Syntheses of Benzofuranoquinolines and Analogues via Photoinduced Acceptorless Dehydrogenative Annulation of o-Phenylfuranylpyridines

Article abstract of DOI:10.1021/acs.orglett.9b03556

A strategy for the syntheses of benzofuranoquinolines and its analogues via the irradiation of o-phenylfuranyl/thienylpyridines/pyrimidines in DCM with UV light at rt under an argon atmosphere is described. The mechanism of this reaction through the proce

Full text of DOI:10.1021/acs.orglett.9b03556

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Syntheses of Benzofuranoquinolines and Analogues via
Photoinduced Acceptorless Dehydrogenative Annulation of
o‑Phenylfuranylpyridines
Jinming Fan,† Wei Zhang,† Wangxi Gao, Tao Wang, Wei-Liang Duan, Yong Liang,‡
and Zunting Zhang*
Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an 710119, People’s Republic of China
S
* Supporting Information
ABSTRACT: A strategy for the syntheses of benzofuranoquinolines and its
analogues via the irradiation of o-phenylfuranyl/thienylpyridines/pyrimidines in
DCM with UV light at rt under an argon atmosphere is described. The
mechanism of this reaction through the process of 6π-electrocyclization, [1,5]-
hydrogen shift, and 1,3-eneamine tautomerism leading to H2 evolution was
elucidated. Notably, the syntheses of cis-8b-methyl-8b,13a-dihydrobenzo[f]-
benzofuro[3,2-h]quinolone via the photoinduced rearrangement of 2-(3-
methylbenzofuran-2-yl)-3-phenylpyridine relevant to the mechanism of this
reaction highlights the importance of the developed methodology.
Scheme 1. Background on Photoinduced Intramolecular
Cross-Coupling Hydrogen Evolution Reactions and
Synthesis of Benzo[f]thieno[3,2-h]isoquinoline
ifferent from all-carbon polycyclic aromatic hydrocarbons
(PAHs), fused heterocyclic polycyclic aromatic hydro-
D
carbons have a large degree of conjugation and contain
heteroatoms, such as N, O, and S in the ring system, enhancing
their photoluminescence properties, electron injection, and
transport capabilities in organic material applications.1 They are
often used to design fluorescent chemical sensors.2 In addition,
chelation of certain fused heterocyclic aromatic hydrocarbons
with metal ions by formation complexes could improve their
physicochemical properties.3 Therefore, the synthesis of fused
heterocyclic aromatic compounds is of great significance.
Photoinduced organic reactions have been playing significant
roles in green chemistry, especially in the construction of
polycyclic aromatic compounds, which would be difficult to
access with standard chemistry reactions in the ground state.4
Furthermore, the protecting groups are not always necessary
during the photochemical reactions,5 which usually occur at rt or
even below. This milder reaction condition is the advantage over
thermochemical reactions. Recently, photoreactions have been
used as a practical synthesis method in the fields of organic and
material chemistry.6 An intramolecular cross-coupling hydrogen
evolution (CCHE) reaction is one of the good strategies to
synthesize fused heterocyclic aromatic hydrocarbons, but very
few examples are reported. In 2014, Reiser et al. reported an Ir-
catalyzed light-induced dehydrogenative annulation of α-
heteroarylchalcones for the synthesis of naphtho[2,1-b]furans
(Scheme 1a).7
1b).8,9 Moreover, irradiation of N-phenylbenzothioamides with
blue LED in the presence of Ru(bpy)3(PF6)2 and Co(III)-
(dmgH)2(4-NMe2Py)Cl gave the dehydrocyclization product,
benzothioazoles (Scheme 1c).10 All of these transition-metal-
catalyzed intramolecular CCHE reactions were carried out with
Also, cyclization of methyl-3-(phenylamino)but-2-enoate and
ethyl-3-phenyl-3-(phenylamino)acrylate in the presence of
Pd(OAc)2/[Ir(bpy)(ppy)2]PF6 and Ir(ppy)3/Co(dmgH)2(4-
CO2Mepy)Cl gave methyl-2-methyl-1H-indole-3-carboxylate
and ethyl-2-phenyl-1H-indole-3-carboxylate by the photo-
induced annulation hydrogen evolution, respectively (Scheme
Received: October 8, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03556
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
visible light irradiation at rt, and hydrogen was formed as the
only byproduct.11
Table 1. Optimization of Reaction Conditions
Quinoline and isoquinoline derivatives are important
heterocyclic compounds and could be found in natural products
and drugs.12 Dibenzo[f,h]quinoline and its analogues, the fused
polycyclic derivatives of quinoline and isoquinoline, have not
been previously explored due to their limited accessibility13 but
exhibit unprecedented chemical and physical properties. For
example, benzo[f ]thieno[3,2-h]isoquinoline units have been
used in electronic devices. The current method for the synthesis
of benzo[f]thieno[3,2-h]isoquinoline requires transition metals,
high temperature (150 °C), and extended reaction time (48 h),
which significantly limits its application (Scheme 1d).14
Given our interest in photoinduced annulation15 as well as the
existence of limited literature reports on intramolecular
dehydrogenative annulation, in this letter, we report a strategy
for the synthesis of benzofuranoquinolines and their analogues
via the photoinduced acceptorless dehydrogenative intermo-
lecular annulation of o-phenylfuranylpyridine and its analogues
(Scheme 2). The protocol eliminates the use of a transition
b
c
entry
solvent
EtOH
MeOH
95% EtOH
DCE
ACE
ACN
DCM
TCM
benzene
DEE
DMF
hexane
toluene
DCM
time (h)
yield (%)
1
2
3
4
5
6
7
8
8
10
8
26
28
30
33
35
40
63
60
5
12
12
2
3.5
8
10
10
10
10
2
9
19
10
11
12
13
NR
NR
trace
trace
NR
d
14
Scheme 2. Synthesis of Benzofurano/Thienoquinolines,
Isoquinolines, and Quinazolines via the Photoinduced
Acceptorless Dehydrogenative Intermolecular Annulation of
o-Phenylfuranylpyridines and Analogues
a
Irradiation of 1a (0.25 mmol) in various solvents (50 mL, 5 mM)
with a high-pressure mercury lamp (500 W) at rt. Reaction time was
determined by the complete consumption of 1a as indicated by thin-
layer chromatography (TLC). Isolated yields. In the open air.
b
c
d
decomposed substrate, and no 2a was obtained (entry 14).
Thus, irradiation of 1a (5 mM) in DCM at ambient temperature
under an Ar atmosphere was determined to be the optimal
condition.
With the optimized condition in hand, the scope of 3-furanyl/
thienyl-2-phenylpyridines 1 was examined and is shown in
Scheme 3. Irradiation of 3-furanyl-2-phenylpyridines 1a, 1d−1i,
1k, 1m−1p, 1r, and 2-phenyl-3-thienylpyridines 1b−1c, 1j, 1l,
and 1q with a 500 W high-pressure mercury lamp gave
benzofurano/thienoquinolines 2a−2r in 52−82% yields. The
dehydrogenative intermolecular annulation of 1 tolerated Me,
OMe, COMe, F, CN, and CF3 groups. In general, substrates
with electron-withdrawing groups (COMe, F, CN, CF3) at the
R2 position gave products in better yields than the substrates
bearing an electron-donating group (Me, OMe). It is note-
worthy that substrate 1m gave the regioselective intramolecular
annulation product 2m in 82% yield. Similar regioselectivity for
3-furanyl-2-naphthalenylpyridine 1o was observed, whereas
intramolecular annulation of 2n gave the product with less
steric hindrance. Moreover, various substrates bearing an
electron-donating group (methyl) at the pyridine moiety
(1p−1r) also gave products in good yields.
Various pyridine and pyrimidine substrates, such as 3-(furan-
2-yl)-4-phenylpyridines 3a−3c, 3-phenyl-4-(thien-2-yl) pyri-
dines 3d, 4-(furan-2-yl)-3-phenylpyridines 4a−4e,h, 3-phenyl-
4-(thien-2-)ylpyridines 4f,g, 5-(furan-2-yl)-4-phenylpyrimi-
dines 7a,c, and 4-phenyl-5-(thien-2-yl)pyrimidines 7b,d were
screened, and the results are summarized in Scheme 4. As the
yields of 5a−5d (43−66%) are almost the same as those for 2a,
2b, and 2d, the position of nitrogen in the pyrimidine ring does
not really affect the annulation efficiency. However, the presence
of an acetyl group at the 4′-position of the phenyl ring (3c)
decreased the yield of 5c to 43%. Substrates 4a−4h with an
electron-donating group (Me, OMe) at the R2 position gave
annulation products in better yields than the substrates bearing
an electron-withdrawing group (COMe, CF3). Analogous
metal catalyst and high temperature, proceeds smoothly without
additives, and is atom-efficient. as well. In this work, based on the
1H NMR monitoring and isolation of intermediates and
detection of H2, the mechanism of acceptorless dehydrogenative
intermolecular annulation was disclosed for the first time. The
reaction proceeded through the process of 6π-electrocyclization,
[1,5]-hydrogen shift, 1,3-eneamine tautomerism, providing two
inner H atoms, and H2 evolution. Inspired by the mechanism of
acceptorless dehydrogenative intermolecular annulation, cis-8b-
methyl-8b,13a-dihydrobenzo[f ]benzofuro[3,2-h]quinoline was
successfully synthesized via the photorearrangement reaction of
2-(3-methylbenzofuran-2-yl)-3-phenylpyridine.
In the initial investigations, 3-(furan-2-yl)-2-phenylpyridine
1a was chosen to screen reaction conditions, and the selected
optimization results are summarized in Table 1. Irradiation of 1a
in EtOH with a high-pressure mercury lamp (500 W) under an
Ar atmosphere at rt for 8 h gave benzo[h]furo[2,3-f ]quinoline
2a in 26% (Table 1, entry 1). The yields of 2a were not
significantly improved in MeOH and 95% EtOH (28% and
30%) (entries 2 and 3). Similar or even worse results were
obtained in aprotic solvents, such as 1,2-dichloroethane (DCE),
acetone (ACE), acetonitrile (ACN), benzene, diethyl ether
(DEE), N,N-dimethylformamide (DMF), hexane, and toluene
(entries 4−6 and 9−13). To our delight, the yields of 2a were
boosted to 63 and 60% in dichloromethane (DCM) and
chloroform (TCM), and the reaction time was significantly
shortened to 2 and 3.5 h (entries 7 and 8). It is noteworthy that
irradiation of 1a in DCM with open air led to a completely
B
DOI: 10.1021/acs.orglett.9b03556
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a
Scheme 3. Scope of Benzofurano/Thienoquinolines
Scheme 4. Scope of Benzofurano/Thienoisoquinolines and
Quinazolines
a
a
Irradiation of 1 (5 mM) in DCM (50 mL) with a high-pressure
mercury lamp (500 W) at rt under Ar atmosphere. Isolated yield.
Reaction time was determined by the complete consumption of 1 as
indicated by TLC.
a
Irradiation of 3, 4, or 7 (5 mM) in DCM (50 mL) with a high-
pressure mercury lamp (500 W) at rt under an Ar atmosphere.
Isolated yield. Reaction time was determined by the complete
consumption of 3, 4, or 7 as indicated by TLC.
treatment of 4-phenyl-5-furano/thienopyrimidines 7a−7d
under the optimal condition also yielded 8a−8d in 61−79%.
To gain insight into the reaction mechanism, a series of
experiments were designed and performed (Scheme 5). To our
surprise, irradiation of 4-(benzofuran-2-yl)-3-phenylpyridine 4i
in DCM at rt under an Ar atmosphere with a high-pressure
mercury lamp (500 W) for 2 h gave dehydrogenative annulation
product benzo[h]benzofuro[2,3-f ]isoquinoline 6i (36%) along
with the intermediate cis-8b,13a-dihydrobenzo[h]benzofuro-
[2,3-f ]isoquinoline(cis-8b,13a-DHBBI) 6j (20%) (Scheme 5,
eq 1). Both 6i and 6j were successfully isolated and characterized
by NMR, HRMS, and IR. Moreover, irradiation of intermediate
6j under the same condition for 1 h led to the formation of 6i in
an 87% yield, which resulted from elimination of a H2 molecule
from 6j. To further monitor the formation and disappearance of
intermediate 6j in detail, the kinetic study was performed. The
nuclear magnetic tube containing 10 mg of 4i and 0.4 mL of
CDCl3 was irradiated with a high-pressure mercury lamp at rt
under an Ar atmosphere for 0, 15, 90, and 180 min, and the 1H
NMR data were collected. As it is shown in the stacked 1H NMR
spectrum, 4i was initially converted into intermediate 6j, which
was consumed to give dehydrogenation product 6i (results are
displayed in Supporting Information Figure S3).
Scheme 5. Mechanism Investigations
Furthermore, analogous treatment of 2-(benzofuran-2-yl)-3-
phenylpyridine 1s with a high-pressure mercury lamp for 30 min
gave a separable mixture of trans-8b,13a-dihydrobenzo[f ]-
benzofuro[3,2-h]quinoline (trans-8b,13a-DHBBQ) 2s and cis-
8b,13a-dihydrobenzo[f ]benzofuro[3,2-h]quinoline (cis-8b,13a-
DHBBQ) 2t in an 82% yield (Scheme 5, eq 2). Luckily, the
structure of both 2s and 2t were identified by single-crystal X-ray
diffraction. However, treatment of 2-([1,1′-biphenyl]-2-yl)furan
9a under the same conditions (500 W high-pressure mercury
lamp) failed to provide dehydrogenative annulation to 10a
(Scheme 5, eq 3). Thus, it is easy to conclude that the presence
of nitrogen in the pyridine and pyrimidine ring is critical.
In the process of photoinduced cyclization of diarylethenes,
the [1,5]-hydrogen shift intermediates, cis-4a,10a-dihydrophe-
C
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nanthrene derivatives, are rare16 and very little is known about
the nature of these photochemically generated intermediates
due to their short lifetime and poor stability. In this work, on the
basis of our experimental results and previous work, a detailed
mechanism for the formation of dehydrogenative annulation
products was proposed and presented in Scheme 6. It is believed
7). Moreover, the structure of 2v has been elucidated by single-
crystal X-ray diffraction.
Scheme 7. Synthesis of 2v with a cis-Geometry via
Photoinduced Rearrangement of 1t
Scheme 6. Plausible Reaction Mechanism
In summary, the strategy for the synthesis of benzofurano/
thienoquinolines, isoquinolines, and quinazolines by irradiation
of o-phenylfuranyl/thienylpyridines/pyrimidines in the DCM
with a high-pressure mercury lamp at rt under an argon
atmosphere was demonstrated. As the photoinduced direct
oxidative annulation of o-phenylfuranylpyridine and its ana-
logues does not require the use of transition metal catalysts and
oxidants, it is believed to be an acceptorless dehydrogenative
reaction. The stepwise elucidation with detailed experimental
data support for each single step is reported for the first time.
that irradiation of 1a with a high-pressure mercury lamp led to
the formation of intermediate trans-7a,7b-dihydrobenzo[h]-
furo[2,3-f ]quinoline (trans-7a,7b-DHBFQ) via 6π-electrocycli-
zation, which is the first step, a typical well-known cyclization for
diarylethenes.16,17 Second, trans-7b,11a-dihydrobenzo[h]furo-
[2,3-f ]quinoline (trans-7b,11a-DHBFQ) was formed via a
suprafacial [1,5]-H shift, driven by the rearomatization of the
furan ring. The process of the [1,5]-H shift has been successfully
validated by the irradiation of 1s with further isolation and
characterization of trans-8b,13a-DHBBQ 2s (Scheme 5 eq 2).
Third, the 1,3-eneamine tautomerization of trans-7b,11a-
DHBFQ led to the formation of 1,7b-dihydrobenzo[h]furo-
[2,3-f ]quinoline (1,7b-DHBFQ) and then was quickly con-
verted to the more stable cis-7b,11a-DHBFQ.
It is noteworthy to restate that the stable cis-isomers 6j and 2t
have been successfully isolated and identified under the
developed condition (Scheme 5, eqs 1 and 2). Moreover, due
to lack of eneamine tautomerization for 9a, no dehydrogenative
annulation product 10a was detected (Scheme 5, eq 3), which
supports the eneamine tautomerization step in the proposed
mechanism.
With the restoration of aromaticity for the benzene ring and
formation of the entire conjugated system, the annulation
product 2a was obtained by the evolution of a hydrogen
molecule (H2) from intermediate cis-7b,11a-DHBFQ. This step
has been further validated by the transformation of cis-8b,13a-
DHBBI 6j to 6i (Scheme 5, eq 1). Moreover, the H2 byproduct
was also successfully detected by gas chromatography during the
irradiation of 4-(3-(furan-2-yl)pyridin-2-yl)benzonitrile 1h in a
quartz tube (Scheme 5, eq 4; results are shown in the Supporting
Information). As the annulation condition does not require any
transition metals, oxidants, and other additives, compared to the
literature reports,7−11 we believe that the photoinduced
intermolecular annulation of o-phenylfuran-2-ylpyridines and
their analogues proceeds via an acceptorless dehydrogenative
process.
To further validate the proposed acceptorless dehydrogen-
ative annulation mechanism, 2-(3-methylbenzofuran-2-yl)-3-
phenylpyridine 1t was subjected to the optimal condition. As it is
much more difficult to eliminate a molecule of methane, cis-8b-
methyl-8b,13a-dihydrobenzo[f ]benzofuro[3,2-h]quinoline
(2v) was isolated in 77% yield along with the enamine
tautomeric product 2u in 18% yield (total yield of 95%, Scheme
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b03556.
Experimental procedures and detailed characterization
data of all new compounds (PDF)
Accession Codes
CCDC 1911401, 1934847, and 1939728 contain the supple-
mentary 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.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhangzt@snnu.edu.cn.
ORCID
Jinming Fan: 0000-0001-5187-4136
Tao Wang: 0000-0002-1494-5297
Wei-Liang Duan: 0000-0003-2791-1931
Yong Liang: 0000-0002-7225-7062
Zunting Zhang: 0000-0003-0806-2452
Present Address
‡Department of Molecular Medicine, Beckman Research
Institute of City of Hope, Duarte, California 91010, United
States.
Author Contributions
†J.F. and W.Z. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for financial support from the National Natural
Science Foundation of China (No. 21672132) and the Shaanxi
■
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