Cationic Cobalt(III) Catalyzed Indole Synthesis: The Regioselective Intermolecular Cyclization of N-Nitrosoanilines and Alkynes

Article abstract of DOI:10.1002/anie.201511002

The unique regioselectivity and reactivity of cobalt(III) in the direct cyclization of N-nitrosoanilines with alkynes for the expedient synthesis of N-substituted indoles is demonstrated. In the presence of a cobalt(III) catalyst, high regioselectivity was observed when using unsymmetrical meta-substituted N-nitrosoanilines. Moreover, internal alkynes bearing electron-deficient groups, which are almost unreactive in the [Cp?Rh^III]-catalyzed system, display good reactivity in this transformation.

Full text of DOI:10.1002/anie.201511002

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International Edition: DOI: 10.1002/anie.201511002
German Edition: DOI: 10.1002/ange.201511002
À
C H Activation
Cationic Cobalt(III) Catalyzed Indole Synthesis: The Regioselective
Intermolecular Cyclization of N-Nitrosoanilines and Alkynes
Yujie Liang and Ning Jiao*
Abstract: The unique regioselectivity and reactivity of cobalt-
(III) in the direct cyclization of N-nitrosoanilines with alkynes
for the expedient synthesis of N-substituted indoles is demon-
strated. In the presence of a cobalt(III) catalyst, high regiose-
lectivity was observed when using unsymmetrical meta-sub-
stituted N-nitrosoanilines. Moreover, internal alkynes bearing
electron-deficient groups, which are almost unreactive in the
[Cp*RhIII]-catalyzed system, display good reactivity in this
transformation.
More recently, by using the N-nitrosoanilines as starting
materials, which are readily prepared through a very simple
process (Scheme 1a),[10] an interesting rhodium-catalyzed
I
ndole motifs are widespread in natural products, bioactive
compounds, and other functional molecules.[1] In light of their
importance, numerous useful methods have been developed
for their preparation,[2] including the well-established Fischer
indole synthesis.[3] Alternatively, intermolecular cyclization of
ortho-halogenated anilines with alkynes is the frequently used
in the construction of indoles.[4] Importantly, the directed
À
transition-metal-catalyzed C H bond functionalization strat-
egy has been employed in cyclization reactions for broader
substrate scope and higher atom economy.[5] According to this
strategy, the precious transition-metal-catalyzed (e.g., palla-
diumn, rhodium, ruthenium, gold) intermolecular cyclizations
Scheme 1. Cobalt-catalyzed regioselective indole synthesis through
À
C H functionalization. Cp*=C5Me5.
À
of anilines, or its derivatives, with alkynes through C H
annulation for the construction of indoles have been devel-
indole synthesis through the intermolecular cyclization with
alkynes has been independently disclosed by the groups of
Zhu[11a] and Huang[11b] (Scheme 1b). Although [Cp*RhIII]
proved to be effective in this transformation, there are still
some limitations which need to be addressed: 1) High loading
of the expensive rhodium catalyst was used; 2) The regiose-
lectivity was not controlled in the rhodium catalysis. When
using the unsymmetrical meta-substituted N-nitrosoanilines
as substrates, typically a pair of inseparable isomeric indole
products was obtained (Scheme 1b); 3) The internal alkynes
bearing electron-deficient groups were not effective. These
issues may limit its application in organic synthesis. If these
problems could be addressed by either an inexpensive and
stable earth-abundant iron(III) or cobalt(III) catalyst, it is
definitely attractive but still a challenging issue. Herein, we
describe a novel highly regioselective, cobalt-catalyzed cyc-
oped.[6]
In the past decades, the development of the first-row
transition-metal-catalyzed transformation has attracted
attention because these base metals are earth-abundant,
inexpensive, have variable oxidation states, and are less toxic,
etc.[7] In contrast to the instability and the sensitivity of the
low-valent earth-abundant metal catalysts to air and moisture,
the high-valent earth-abundant metals such as iron(III)[8] and
cobalt(III)[9] are stable and easily prepared, and have
À
exhibited incredible potential in C H activation in recent
À
years. Despite these elegant works, the intermolecular C H
cyclization of aniline derivatives with alkynes, catalyzed by
either iron(III) or cobalt(III), has not been achieved.
À
lization of N-nitrosoanilines and alkynes through C H bond
cleavage (Scheme 1c). To the best of our knowledge, this
[*] Y. Liang, Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs, School of
Pharmaceutical Sciences, Peking University
Xue Yuan Rd. 38, Beijing 100191 (China)
http://sklnbd.bjmu.edu.cn/nj/
reaction is the first cobalt(III)-catalyzed construction of
À
indoles through an intermolecular C H cyclization of
simple aniline derivatives with alkynes (Scheme 1c).
With our continued interest in the use of inexpensive
E-mail: jiaoning@pku.edu.cn
metals, such as iron, copper, manganese, and cobalt, to
Dr. N. Jiao
[12]
À
catalyze C N bond-formation reactions and the efficient
State Key Laboratory of Organometallic Chemistry, Chinese Academy
of Sciences, Shanghai 200032 (China)
synthesis of indoles,[6b] we initially tried to execute the
intermolecular cyclization of N-methyl-N-phenylnitrous
amide (1a) with diphenylacetylene (2a) by using iron(III)
Supporting information for this article can be found under http://dx.
doi.org/10.1002/anie.201511002.
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Table 1: Optimization studies.[a]
Entry Catalyst
Additive (equiv) Solvent Yield [%][b]
1
2
3
4
5
6
7
8
FeF3
Fe(OTf)3
[Cp*Co(CO)I2]
[Cp*Co(MeCN)3](SbF6)2
[Cp*Co(MeCN)3](SbF6)2
[Cp*Co(MeCN)3](SbF6)2
[Cp*Co(MeCN)3](SbF6)2 LiOAc (2)
[Cp*Co(MeCN)3](SbF6)2 NaOAc (2)
[Cp*Co(MeCN)3](SbF6)2 KOAc (2)
[Cp*Co(MeCN)3](SbF6)2 AgOAc (2)
[Cp*Co(MeCN)3](SbF6)2 Cu(OAc)2 (2)
[Cp*Co(MeCN)3](SbF6)2 Cu(OAc)2 (1)
[Cp*Co(MeCN)3](SbF6)2 Zn(OAc)2 (2)
[Cp*Co(MeCN)3](SbF6)2 Fe(OAc)2 (0.5)
–
–
–
–
–
–
DCE
DCE
DCE
DCE
MeOH
AcOH
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
–
–
–
7
–
–
–
–
–
<5
73
66
46
51
–
9
10
11
12
13
14
15
–
Cu(OAc)2 (2)
Scheme 2. Reactions of diverse substituted N-nitrosoanilines with
diphenylacetylene. [a] [Cp*Co(MeCN)3](SbF6)2 (10 mol%) was used,
49 h.
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst
(5.0 mol%), additive (0.5–2.0 equiv), solvent (1 mL), 808C for 24 h.
[b] Yield of isolated product. DCE=1,2-dichloroethane, Tf =trifluoro-
methanesulfonyl.
tant substitution patterns to be incorporated into the indole
skeleton. Whereas this reaction seems sensitive to steric
hindrance, the substrate bearing an ortho-substituent
retarded the reaction (3p).
and cobalt(III) catalysts (Table 1). Unfortunately, some iron
salts, and the [Cp*Co(CO)I2] were totally unreactive
(entries 1–3). Delightfully, the use of the cationic [Cp*Co-
(MeCN)3](SbF6)2 afforded the desired product 3a in 7% yield
(entry 4). Other solvents such as MeOH and HOAc were less
effective (entries 5 and 6). Further screening revealed that the
key to achieving this transformation in decent yield is the
identification of a suitable base, and we found that adding
Cu(OAc)2 improved the yield notably, whereas the efficiency
decreased significantly when bases such as LiOAc, NaOAc,
KOAc, and AgOAc were used (entries 7–12). It is noteworthy
that the addition of Zn(OAc)2 and Fe(OAc)2 can furnish the
desired 3a in moderate yields, and thus convinced us that this
reaction indeed proceeded without an external oxidant, and
that the Cu(OAc)2 served as a base, not an oxidant in this
transformation (entries 13 and 14). No reaction was observed
in the absence of the cobalt catalyst (entry 15).
The superior regioselectivity of [Cp*CoIII] compared to
that of [Cp*RhIII] was observed when unsymmetrical meta-
substituted N-nitrosoanilines were employed (Scheme 3). For
example, the meta-bromo- and meta-chloro-substituted N-
nitrosoanilines gave the desired product with impressively
high regioselectivity (i.e., only one isomer was obtained;
4a,b). In comparison, by using the [Cp*RhIII]-catalyzed
system for the same substrates, inseparable mixtures,
(1:1.15) and (1:1.64), were obtained, respectively.[11a] Addi-
tionally, meta-methyl-substituted N-nitrosoaniline also under-
went the cyclization with high site-selectivity (4c). It is worthy
mentioning that our method is not only compatible with
diverse monosubstituted, but also disubstituted N-nitrosoani-
lines as well (4d–i). Among them, the dihalogen-substituted
With the optimized reaction conditions in hand, the
substrat scope was then investigated. The generality of the
reaction is showcased by the remarkable compatibility with
various functional groups on the phenyl ring and the tolerance
of different N-substituents (Scheme 2). When using other N-
phenyl- or N-benzyl-substituted substrates, slightly decreased
yields were obtained, and may be attributed to steric effects
(3b,c). Interestingly, the fused tricyclic indole framework,
which is widely present in natural products, can be easily
constructed using this method (3d). It is important to stress
that N-nitrosoanilines bearing a variety of important func-
tional groups, such as the halogens (F, Cl, Br, I) and phenyl,
alkyl groups (tBu, Me), trifluoro methoxy (OCF3), and
electron-deficient (CF3, CO2Me, CN) groups, at the para-
position of the phenyl ring generally react well under the
present catalytic system (3e–o), thus allowing many impor-
Scheme 3. Reactions of meta-substituted N-nitrosoanilines with diphe-
nylacetylene. [a] [Cp*Co(MeCN)3](SbF6)2 (10 mol%) was used.
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N-nitrosoanilines were found to be efficient reaction partners,
thus offering products suitable for further structural elabo-
ration (4g–i). The reason for the excellent regioselectivity
may be attributed to the smaller ionic radius of cobalt
compared to that of rhodium, thus resulting in a stronger
steric repulsion between the substrate and the Cp* ligand of
[Cp*CoIII]. Therefore, it is much easier for [Cp*CoIII] to
distinguish between the two different ortho sites in the
unsymmetrical meta-substituted N-nitrosoanilines.[9k]
We next tested the scope with respect to the internal
alkynes (Scheme 4). To our delight, various symmetric diaryl-
Scheme 5. Mechanistic studies.
To gain more insight into the mechanism, some experi-
ments were conducted (Scheme 5). A significant level of
deuterium incorporation (20%) was observed at the ortho-
position of 1k when it was subjected to the present cobalt-
catalyzed system in CD3OD, in the absence of an alkyne, thus
À
indicating that the cobalt-mediated C H bond cleavage is
reversible (Scheme 5a). Moreover, a competition reaction
was also performed to determine the electronic preference of
this transformation. When 1e was and 1k were reacted at the
same time, the reaction favored the electron-deficient N-
nitrosoaniline in a 1.33:1 ratio (Scheme 5b), thus implying
Scheme 4. Scope with respect to the internal alkynes. [a] 1 (0.2 mmol),
2 (0.3 mmol), catalyst (5 mol%), Cu(OAc)2 (0.4 mmol), DCE (1 mL),
808C, 24 h. [b] 1 (0.2 mmol), 2 (0.3 mmol), catalyst (10 mol%), Cu-
(OAc)2 (0.4 mmol), DCE (1 mL), 1008C, 36 h. [c] The number within
the parentheses is the yield obtained when [Cp*Rh(MeCN)3](SbF6)2
was used as the catalyst. [d] 1 (0.2 mmol), 2 (0.4 mmol), catalyst
(10 mol%), Cu(OAc)2 (0.4 mmol), DCE (1 mL), 1208C, 24 h. [e] 1
(0.4 mmol), 2 (0.2 mmol), catalyst (10 mol%), Cu(OAc)2 (0.4 mmol),
DCE (1 mL), 1208C, 24 h.
À
that the C H activation probably occurs by a concerted
metalation-deprotonation (CMD) mechanism.[14] Moreover,
a notable primary kinetic isotope effect was observed (KH/
À
KD = 4.0), thus suggesting that C H bond cleavage should be
involved in the rate-limiting step (Scheme 5c).
Based on the mechanistic studies and the relevant
reports,[9,11] a plausible mechanism is proposed (Scheme 6).
The reaction begins with an anion exchange to generate the
substituted alkynes can react smoothly with 1a to generate
the desired products (5a–d). More attractively, according to
the previous reports[11] and our observations,[13] the reaction is
less efficient under the [Cp*RhIII]-catalyzed reaction condi-
tions with an internal alkyne bearing either two electron-
deficient groups or one alkyl group and one electron-deficient
group, such as an ester group (COOEt). In contrast, it is
noteworthy to find that these alkynes displayed good
reactivity in the present protocol, furnishing the desired
product in synthetically acceptable yield (5e–g). In addition,
unsymmetrical alkyl aryl alkynes and carboxy aryl alkynes
could also participate in this reaction, giving the correspond-
ing products in reasonable yields as well as good to excellent
regioselectivity (5 f–j). Surprisingly, for the unsymmetrical
alkyl aryl alkynes, different selectivity was observed between
[Cp*CoIII] and [Cp*RhIII][11] catalysis. For the [Cp*CoIII]
catalyst, products with an aryl group at the 3-position were
favored (5h–j; and further confirmed by HMBC, HSQC and
NOESY experiments). Gratifyingly, the reaction of an alkyl
alkyne also proceeded efficiently (5k).
Scheme 6. Proposed catalytic cycle.
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À
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[15]
À
À
C N bond and break the N N bond with the aid of HOAc.
The product 3a is thus formed and the active catalyst
[Cp*Co(OAc)2] is regenerated to complete the catalytic cycle.
In summary, we have demonstrated an efficient and direct
cobalt-catalyzed cyclization of N-nitrosoanilines with alkynes
and it provides expedient access to various N-substituted
indoles. The unique regioselectivity and reactivity of cobalt-
(III) in the direct cyclization of N-nitrosoanilines with alkynes
is demonstrated for the first time. Moreover, electron-
deficient internal alkynes can be tolerated in the present
À
protocol. Mechanistic studies revealed that the C H activa-
tion is reversible through a concerted metalation-deprotona-
tion (CMD) mechanism. Considering the cost-effective
nature of earth-abundant cobalt and the diversity of acces-
sible covalent linkages for the nitroso group, as well as the
excellent regioselectivity of the present protocol, this method
should be of high synthetic utility. Further applications of this
transformation are ongoing in our laboratory.
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Acknowledgments
Financial support from the National Basic Research Program
of China (973 Program) (grant No. 2015CB856600), the
National Natural Science Foundation of China (Nos.
21325206, 21172006), and the National Young Top-notch
Talent Support Program are greatly appreciated. We thank
Kai Wu in this group for reproducing the results for 4 f and 5e.
À
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À
Keywords: C H activation · cobalt · indoles ·
reaction mechanisms · regioselectivity
How to cite: Angew. Chem. Int. Ed. 2016, 55, 4035–4039
Angew. Chem. 2016, 128, 4103–4107
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Received: November 26, 2015
Revised: January 1, 2016
Published online: February 17, 2016
Angew. Chem. Int. Ed. 2016, 55, 4035 –4039
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4039