Free-Amine-Directed Iridium-Catalyzed C?H Bond Activation and Cyclization of Naphthalen-1-amines with Diazo Compounds Leading to Naphtho[1,8-bc]pyridines

Article abstract of DOI:10.1002/adsc.201801598

Iridium-catalyzed C?H activation and cyclization of naphthalen-1-amines with diazo compounds leading to naphtho[1,8-bc]pyridines have been developed. Different from the previous free-amine-directed C?H functionalization with diazo compounds that relied on the coordination of lone pair electrons or in situ formation of imine, this transformation passes through a five-membered iridacycle intermediate containing an N?Ir σ-bond. It offers an alternative approach for the synthesis of useful diverse naphtho[1,8-bc]pyridine derivatives in mild conditions. (Figure presented.).

Full text of DOI:10.1002/adsc.201801598

Title: Free-Amine-Directed Iridium-Catalyzed C–H Bond Activation
and Cyclization of Naphthalen-1-amines with Diazo Compounds
Leading to Naphtho[1,8-bc]pyridines
Authors: Kelu Yan, Yongxue Lin, Yong Kong, Bin Li, and Baiquan
Wang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801598
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10.1002/adsc.201801598
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Free-Amine-Directed Iridium-Catalyzed C–H Bond Activation
and Cyclization of Naphthalen-1-amines with Diazo
Compounds Leading to Naphtho[1,8-bc]pyridines
Kelu Yan,a Yongxue Lin,b Yong Kong,b Bin Li,a and Baiquan Wanga,c,*
a
State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071,
People’s Republic of China
Fax: (+86)-22-23504781; e-mail: bqwang@nankai.edu.cn
SINOPEC Research Institute of Petroleum Engineering, Beijing 100101, People’s Republic of China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
c
Sciences, Shanghai 200032, People’s Republic of China
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
still an urgent need to establish new methods to
synthesize naphtho[1,8-bc]pyridine derivatives with
various substituents.
Abstract: Iridium-catalyzed C–H activation and
cyclization of naphthalen-1-amines with diazo compounds
leading to naphtho[1,8-bc]pyridines have been developed.
Different from the previous free-amine-directed C−H
functionalization with diazo compounds that relied on the
coordination of lone pair electrons or in situ formation of
imine, this transformation passes through a five-membered
iridacycle intermediate containing an N-Ir σ-bond. It offers
an alternative approach for the synthesis of useful diverse
naphtho[1,8-bc]pyridine derivatives in mild conditions.
In the past decades, transition-metal-catalyzed
direct C–H bond functionalization has flourished as a
powerful and atom-economical method to construct
C–C and C–X bonds without prior functionalization.[4]
Free amino groups which widely occur in natural
products and synthetic compounds are one of the best
directing and assisting groups for C−H
functionalization reactions catalyzed by Pd(II),[5]
Rh(III),[6] Ir(III),[7] Ru(II),[8] and other metals.[9] At
the same time, diazo compounds have been widely
employed as powerful cross-coupling partners for the
construction of various organic frameworks by
transition-metal-catalyzed conversions.[10] However,
investigations on the transformations of free-amine-
directed C−H functionalization with diazo
Keywords: CH activation; diazo compounds; iridium;
naphtho[1,8-bc]pyridines
Seeking novel, practical, and efficient methods for
the construction of nitrogen-containing fused
aromatics have attracted much attention for their
broad applications in biological, photo- and
electrochemical
materials.[1]
Naphtho[1,8-
bc]pyridines are an important class of nitrogen-
containing heterocyclic compounds. They possess
potential applications as organic semiconductors and
luminescence materials for their important
electrochemical and photochemical properties.[2]
Therefore, the development of novel and efficient
methods for the synthesis of naphtho[1,8-
bc]pyridines is of great importance. However,
investigations on the synthetic methods for the
formation of naphtho[1,8-bc]pyridine motifs are
rather limited. Very recently, our group reported an
elegant rhodium-catalyzed C–H activation and
annulations of β–enaminonitriles with alkynes
leading to substituted naphtho[1,8-bc]pyridines.[3]
Despite some great advantages of this conversion, the
reaction yields based on aliphatic alkynes decreased
seriously and the regioselectivity for unsymmetrical
alkynes is also difficult to control. Therefore, there is
Scheme 1. Free-amine-directed C−H functionalization
with diazo compounds
1
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10.1002/adsc.201801598
Advanced Synthesis & Catalysis
compounds are rather limited. In 2016, Huang and
coworkers developed a Rh(III)-catalyzed free-amine-
directed C−H functionalization/amidation reaction for
synthesis of azepinone derivatives (Scheme 1a).[6k]
This reaction relies on the coordination of the lone
the substrate, the corresponding product 3oa was
obtained in 62% yield. Meanwhile, the study of
substrate with a substituent at the C5-position was
also examined and the corresponding product 3pa
was obtained in 51% yield. In addition, the reactions
pair electrons of the N atom on the free amino groups. of pyren-1-amine (1q) and anthracen-9-amine (1r)
Subsequently, Cheng,[6l] Li,[6m] and Cui[6n] reported
similar Rh(III)-catalyzed annulations of
with 2a provided the desired products 3qa and 3ra in
55% and 70% yields, respectively.
benzylamines with diazo compounds toward
isoquinolines (Scheme 1b). In these three reactions,
benzylamines needed to react with acetone or be
oxidized to imines, and the in situ formed imines
have been proposed to be the real directing group. In
addition, all of this kind of conversions employ a
Rh(III) catalyst. In contrast Ir(III)-catalyzed C–H
activation reactions[11] employing diazo compounds
as coupling partners were reported not as much as
Rh(III)-catalyzed reactions. Following our growing
interest in building heterocyclic compounds using
diazo compounds[11h,12] and iridium-catalyzed C–H
bond functionalizations,[13] we herein report a free-
Table 1. Substrate scope of naphthalen-1-amines.[a]
amine-directed
iridium-catalyzed
C–H
bond
activation and cyclization of naphthalen-1-amines
with diazo compounds leading to naphtho[1,8-
bc]pyridines in mild conditions (Scheme 1c).
Different from the previous free-amine-directed C−H
functionalization with diazo compounds that relied on
the coordination of lone pair electrons or in situ
formation of imine, this reaction passes through a
five-membered iridacycle intermediate containing an
N-Ir σ-bond.
By treating naphthalen-1-amine (1a) (42.9 mg, 0.3
mmol,
1.5
equiv.)
with
2-diazo-5,5-
dimethylcyclohexane-1,3-dione (2a) (33.2 mg, 0.2
mmol, 1.0 equiv.) in the presence of
[Cp*Ir(CH3CN)3](SbF6)2 (10 mol%) and HOAc (1.0
equiv.) in N,N-dimethylformamide (DMF) (1.0 mL)
at room temperature for 12 h under Ar, product 9-
dimethyl-9,10-dihydro-7H-benzo[kl]acridin-11(8H)-
one (3aa) was obtained in 83% yield (for detailed
optimization studies, see Table S1 in the ESI). The
[a]
Reaction conditions: 1a (0.3 mmol), 2 (0.2 mmol)
[Cp*Ir(CH3CN)3](SbF6)2 (10 mol%), HOAc (0.2 mmol),
DMF (1.0 mL), r.t., 12 h, under Ar, isolated yield.
Then, we investigated the scope of diazo
compounds with naphthalen-1-amine (1a) as the
reaction partner (Table 2). By treating 1a with
different six-membered cyclic diazo compounds,
products 3ab–3ad were obtained in good yields (63–
85%). The reaction of 1a with a meta-substituted six-
membered cyclic diazo compound 2e afforded a
mixture of two isomers in a ratio of 1:0.8.
Unfortunately, we failed to separate this pair of
isomers. Product 3af was obtained in 51% yield when
five-membered cyclic 2-diazo-1,3-dione (2f) was
employed in this reaction. Furthermore, different
kinds of open chain diazo substrates were also
investigated, and the desired products 3ag–3ao
containing diverse substituents such as alkyl, ether,
ketone, phenyl, and ester were given in 41–92%
yields. These regioselective products with various
alkyl groups are difficult to prepare for the reactions
employing alkynes as coupling partners.[3]
1
structure of 3aa was confirmed by its H, 13C NMR
spectra and high-resolution mass spectrometry.
With the establishment of the optimized conditions,
we explored the applicability of the scope of
diversely substituted naphthalen-1-amines (1a−1r),
and the results are summarized in Table 1. To our
delight, most reactions proceeded smoothly to afford
corresponding products (3aa–3ra) in moderate to
good yields. Substituted naphthalen-1-amines bearing
alkyl, phenyl, and 2-naphthyl groups at the C4-
position afforded corresponding products (3ba–3ga)
in good yields (73–84%). While, 4-methoxyl and
electron-withdrawing groups substituted naphthalen-
1-amines afforded corresponding products (3ha–3ka)
in moderate yields (41–59%). Delightfully, the
substrate bearing a thiophene/furan ring also gave the
desired products (3la, 3ma) in 65–71% yields.
Subsequently, the ortho-substituted naphthalen-1-
amine (1n) was also explored and the expected
product (3na) was obtained in 78% yield. When 2,3-
dibromonaphthalen-1-amine (1o) was employed as
To gain more insight into the mechanism, a series
of preliminary mechanistic experiments have been
conducted.
A
hydrogen–deuterium exchange
experiment of 1a using 0.2 mL of AcOD or D2O was
2
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10.1002/adsc.201801598
Advanced Synthesis & Catalysis
Table 2. Substrate scope of diazo compounds.[a]
obtained in 17% yield (Scheme 2f). When
intermediate A-1 was used as the catalyst instead of
[Cp*Ir(CH3CN)3](SbF6)2 in standard conditions, the
desired product 3ka was obtained in 56% yield
(Scheme 2g). These results supported the idea that the
reaction may engage a cyclometalation step and a
five-membered iridacycle intermediate containing an
N-Ir σ-bond probably was an active species in this
reaction. In addition, the intermolecular competition
experiment between electronically differentiated 4-
methylnaphthalen-1-amine
(1b)
and
4-
bromonaphthalen-1-amine (1j) with 2a was
performed to determine the electronic preference of
1
the reaction. The H NMR spectrum of the obtained
products showed that the corresponding products 3ba
and 3ja were produced in a ratio of 1:0.45. A
competition
experiment
starting
(1b)
from
and
4-
4-
methylnaphthalen-1-amine
nitronaphthalen-1-amine (1k) with 2a was also
performed, and the corresponding products 3ba and
3ka were produced in a ratio of 1:0.31. Both results
indicated that the reaction favored the electron-rich
naphthalen-1-amines (Scheme 2h).
[a]
Reaction conditions: 1a (0.3 mmol), 2 (0.2 mmol)
[Cp*Ir(CH3CN)3](SbF6)2 (10 mol%), HOAc (0.2 mmol),
DMF (1.0 mL), r.t., 12 h, under Ar, isolated yield.
performed under standard conditions, and no H/D
exchange was detected (Scheme 2a). This result
indicated that the cleavage of the C–H bond at the
C8-position of naphthalen-1-amine was an
irreversible process. Then, a deuterium competition
experiment between substrates 1a and 1a-d7
illustrated a kinetic isotope effect (KIE) of 4.0
(Scheme 2b). Meanwhile, two parallel independent
reactions of 1a and 1a-d7 were performed, and a KIE
of 2.3 was observed (Scheme 2c). Both results
indicated that the C–H bond cleavage might be
involved in the rate-determining step. An iridacycle
intermediate A-1 was isolated in 99% yield by the
reaction of 4-nitronaphthalen-1-amine (1k) with
[Cp*IrCl2]2 and pyridine in the presence of
triethylamine at room temperature (Scheme 2d). Its
1
structure was fully characterized by H and 13C NMR
spectra and high-resolution mass spectrometry.
However, when the reaction of 1k with [Cp*IrCl2]2
was conducted under the standard catalytic conditions,
no iridacycle intermediate was detected. The
formation of A-1 could be observed only in trace
amount in the presence of pyridine (Scheme 2e) and
detected by ESI–HRMS from the reaction mixture
(see 5e in the SI). This may be because the catalytic
conditions are not conducive to the formation of a
stable A-1 but to the formation of an unstable
iridacycle intermediate that facilitates conversion to a
product. By treating intermediate A-1 with 2a in
standard conditions, the final product 3ka was
Scheme 2. The mechanism study experiments.
3
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10.1002/adsc.201801598
Advanced Synthesis & Catalysis
Afterwards, it was diluted with CH2Cl2 (5 mL) and filtered
through a short pad of Celite. The sealed tube and Celite
pad were washed with an additional 30 mL of CH2Cl2. The
filtrate was concentrated in vacuo, and the resulting residue
was purified by flash column chromatography on silica gel
with EtOAc/petroleum ether.
On the basis of known transition-metal-catalyzed
C–H bond activation reactions[4] and our group’s
related research,[11h,12] a possible mechanism is
proposed for this present catalytic reaction (Scheme
3). The first step is likely to be a ligand exchange
between [Cp*Ir(CH3CN)3](SbF6)2 and HOAc to form
the catalytic species [Cp*Ir(OAc)(MeCN)](SbF6),
which then coordinates with 1a and subsequently
occurs C(sp2)–H bond activation to form a five-
membered iridacycle intermediate A by releasing
HOAc. Then A reacts with the diazo compound 2a to
afford the iridium-carbene intermediate B with
extrusion of nitrogen. Migratory insertion of the
carbene into the Ir–C bond generates the intermediate
C. Protonolysis of C leads to the intermediate D and
release catalytic species for next catalytic cycle.
Finally, condensation of intermediate D generates the
final product 3aa and water.
Acknowledgements
We thank the National Natural Science Foundation of China (No.
21672108) and the Natural Science Foundation of Tianjin (No.
16JCZDJC31700) for financial support.
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Scheme 3. Proposed mechanistic pathway.
In conclusion, we have successfully developed a
novel and efficient iridium-catalyzed C–H activation
and cyclization of naphthalen-1-amines with diazo
compounds to build substituted naphtho[1,8-
bc]pyridines. Various regioselective products were
prepared in moderate to good yields under mild
conditions. Several related mechanistic studies have
been performed. Further investigations and
applications on electrochemical and photochemical
properties of the obtained compounds in this work are
in progress in our laboratory.
Experimental Section
A mixture of substituted naphthalen-1-amines (1) (0.3
mmol, 1.5 equiv.), diazo compounds (2) (0.2 mmol, 1.0
equiv.), [(Cp*Ir(CH3CN)3](SbF6)2 (18.4 mg, 0.02 mmol,
10.0 mol%), HOAc (12.0 mg, 0.2 mmol, 1.0 equiv.) were
weighted in a Schlenk sealed tube equipped with a stir bar.
Dry DMF (1.0 mL) was added and the mixture was stirred
at room temperature for 12 h under Ar atmosphere.
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10.1002/adsc.201801598
Advanced Synthesis & Catalysis
COMMUNICATION
Free-amine-directed iridium-catalyzed C–H bond
activation and cyclization of naphthalen-1-amines
with diazo compounds leading to naphtho[1,8-
bc]pyridines
Adv. Synth. Catal. Year, Volume, Page 1 - Page 5
Kelu Yan, Yongxue Lin, Yong Kong, Bin Li, and
Baiquan Wang*
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