Heterocycles
A Copper-Catalyzed Decarboxylative Amination/Hydroamination
Sequence: Switchable Synthesis of Functionalized Indoles
Tian-Ren Li, Bei-Yi Cheng, Ya-Ni Wang, Mao-Mao Zhang, Liang-Qiu Lu,* and Wen-Jing Xiao
Abstract: A copper-catalyzed decarboxylative amination/
hydroamination sequence of propargylic carbamates with
various nucleophiles is described for the first time. It features
an earth-abundant metal catalyst, mild reaction conditions, and
high efficiency. Further treatments of the resultant key
intermediates using an acid or a base in one pot enable the
controllable and divergent synthesis of two types of function-
alized indoles. Moreover, experiments to demonstrate the
synthetic potential of this methodology are performed.
tization sequence to afford either 3-acetoxy- or 3-methoxy-2-
homoallyl indoles.[3a] In 2010, the group of Chan reported an
impressive method for producing 3-aryl-2-vinyl and 2-
hydroxymethyl indoles, where a cationic gold(I) catalyst was
used to promote either an intramolecular hydroamination/
dehydration or SN2’ substitution sequence.[3b] Recently, Tang
et al. developed several prominent sequential reactions to
achieve functionalized indoles via either rhodium(I) or
platinum(II) carbene intermediates.[3g,4b] Despite these
impressive advances, many methods usually involve intra-
molecular processes with precisely designed substrates and/or
frequently employ precious metals as the reaction catalysts.
Therefore, it is highly desirable to develop an alternative
route to structurally diverse indoles with earth-abundant
metals as the catalyst.
T
he development of new methodologies which enable facile
access to valuable heterocycles remains a subject of intensive
research in chemical sciences. In this context, the construction
of the indole scaffold, a nitrogen-containing heterocyclic
motif prevalent in numerous natural products and pharma-
ceutical agents, is still under active investigation.[1,2] In the
past decade, transition-metal-catalyzed sequential reactions
of aniline-bearing propargyl alcohols and their derivatives
have been demonstrated to be a highly efficient and fruitful
strategy for constructing functionalized indoles (Figure 1).[2–5]
Because of the pioneering work from the groups of
Murahashi[5a] and Godfrey[5b] in 1994, the catalytic trans-
formation of propargylic esters by a highly active copper
allenylidene species (Figure 2) has been identified as a power-
Figure 1. Pervious work on transition-metal-catalyzed sequential reac-
tions for the synthesis of functionalized indoles. [M]=metal complex
with Pt, Au, Rh, Pd, Ag, etc.; X=H, Me, or Ac; Y=H, Me, or CO2Et.
FG=functional group.
Figure 2. Proposal of this work: Copper-catalyzed sequential reaction
for the switchable synthesis of functionalized indoles.
For example, in 2007, Fensterbank, Malacria, and co-workers
reported an elegant example of a platinum-catalyzed intra-
molecular hydroamination/aza-Cope rearrangement/aroma-
ful tool to introduce productive alkyne functional groups.[6]
However, copper-catalyzed sequential transformations via
this key intermediate remain underdeveloped. In 2010, the
group of Nishibayashi disclosed the first example of a copper-
catalyzed sequential propargylic amination/Diels–Alder reac-
tion.[7] Recently, Hu and co-workers successfully developed
a few reactions of propargylic esters with substrates bearing
two nucleophilic components.[8] These transformations have
nicely demonstrated the potential of the copper allenylidene
intermediate in the construction of heterocyclic architec-
tures.[7,8] As part of our continuing efforts on heterocycle
synthesis,[9] we recently designed a new propargyl carbamate
[*] T.-R. Li, B.-Y. Cheng, Y.-N. Wang, M.-M. Zhang, Prof. Dr. L.-Q. Lu,
Prof. Dr. W.-J. Xiao
Key Laboratory of Pesticide & Chemical Biology Ministry of
Education, College of Chemistry, Central China Normal University
152 Luoyu Road, Wuhan, Hubei 430079 (China)
E-mail: luliangqiu@mail.ccnu.edu.cn
Prof. Dr. W.-J. Xiao
Collaborative Innovation Center of Chemical Science and Engineer-
ing (Tianjin) (China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605900.
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reagent (Figure 2)[10] and hypothesized that in the presence of
a copper catalyst, this reagent could undergo an intermolec-
ular decarboxylative amination reaction to generate a prop-
argylamine through a reactive copper allenylidene species. If
successful, an intramolecular alkyne hydroamination process
would afford the pivotal 3-aminoindoline intermediate, from
which two different indole derivatives are expected, either by
a one-pot, acid-mediated aza-Cope rearrangement or a base-
mediated 1,3-proton shift.
Lewis acid BF3·Et2O to the reaction system when the
substrate 1a was consumed, the aza-Cope rearrangement of
3aa did indeed occur, and readily afforded the final indole
product 4aa in 88 and 94% yield, respectively (entries 7 and
8). When Cs2CO3 was added instead of the acid, the 1,3-
proton-transfer process also proceeded well. Another indole
product, 5aa, was given in 86% yield (entry 9). The structures
of the indole products 4aa and 5aa were definitively
confirmed by X-ray analysis.[11]
We initially chose the propargylic carbamate 1a and
indoline 2a as model substrates to examine the feasibility of
the proposal and optimize reaction conditions (Table 1).
Having identified the optimal reaction conditions, we
probed the generality of this methodology for divergent
indole synthesis. As illustrated in Scheme 1, various 2-ortho-
Table 1: Optimization of the reaction conditions.[a]
Entry
Change from the standard
reaction conditions
Yield [%]
4aa
3aa
5aa
1
2
3
4
5
6
none
bpy instead of box
dppe instead of box
Cu(OAc)2 instead of CuI
Cu(OTf)2 instead of CuI
Cs2CO3 instead of iPr2NEt
TFA added when 1a consumed
BF3·Et2O added when 1a consumed
Cs2CO3 added when 1a consumed
97
55
34
64
80
–
–
30
–
–
–
–
–
–
–
complex mixture
–
–
–
7[b]
8[b]
9[c]
88
94
–
–
–
86
[a] Standard reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), CuI
(5 mol%), ligand (6 mol%), and iPr2NEt (0.4 mmol, 2.0 equiv) in MeOH
(2 mL) at ambient temperature for 0.5 h. Yield is that of isolated product.
[b] Stirred at 658C for 0.5 h after 10 equiv of acid were introduced.
[c] Stirred at 658C for 2 h after 2 equiv of base were introduced.
TFA=trifluoroacetic acid.
Scheme 1. Indole synthesis by copper-catalyzed sequential reactions
and acid-mediated aza-Cope rearrangements. Reaction conditions A:
see entry 8 in Table 1. Yield is that of the isolated product. [a] 1108C in
toluene instead of 658C in methanol.
anilinemethyl indoles were produced in good to high yields
through a copper-catalyzed sequential reaction followed by
a BF3-mediated aza-Cope rearrangement. In addition to
various secondary anilines (4aa–ad: 77–94% yields), primary
anilines with different substituents on the benzene ring also
participated in this one-pot reaction, thus affording the
corresponding products in 81–90% yields (4ae–ai). In the
case of electron-deficient 4-fluoroaniline as the substrate,
a higher temperature was needed to promote the rearrange-
ment process (4af). A chiral amino-acid-derived secondary
amine, methyl N-phenyl phenylalaninate, also served as an
efficient substrate, thus producing the corresponding indole
product 4aj in 89% yield. Moreover, we surveyed the scope
of the propargylic carbamates in this transformation. For
instance, variation of the propargylic carbamates, including
the substituent position on the benzene ring and its electronic
character, was compatible with the reaction conditions A,
thus yielding the final products with generally high reaction
efficiencies (4be–ge). Because of the significance of fluorine
Upon evaluating various reaction parameters, we determined
that the combination of CuI and a bisoxazoline ligand (box) in
methanol can efficiently catalyze the propargylative amina-
tion/alkyne hydroamination sequence at ambient temper-
ature, thus furnishing 3-aminoindoline 3aa in 97% yield
(entry 1). Replacement of the box ligand with the nitrogen-
containing ligand 2,2’-dipyridyl (bpy) adversely affected the
reaction efficiency (entry 2). Reaction with the bidentate
phosphine ligand 1,2-bis(diphenylphosphanyl)ethane (dppe)
provided a mixture of indole and indoline products (entry 3).
We observed that CuI was superior to other copper catalyst
precursors (entries 4 and 5) and that the use of the inorganic
base Cs2CO3 only resulted in a complex mixture. As expected,
after the introduction of either the Brønsted acid TFA or
2
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substitution in biological, pharmaceutical, and materials
To demonstrate the synthetic utility of these methods, we
science,[12] propargylic carbamates bearing either CF3 or
fluorine groups were tested under the present reaction
conditions. To our delight, the corresponding fluoro-contain-
ing indoles were produced in high yields (4de: 91%, 4ee:
88%, 4 fe: 88%).
performed a gram-scale reaction of 1a and 2e while simulta-
neously reducing the catalyst loading to 1 mol%. To our
delight, no erosive effect on the reaction efficiency was
observed (Scheme 3a, 4ae: 89%). Additionally, 2-aniline-
In parallel, we examined the generality of the copper-
catalyzed sequential reaction followed by a base-promoted
1,3-proton shift process. As shown in Scheme 2, many amine
Scheme 3. Synthetic utility of methodology. [a] Reaction conditions A.
[b] Reaction conditions B, where Cs2CO3 was replaced with KOtBu.
methyl indole 6ae, an important precursor in the total
synthesis of the natural alkaloid 10H-indolo[3,2-b]quino-
line,[13] was obtained in 91% yield through a reductive
detosylation step.[14] Note that this compound can also be
converted into a useful benzazepinoindoles through a catalytic
asymmetric Pictet–Spengler reaction.[15] In addition, we chose
three commercially available prescribed drugs: tetracaine,
metoprolol, and sertraline, as a platform to further demon-
strate the potential of this copper-catalyzed sequential
reaction, as diversification of commercial drugs is well
accepted to be a fruitful approach to pharmaceutical candi-
date discovery.[16] Subjecting these drug molecules to our
standard reaction conditions, we successfully synthesized
medicinally relevant compounds 7a–c in high yields (Sche-
me 3b).
A plausible mechanism for the reaction of the propargylic
carbamates 1 and nucleophiles 2 has been proposed to
illustrate the pathway to the key indoline intermediates 3.[17]
As depicted in Figure 3, the copper catalyst initially reacts
with 1 to generate the copper acetylide complex A in the
presence of iPr2NEt. Subsequently, the decarboxylation
reaction of A followed by a protonation process generates
the copper allenylidene complex B and its resonance struc-
ture C. The capture of this reactive species with 2, followed by
an iPr2NEt-mediated proton shift of D, results in the
intermediate E and regenerates the copper catalyst.[8a,18] In
the second catalytic cycle, the copper catalyst can act as
a p acid to activate the alkyne competent in complex F,[3h,19]
thus yielding the intermediate G through an intramolecular
alkyne hydroamination reaction. Finally, 3 is produced after
Scheme 2. Indole synthesis by the copper-catalyzed sequential reac-
tions and base-mediated 1,3-proton shift. Reaction conditions B: see
entry 9 in Table 1. Yield is that of the isolated product. [a] nBu4NCl and
NaHCO3 at ambient temperature, instead of Cs2CO3 at 658C. [b]
KOtBu, instead of Cs2CO3.
nucleophiles, including aromatic N-methyl aniline and ali-
phatic secondary amines, were suitable for this transforma-
tion. A wide range of structurally diverse 3-aminoindole
products were achieved in 84–99% yields (5ab and 5aj–am).
2-Iodoaniline was also tested for the one-pot sequential
reaction, and the indole product 5an was synthesized in 90%
yield at ambient temperature when Cs2CO3 was replaced with
nBu4NCl and NaHCO3.[10] Chiral secondary amines were also
efficient substrates for this transformation, thus affording the
corresponding indole derivatives in high yields (5ao: 77%,
5ap: 94%). In addition, the reactivity of various propargylic
carbamates were explored and an array of 3-aminoindoles
with different substitutions were prepared in 75–94% yields
(5ca–fa and 5ha). In addition to amines, we were pleased to
find that other nucleophiles, such as methanol, trifluoroetha-
nol, and an acetophenone-derived enamine, can participate in
this reaction efficiently, thus delivering the corresponding
indole products in good yields (5aq–as).
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[2] For select reviews on indole synthesis, see: a) G. R. Humphrey,
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[5] For pioneering work, see: a) Y. Imada, M. Yuasa, I. Nakamura,
S.-I. Murahashi, J. Org. Chem. 1994, 59, 2282; b) J. D. God-
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[6] For a recent reviews, see: a) D.-Y. Zhang, X.-P. Hu, Tetrahedron
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Figure 3. Proposed mechanism to generate the key indoline intermedi-
ate 3.
protonation of the intermediate H, accompanied by the
regeneration of the active copper catalyst. The key indoline
intermediate, that is, 3aa, can be isolated and its structure has
been unambiguously established by X-ray analysis.[11]
In summary, we have successfully developed an unprece-
dented copper-catalyzed decarboxylative amination/hydro-
amination sequence of propargylic carbamates with N-, O-,
and C-centered nucleophiles. Variation of the reaction con-
dition allows the switchable synthesis of two types of
functionalized indoles with high efficiencies and complete
chemoselectivities. This process has been successfully
employed in the formal synthesis of the natural alkaloid
10H-indolo[3,2-b]quinolone, and its utility has been further
demonstrated by the derivatization of some prescribed drugs.
[7] a) G. Hattori, Y. Miyake, Y. Nishibayash, ChemCatChem 2010,
2, 155 – 158.
[8] a) C. Zhang, X.-H. Hu, Y.-H. Wang, Z. Zheng, J. Xu, X.-P. Hu, J.
Am. Chem. Soc. 2012, 134, 9585 – 9588; b) F.-L. Zhu, Y.-H.
Wang, D.-Y. Zhang, J. Xu, X.-P. Hu, Angew. Chem. Int. Ed. 2014,
53, 10223 – 10227; Angew. Chem. 2014, 126, 10387 – 10391; c) D.-
Y. Zhang, L. Shao, J. Xu, X.-P. Hu, ACS Catal. 2015, 5, 5026 –
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Acknowledgments
We are grateful to the National Natural Science Foundation
of China (NO. 21232003, 21472057 and 21572074) and other
financial supports (NO. 201422, CCNU15A02007 and
2015CFA033) for support of this research.
[9] Select work from our group, see: a) L.-Q. Lu, J.-R. Chen, W.-J.
Xiao, Acc. Chem. Res. 2012, 45, 1278 – 1293; b) T.-R. Li, F. Tan,
L.-Q. Lu, Y. Wei, Y.-N. Wang, Y.-Y. Liu, Q.-Q. Yang, J.-R. Chen,
D.-Q. Shi, W.-J. Xiao, Nat. Commun. 2014, 5, 5500; c) Y. Wei, L.-
Q. Lu, T.-R. Li, B. Feng, Q. Wang, W.-J. Xiao, H. Alper, Angew.
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Yuan, K. Zhang, B.-J. Li, Y. Lan, W.-J. Xiao, Angew. Chem. Int.
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e) Q. Wang, T.-R. Li, L.-Q. Lu, M.-M. Li, K. Zhang, W.-J. Xiao, J.
Am. Chem. Soc. 2016, 138, 8360 – 8363.
Keywords: alkynes · copper · hydroamination · indoles ·
synthetic methods
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4
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data can be obtained free of charge from The Cambridge
[17] Please see the Supporting Information for control experiments
Crystallographic Data Centre.
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Received: June 17, 2016
Published online: && &&, &&&&
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Communications
Heterocycles
Switch-hitter: This work describes
a copper-catalyzed decarboxylative ami-
nation/hydroamination sequential reac-
tion. The one-pot treatment of an indoline
intermediate with either an acid or a base
enables the switchable synthesis of two
types of functionalized indoles in gener-
ally high yields and with complete che-
moselectivities (36 examples and up to
99% yield).
T.-R. Li, B.-Y. Cheng, Y.-N. Wang,
M.-M. Zhang, L.-Q. Lu,*
W.-J. Xiao
&&&& — &&&&
A Copper-Catalyzed Decarboxylative
Amination/Hydroamination Sequence:
Switchable Synthesis of Functionalized
Indoles
6
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