Nickel-Catalyzed Synthesis of Benzo[ b]naphtho[1,2- d]azepine via Intramolecular Radical Tandem Cyclization of Alkyl Bromide-Tethered Alkylidenecyclopropanes

Article abstract of DOI:10.1021/acs.orglett.8b02699

A Ni(II)-catalyzed tandem cyclopropane ring opening and radical alkylation of the aromatic ring using unactivated alkyl bromide-tethered alkylidenecyclopropanes (ACPs) have been described in this paper. This ring-forming process exhibits a broad substrate scope with a variety of primary alkyl bromides and aromatic rings, affording diversified benzo[b]naphtho[1,2-d]azepine derivatives in moderate-to-excellent yields under mild conditions. Plausible reaction mechanisms have been proposed on the basis of several control experiments, including the deuterium labeling examinations. Further derivatization of the obtained polycyclic product has also been performed.

Full text of DOI:10.1021/acs.orglett.8b02699

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Nickel-Catalyzed Synthesis of Benzo[b]naphtho[1,2‑d]azepine via
Intramolecular Radical Tandem Cyclization of Alkyl Bromide-
Tethered Alkylidenecyclopropanes
Bo Jiang, Jia-Xin Liu, Yin Wei, and Min Shi*,†,‡
†State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, People’s Republic of China
‡State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: A Ni(II)-catalyzed tandem cyclopropane ring opening and
radical alkylation of the aromatic ring using unactivated alkyl bromide-
tethered alkylidenecyclopropanes (ACPs) have been described in this
paper. This ring-forming process exhibits a broad substrate scope with a
variety of primary alkyl bromides and aromatic rings, affording diversified
benzo[b]naphtho[1,2-d]azepine derivatives in moderate-to-excellent yields
under mild conditions. Plausible reaction mechanisms have been proposed
on the basis of several control experiments, including the deuterium labeling
examinations. Further derivatization of the obtained polycyclic product has also been performed.
any natural products containing the benzazepine core
scaffold widely exist in many biologically active
molecules and photoelectric materials such as antitumor
agent, N-benzylkenpaullone, and photochromic oxepine, as
shown in Figure 1.1 Moreover, the biological activities of some
Transition metal catalysts have proven useful in selective
intramolecular aromatic C−H alkylations using activated alkyl
halides.5,6 It is also well-known that, in transition metal-
catalyzed intermolecular aromatic C−H alkylations, a directing
group appended at the aromatic substrate is required to react
with unactivated alkyl halides.7,8 However, a transition metal-
catalyzed intramolecular tandem radical C−H alkylation of
arenes and heteroarenes using unactivated alkyl halides has not
been reported thus far and would be highly desirable in the
synthesis of polycyclic skeletons.
M
Recently, Lei’s group reported the nickel(0)-catalyzed
aromatic C−H alkylation with activated secondary or tertiary
alkyl bromides for the construction of indolones. This reaction
undergoes a Ni(I)/Ni(II) catalytic cycle via a SET process, and
the radical of the aromatic ring is oxidized by the Ni(II)
species to give the product (Scheme 1, eq 1).9 In 2015,
Alexanian and co-workers also reported palladium(0)-catalyzed
ring-forming aromatic C−H alkylations with unactivated alkyl
halides, affording the cyclized products in moderate to good
yields (Scheme 1, eq 2).10 On the basis of these successful
examples, we envisaged that the benzo[b]naphtho[1,2-d]-
azepine scaffold could also be established from the intra-
molecular radical tandem cyclization of alkyl bromide-tethered
alkylidenecyclopropanes (ACPs) in the presence of the Ni(II)
catalyst (Scheme 1, eq 3, this work). Herein, we wish to report
the details in this context.11
Figure 1. Selected examples of biologically active product and
photoelectric material with benzo[b]naphtho[1,2-d]azepine.
modified benzazepine-containing compounds, such as N-
benzylkenpaullone, have also been intensively studied both in
vitro and vivo.2 Therefore, it is not surprising that much
attention has been paid to developing a more general and
efficient strategy to afford such azacyclic skeletons.
The Friedel−Crafts alkylation is an important synthetic
approach to polycyclic carbo- or heterocycles via the
cyclization of aromatic substrates with simple alkyl halides.3
However, there are some limitations for substrate scopes such
as the requirement of electron-rich aromatic substrates and the
utilization of stoichiometric amounts of strong Lewis acids.
Recently, an alternative approach has been found via the
radical-mediated homolytic aromatic substitutions by which
the electronic activation of substrate is not required as
compared to the standard Friedel−Crafts alkylation of
aromatic systems.4
Initially, we attempted to optimize the reaction conditions
for this nickel(II)-catalyzed intramolecular tandem cyclization
of substrate 1a. As shown in Table 1, the reaction was
Received: August 23, 2018
© XXXX American Chemical Society
A
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and L4 were tested, revealing that L1 was the best choice. This
may be due to L2, L3, and L4 being difficult to generate
LNi(II)Br2 species because L2 and L4 are the electron-
deficient nitrogen-containing ligands and L3 is a flexible
nitrogen-containing ligand (entries 5−7). In the absence of L1,
2a could not be detected under the standard conditions (entry
8). Moreover, without Ni(II) catalyst or Zn additives, no
reaction occurred (entries 9 and 10). Subsequently, the solvent
effect was examined using NiBr2(DME) as the catalyst, and it
was found that the employed solvents had a significant impact
on the reaction outcomes. Using DMAc and DMSO as the
solvents afforded 2a in 74 and 70% yields, respectively, and
other solvents such as DMA, THF, DCE, MeCN, and toluene
were not suitable media for this reaction (entries 11−17).
Raising the reaction temperature to 100 °C produced 2a in
66% yield (entry 18). Lowering the reaction temperature to 50
°C and room temperature (25 °C) furnished 2a in 56 and 32%
yields, respectively (entries 19 and 20). The structure of 2a has
been unambiguously determined by X-ray diffraction. The CIF
data are presented in the Supporting Information (SI), and the
ORTEP drawing of 2a is shown in Table 1.12 On the basis of
the above reaction conditions, we identified that the reaction
should be carried out in DMF at 80 °C using NiBr2(DME) (10
mol %) as the catalyst and L1 as the ligand in the presence of
Zn additives.
Scheme 1. Transition-Metal-Catalyzed Radical Alkylation of
Aromatic Ring and This Work
Table 1. Optimization of Reaction Conditions for Substrate
1a
With the determination of the best conditions for the
production of 2a, we next investigated the substrate scope of
this transformation using various substrates 1 (0.4 mmol, 1.0
equiv) under the optimal reaction conditions. The results are
summarized in Scheme 2. Phenyl, electron-donating methoxyl,
and alkyl group-substituted substrates 1b−e, 1i, and 1j as well
as 1m and 1n were compatible in this transformation, giving
the desired products 2b−e, 2i, and 2j as well as 2m and 2n in
56−86% yields, but the products 2i and 2j were obtained as
two regioisomers along with a ratio of 1:1. We found that
Scheme 2. Substrate Scope of ACPs 1 under the Optimal
a b
,
Conditions
a
Reaction conditions: 1a (0.2 mmol), cat. (10 mol %), Zn (2.0
equiv), ligand (10 mol %), dry solvent (2.0 mL). N.R. = no reaction,
DMF = N,N-dimethylformamide, DMAc = N,N-dimethylamide
b
c
acetate, DMA = N,N-dimethylaniline. Isolated yields. Yield
1
determined by H NMR spectroscopic analysis of the crude reaction
mixture using 1,3,5-trimethoxybenzene as an internal standard. 1a
d
e
decomposed. Without addition of Zn.
conducted upon treating 1.0 equiv of 1a with 2.0 equiv of Zn
in the presence of NiCl2(DME) (10 mol %) and L1 (10 mol
%) in N,N-dimethylformamide (DMF) at 80 °C, affording
desired product 2a in 56% yield (entry 1). To improve the
reaction outcome, we examined the nickel catalysts with
different counteranions such as OAc and Br (entries 1−4) and
found that the best catalyst is NiBr2(DME), giving 2a in 75%
yield. Next, other nitrogen-containing ligands such as L2, L3,
a
Reaction conditions: 1 (0.4 mmol), NiBr2(DME) (10 mol %), Zn
b
(2.0 equiv), 1,10-phen (10 mol %), DMF (4.0 mL). Isolated yields.
c
Zn (4.0 equiv).
B
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introducing electron-withdrawing groups such as halogen
atoms F, Cl, Br, and the trifluoromethyl group (CF3) into
both of the aromatic rings in substrate 1 produced the desired
hydroquinoline derivatives 2f−h, 2i, and 2o in 40−70% yields.
Substrates 1g and 1h bearing strongly electron-withdrawing F
and CF3 groups gave the corresponding products in lower
yields, presumably due to the electronic effect. However,
introducing the methyl group and chlorine atom at the ortho-
position such as substrates 1k and 1r only provided complex
mixtures presumably due to the steric effect. Naphthyl and
benzothiophenyl groups incorporating substrates 1x and 1w
were also tolerated, giving the desired products 2w and 2x in
62 and 23% yields, respectively. In the case of substrate 1s
having a thiophene moiety, a complex mixture was also
afforded under the standard conditions. Moreover, we also
examined the other protecting groups (Pg) such as Ms and Ns
in this transformation and found that substrate 1p afforded
desired product 2p in 80% yield, but no reaction occurred if
using alkyl bromide-tethered alkylidenecyclopropane 1q (Pg =
Ns) as substrate under the standard conditions. In general, an
electron-rich substituent on benzene ring facilitated the
reaction, affording the desired product in higher yield.
Substrate 1t, in which the oxygen atom was used as a linker,
delivered desired product 2t in 56% yield. To further extend
the substrate scope of this reaction, we used substrates 1u and
1v having a (CH2)3 and (CH2)4 carbon chain in this
transformation, affording desired products 2u and 2v bearing
eight- and nine-membered rings in 59 and 74% yields,
respectively. The use of alkyl bromide-tethered alkylidenecy-
clobutane 1y as substrate produced cyclobutane ring-opened
product 2y in 45% yield under the standard conditions. For
substrate 1z bearing a methyl group rather than a benzene ring,
the corresponding cyclopropane ring-opened product 2z was
given in 44% yield in the presence of 4.0 equiv of a Zn additive.
More interestingly, we found that using alkyl bromide-
tethered alkylidenecyclopropane 3 produced the correspond-
ing cyclopropane remaining products 4 in moderate to good
yields at 120 °C under otherwise identical conditions. As
shown in Scheme 3, a variety of substrates were suitable in this
The gram-scale synthesis and further derivatization of 2a
have been conducted, as shown in Scheme 4. The use of 4.0
Scheme 4. Gram-Scale Synthesis and the Further
Transformation of 2a
mmol 1a could give 2a in 70% yield (1.12 g) under the
standard conditions. In addition, the oxidation of 2a produced
desired product 5a in 84% yield in the presence of 2.0 equiv of
m-CPBA, and the aromatization of 2a afforded corresponding
product 6a in 50% yield upon treatment with 1,2-dichloro-4,5-
dicyanobenzoquinone (DDQ) (2.0 equiv) in PhCl at 80 °C.
Following our synthetic studies, we performed a number of
control experiments to gain more insight into the reaction
mechanism (Scheme 5). No reaction occurred upon treatment
of 1a with 2.0 equiv of 2,2,6,6-tetramethylpiperidine 1-oxyl
(TEMPO) under the standard conditions, and a trace amount
Scheme 5. Further Mechanistic Investigations
Scheme 3. Substrate Scope of ACPs 3 for the Production of
a
Products 4
a
Reaction conditions: 3 (0.4 mmol), NiBr2(DME) (10 mol %), Zn
(2.0 equiv), 1,10-phen (10 mol %), DMF (4.0 mL). Isolated yields.
transformation to give desired products 4a−d in 53−83%
yields. The structure of 4d has been unambiguously
determined by X-ray diffraction. The CIF data are presented
in the Supporting Information, and the ORTEP drawing of 4d
is shown in Scheme 3.12 Substrate 3e using an oxygen atom as
a linker afforded a complex mixture. The steric effect may
cause a different radical reaction pathway between substrates
1z and 3.
C
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of 2a was formed if 2.0 equiv of di-tert-butylhydroxytoluene
(BHT) was added into the reaction system. When the reaction
vessel was exposed to air, no reaction occurred, and substrate
1a was fully recovered. Furthermore, in the dark, the reaction
also proceeded smoothly to afford 2a in 68% yield, indicating
that the visible light irradiation did not significantly affect the
reaction outcome. All these results suggest a radical pathway
for this transformation. For identifying the hydrogen source in
the transformation of 1z and 3a, isotope labeling experiments
by adding DMSO-d6 to the reaction system were conducted.
We found that the corresponding deuterium incorporation
product [D]-2z was afforded in 43% yield along with 69% D1
content and 28% D2 content (Scheme 5, eq 1). In the case of
substrate 3a, product [D]-4a was produced in 69% yield along
with 43% D content incorporated at the benzene ring (Scheme
5, eq 2). The use of D2O in the reaction of 3a or using D2O to
quench the reaction under the standard conditions in DMF or
DMSO did not give deuterium-incorporated product [D]-4a,
suggesting a radical pathway in these reactions (Scheme 5, eqs
3−5).
intermediate G with the solvent such as DMF or DMSO
afforded product 2y along with regeneration of the nickel(0)
catalyst via the reduction by Zn, or the formed DMSO radical
might go through single electron transfer (SET) with LNi(I)Br
species again, giving a brominated DMSO species. When R is a
phenyl group and n = 1, the corresponding intermediate B
underwent a cyclopropane ring-opening process to afford
radical intermediate C, which produced radical intermediate D
through an intramolecular alkylation. A single electron transfer
(SET) between intermediate D and LNi(I)Br delivered
desired product 2a along with the regeneration of the Ni(0)
species for the next catalytic cycle.
In summary, we have disclosed a novel protocol of Ni(II)-
catalyzed tandem cyclopropane ring-opening and radical
alkylation of the aromatic ring using unactivated alkyl
bromide-tethered alkylidenecyclopropanes (ACPs). This ring-
forming process has a broad substrate scope with a variety of
primary alkyl bromides and aromatic moieties, giving
diversified benzo[b]naphtho[1,2-d]azepine derivatives in mod-
erate to excellent yields under mild conditions. A plausible
mechanism has been proposed on the basis of previous
literature and our own experimental results including the
deuterium labeling examination. Further derivatization of the
obtained polycyclic product 2a could smoothly give the
corresponding epoxidation and aromatization products. We
anticipate that this finding will provide a practical application
for the synthesis of diversified benzo[b]naphtho[1,2-d]azepine
derivatives. The potential utilization and extension of this new
synthetic methodology are currently under investigation in our
laboratory.
According to the previously reported literature7,9 and our
experimental results, a plausible mechanism for this Ni-
catalyzed intramolecular tandem radical cyclization has been
outlined in Scheme 6.13 In the presence of Zn and 1,10-phen
Scheme 6. A Plausible Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02699.
Experimental procedures and characterization data for
all compounds (PDF)
Accession Codes
CCDC 1839133 and 1854478 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
ligand, LNi(II) species was reduced to LNi(0), which reacted
with 1 to give radical species A through single electron transfer
(SET), and the LNi(0) species was oxidized to the Ni(I)
species. When R is a hydrogen atom and n = 1, the
intramolecular radical addition gave intermediate E, which
underwent an intramolecular hydrogen atom transfer to give
intermediate F. The hydrogen atom transfer (HAT) with the
solvent such as DMF or DMSO afforded the final product 4a.
When R is a CH3 group and n = 1, the intramolecular radical
addition gave intermediate H, which underwent an intra-
molecular 1,5-HAT to afford intermediate I. Intermediates H
and I could produce 2z both via an intermolecular HAT with
the solvent such as DMF or DMSO. When R is a phenyl group
and n = 2, the intramolecular radical addition gave
intermediate B, which underwent a cyclobutane ring-opening
process to give intermediate G. The intermolecular HAT of
AUTHOR INFORMATION
Corresponding Author
■
*Fax: (+86)-21-6416-6128; e-mail: mshi@mail.sioc.ac.cn.
ORCID
Min Shi: 0000-0003-0016-5211
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Basic
Research Program of China (973)-2015CB856603, the
Strategic Priority Research Program of the Chinese Academy
of Sciences, Grant No. XDB20000000 and sioczz201808, and
the National Natural Science Foundation of China (20472096,
D
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Letter
(12) CCDC 1839133 for 2a and CCDC 1854478 for 4d contains
the supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(13) (a) Kuang, Y. L.; Wang, X. F.; Anthony, D.; Diao, T. N. Chem.
Commun. 2018, 54, 2558. (b) Yuan, Y. C.; Liu, H. L.; Hu, X. B.; Wei,
Y.; Shi, M. Chem. - Eur. J. 2016, 22, 13059. (c) Fan, X.; Yu, L.-Z.; Wei,
Y.; Shi, M. Org. Lett. 2017, 19, 4476. (d) Matsubara, H.; Kawamoto,
T.; Fukuyama, T.; Ryu, L. Acc. Chem. Res. 2018, 51, 2023.
21372241, 21572052, 20672127, 21421091, 21372250,
21121062, 21302203, 20732008, 21772037, and 21772226).
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