Visible-Light-Induced Arene C(sp 2)-H Lactonization Promoted by DDQ and tert -Butyl Nitrite

Article abstract of DOI:10.1055/s-0039-1691537

A visible-light photocatalytic aerobic oxidative lactonization of arene C(sp ²)-H bonds proceeds in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and tert -butyl nitrite (TBN). Under the optimized conditions, a range of 2-arylbenzoic acids is converted into the corresponding benzocoumarin derivatives in moderate to excellent yields. This method is characterized by its atom economy, mild reaction conditions, the use of a green oxidant and metal-free catalysis.

Full text of DOI:10.1055/s-0039-1691537

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–F
letter
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Y. Wang et al.
Letter
Synlett
Visible-Light-Induced Arene C(sp2)–H Lactonization Promoted by
DDQ and tert-Butyl Nitrite
Yiqing Wanga
Shengpeng Wangb
Bajin Chenb
Meichao Li*a
Xinquan Hua
Baoxiang Hua
Liqun Jina
H
Ar1
DDQ (5 mol%)
TBN (5 mol%)
Ar2
Ar2
Ar1
O2 (0.1 MPa), rt
HO
O
O
O
blue LED light, DCE
easy operation
mild conditions
28 examples, up to 99% isolated yields
Nan Suna
Zhenlu Shen*a
a College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 310014, P. R. of China
limc@zjut.edu.cn
zhenlushen@zjut.edu.cn
b Transfar Zhilian Co., Ltd., Xiaoshan Economy & Technology Develop-
ment Zone, Hangzhou 311215, P. R. of China
Received: 01.11.2019
Accepted after revision: 27.11.2019
Published online: 16.12.2019
veloped by Xu’s group in 2015.6 In addition, Martin group
has reported a tandem C(sp2)–H functionalization/C–O
bond formation catalyzed by iodine(III) reagents generated
in situ.7 However, most of these methods often required
transition-metal catalysts or excess amounts of oxidants,
which might cause metal residue problems and limit the
practical applications of these processes.
DOI: 10.1055/s-0039-1691537; Art ID: st-2019-u0593-l
Abstract A visible-light photocatalytic aerobic oxidative lactonization
of arene C(sp2)–H bonds proceeds in the presence of 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) and tert-butyl nitrite (TBN). Under
the optimized conditions, a range of 2-arylbenzoic acids is converted
into the corresponding benzocoumarin derivatives in moderate to ex-
cellent yields. This method is characterized by its atom economy, mild
reaction conditions, the use of a green oxidant and metal-free catalysis.
Over the years, visible-light-induced photocatalysis has
emerged as an environmentally friendly and powerful tool
to promote bond formation under mild conditions,8 and has
provided new strategies for oxidative intramolecular cy-
clization reactions. The catalysts involved in most photocat-
alytic reactions are transition-metal complexes, especially
ruthenium and iridium polypyridyl complexes.8a,9 In 2015,
Gonzalez-Gomez and co-workers developed a method for
the visible-light photocatalytic oxidative lactonization of 2-
arylbenzoic acids in which NH4S2O8 was used as the stoi-
chiometric oxidant.10 Gilmour and Metternich have report-
ed a direct synthesis of coumarin derivatives from (E)-cin-
namic acids using (–)-riboflavin as the photocatalyst.11 Sub-
sequently, in 2018, the same group reported another
approach for the preparation of benzo-3,4-coumarins di-
rectly from biaryl carboxylic acids photocatalyzed by (–)-ri-
boflavin.12 In 2018, Lei, Luo and Zhu independently devel-
oped a visible-light-promoted arene C–H lactonization in
the presence of a photoredox catalyst and a cobalt cata-
lyst.13 Electrochemistry represents an environmentally be-
nign strategy for the formation of many organic com-
pounds, whilst it has also been utilized for the direct syn-
thesis of aromatic lactones through C–O cyclization.14
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a
powerful oxidant.2f,15 It has been widely used in numerous
organic C–X bond-forming processes, such as C–C,15b C–O,2f
Key words photocatalysis, lactonization, 2-arylbenzoic acids, 2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone, oxygen
It is well known that aromatic lactones are an important
class of motifs in the structures of advanced materials,
pharmaceuticals, agrochemicals and bioactive molecules
such as alternariol, ellagic acid, isodispar B and chrysomy-
cin A (Scheme 1a).1 Consequently, the study of this notable
structure is very meaningful. It is well established that the
C–O bond is one of the most important carbon–heteroatom
bonds, and direct formation of C–O bonds by oxidation of
C–H continues to attract more and more attention.2 Meth-
ods for the C–H functionalization of arenes often involve
transition-metal catalysts.3 There are also many methods
for preparing lactone structures through C–H functionaliza-
tion and C–O lactonization. For instance, in 2013, Wang et
al. reported a Pd(II)/Pd(IV)-catalyzed C–H activation/C–O
cyclization for the preparation of lactones.4 In the same
year, the groups of Gevorgyan and Martin independently
reported copper-catalyzed C–H activation/C–O cyclization
for the formation of aromatic lactones,5 whilst a silver-cata-
lyzed C(sp2)–H activation/C–O cyclization strategy was de-
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
Y. Wang et al.
Letter
Synlett
10 mol% of TBN in 1,2-dichloroethane (DCE) under 0.1 MPa
of oxygen at room temperature with 18 W blue LED light ir-
radiation. To our delight, the desired product, 6H-ben-
zo[c]chromen-6-one (2a), was obtained in 99% isolated
yield after 8 hours (entry 1). When the loadings of DDQ and
TBN were reduced to 5 mol%, the isolated yield of 2a was
decreased to 90% in 8 hours (entry 2). However, a quantita-
tive yield of 2a could be achieved when the reaction time
was prolonged to 12 hours (entry 3). Further reducing the
loadings of DDQ and TBN to 2 mol% each resulted in an iso-
lated yield of 2a of only 65% (entry 4). Furthermore, in-
creasing the amount of TBN to 5 mol% only led to a slight
improvement in the yield of 2a (entry 5). When TBN was
absent, only a 3% yield of 2a was isolated after 12 hours (en-
try 6). As we expected, no desired reaction was observed
without DDQ (entry 7). In addition, only a small amount of
the product was obtained in the absence of light or O2 (en-
tries 8 and 9). As a comparison, we carried out the reaction
under heating in the dark, and the isolated yield of 2a was
7% (entry 10). These results revealed that DDQ, TBN, and
visible light were all essential for the reaction to proceed.
Though DCE is a common solvent in photocatalytic reac-
a. Prevalent lactone structures in organic molecules
OH
OH
O
O
OH
OH
HO
O
O
HO
O
O
O
O
OH
6H-benzo[c]chromen-6-one
alternariol
ellagic acid
O
OH
OH
O
CH2
O
HO
HO
O
O
O
O
O
iPr
OH
chrysomycin A
HO
isodispar B
b. Previous work:
1. transition-metal catalyst
(Pd, Cu, Ag)
2. photoredox catalyst
(NH4S2O8, (–)-riboflavin, Acr+-Mes)
Ar2
Ar2
Ar1
Ar1
O
O
HO
O
3. electrochemical method
c. This work:
Table 1 Optimization of the Reaction Conditionsa
Ar2
Ar2
DDQ/TBN (Cat.)
O2 (0.1 MPa), rt, visible light
Metal free
Ar1
Ar1
DDQ/TBN (cat.)
HO
O
O
O
O2 (0.1 MPa), rt, blue LED light
HO
O
O
O
Scheme 1 Lactones in bioactive molecules and methods for their syn-
thesis
1a
2a
Entry
Solvent
DDQ
(mol%)
TBN
(mol%)
Time
(h)
Yield
(%)b
C–N,15i,k C–P15h,l and C–S,15j because of its hydrogen abstrac-
tion capacity and one-electron reduction potential.16 Fur-
thermore, under visible-light irradiation, DDQ can be excit-
ed into a triplet excited state, which demonstrates strong
oxidizing power.17 We have previously reported an efficient
and environmentally friendly oxidation system comprising
DDQ/tert-butyl nitrite (TBN)/O2.18 This system has been
successfully applied for several oxidation reactions under
thermal conditions or by irradiation with visible light.15i,19
In this system, molecular oxygen is used as the terminal ox-
idant, DDQ is the catalyst and TBN releases NO, which can
act as the bridge between DDQ and molecular oxygen.
Inspired by existing work, we have focused our studies to-
ward the development of a new and convenient strategy for
the visible-light photocatalytic oxidative lactonization of 2-ar-
ylbenzoic acids with DDQ as the photocatalyst, TBN as the co-
catalyst and O2 as the terminal oxidant in order to synthesize
benzocoumarin derivatives under mild conditions, in which
H2O is the sole by-product. This strategy would meet the re-
quirements of green chemistry (Scheme 1c).
1
2
DCE
10
5
5
2
2
5
–
5
5
5
5
5
5
5
5
10
5
5
2
5
–
5
5
5
5
5
5
5
5
5
8
8
99
90
99
65
70
3
DCE
3
DCE
12
12
12
12
12
12
12
12
12
12
12
12
12
4
DCE
5
DCE
6
DCE
7
DCE
NR
4
8c
9d
10e
11
12
13
14
15
DCE
DCE
4
DCE
7
CH2Cl2
EtOAc
THF
94
84
21
51
2
EtOH
toluene
a Reaction conditions: 1a (0.5 mmol), solvent (4.0 mL), 18 W blue LED, O2
(0.1 MPa), rt.
In the initial experiment, 2-phenylbenzoic acid (1a) was
chosen as the model substrate to optimize the photocata-
lytic oxidative lactonization conditions (Table 1). The reac-
tion was evaluated in the presence of 10 mol% of DDQ and
b Yield of isolated product.
c Reaction was performed in the absence of light.
d Reaction under a N2 atmosphere.
e Reaction at 90 °C (oil bath) in the dark.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

C
Y. Wang et al.
Letter
Synlett
tions, a series of other solvents was also tested. Compound
2a was isolated in 94% yield in dichloromethane, whilst the
other solvents (EtOAc, THF, EtOH, and toluene) gave lower
yields (entries 11–15). It was not clear why the yield of 2a
decreased dramatically in THF. The low yield of 2a in tolu-
ene might be due to a side reaction between DDQ and tolu-
ene promoted by visible light.15f
99%). Firstly, substrates with different substituents on the
Ar1 aromatic ring were examined. Both electron-withdraw-
ing groups such as fluoro, chloro, bromo and trifluorometh-
yl, and electron-donating groups such as methyl, tert-butyl,
methoxy, methoxymethyl and phenyl on the 4′ position
were tolerated, furnishing the corresponding benzocouma-
rins 2b–j in acceptable yields. The 2-arylbenzoic acids bear-
ing chloro and trifluoromethyl substituents gave the ex-
pected benzocoumarins 2c and 2e in quantitative yields.
This is especially noteworthy as usually only moderate
yields of 2e were obtained in previous photoredox- and co-
balt co-catalyzed lactonizations of 2-arylbenzoic acids.13
With optimized reaction conditions in hand,20 the gen-
eral applicability of this reaction was investigated and the
results are summarized in Scheme 2. A variety of substitut-
ed 2-arylbenzoic acids could be converted into the corre-
sponding products in moderate to excellent yields (up to
DDQ (5 mol%)
TBN (5 mol%)
Ar2
Ar2
Ar1
Ar1
O2 (0.1 MPa), DCE, blue LED light, rt
HO
O
O
O
O
1
2
F
O
O
F3C
O
O
O
O
O
Cl
O
O
Br
O
2a,12 h, 99%
2b, 48 h, 71%
2c, 12 h, 99%
2d, 12 h, 73%a
2e, 7 h, 99%
O
O
O
O
O
O
O
O
O
O
O
2j, 24 h, 64%b
2f, 8 h, 99%
2g, 12 h, 62%
2h, 48 h, 48%a
2i, 12 h, 63%
∗
∗
O
O
O
O
O
O
O
O
O
O
O
2k, 48 h, 61%a,c
2m, 15 h, 96%
2o, 12 h, 99% (72:26)
2l, 12 h, 99%
2n, 48 h, 74% (88:12)
O
∗
O
S
O
O
O
O
O
O
O
O
O
F
2p, 48 h, 95% (94:6)
2q, 24 h, 62%
2s, 24 h, 85%
2t, 12 h, 99%
2r, 12 h 99%
Cl
F
O
O
O
O
O
O
O
O
O
O
2x, 24 h, 62%a
2y, 48 h, 61%a,c
2v, 12 h, 95%
2u, 12 h, 84%
2w, 12 h, 99%
O
O
S
S
O
O
S
2z, 24 h, 98%d
2aa, 48 h, 74%a,c
Scheme 2 Photocatalytic oxidative lactonization of 2-arylbenzoic acids. Reagents and conditions: 1 (0.5 mmol), DDQ (5 mol%), TBN (5 mol%), DCE (4
mL), O2 (0.1 MPa), 18 W blue LED, rt. The reaction times and isolated yields are reported for each product. a DDQ (10 mol%), TBN (10 mol%). b HFIP was
used as the solvent. c Reaction temperature: 65 °C. d MeCN was used as the solvent.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

D
Y. Wang et al.
Letter
Synlett
O
O
DDQ (5 mol%)
TBN (5 mol%)
O
O
OH
O2 (0.1 MPa)
rt, blue LED light
HO
cannabinol
C5H11
C5H11
C5H11
3
4
isolated yield: 92%
Scheme 3 Synthesis of the key intermediate of cannabinol
When 4′-bromo-2-phenylbenzoic acid (1d) was used as the
substrate, the loadings of DDQ and TBN had to be increased
to 10 mol% such that a 73% isolated yield of 2d could be
achieved. When a strong electron-donating group (OMe)
was attached to the para position of the Ar1 aromatic ring,
the yield of 2h was only 48% after 48 hours in the presence
of 10 mol% each of DDQ and TBN. For 4′-phenyl-2-phenyl-
benzoic acid (1j), which was difficult to dissolve in most
solvents, the reaction had to be performed in hexafluo-
roisopropanol (HFIP). When a methyl substituent was pres-
ent at the ortho position of the Ar1 aromatic ring, the lac-
tonization could also be carried out to afford 2k, being
slight different from the prior report.13a As shown with sub-
strate 2l, a naphthalene ring was tolerated in this reaction.
The multisubstituted lactone product 2m was produced
smoothly in 96% yield from 3′,5′-dimethyl-2-phenylbenzoic
acid (1m). When substrates with multiple reaction sites
were subjected to the reaction, the corresponding benzo-
coumarins 2n,5b,13a 2o5b and 2p13a were obtained in good to
excellent yields and with good regioselectivities. Under the
optimized reaction conditions, the heteroaromatic sub-
strate 1q reacted to give the desired product 2q in 62% iso-
lated yield. To our delight, when the sulfur-containing het-
eroaromatic substrate 1r was employed in this reaction, a
single isomer of 2r was obtained in 99% isolated yield.
In addition, the substituent effect on the Ar2 ring was
also explored. It can be seen that steric hindrance had a
slight influence on the reaction. Although the products 2s–
u were obtained in excellent yields, a longer reaction time
(24 h) was needed for the substrate with a methyl group at
the ortho position of the phenyl group (1s). Substrates with
chloro and fluoro substituents at the 4-position of the Ar2
aromatic ring could be transformed into the target products
2v and 2w in 95% and 99% isolated yields, respectively.
When a phenyl group was present at the 4-position, higher
loadings of the catalysts were required, and the desired
product 2x was isolated in 62% yield. Under harsher reac-
tion conditions, 1-phenyl-2-naphthoic acid (1y) and [3,3′-
bithiophene]-2-carboxylic acid (1aa) gave the correspond-
ing products 2y and 2aa in 61% and 74% isolated yields, re-
spectively. Finally, 3-phenylthiophene-2-carboxylic acid
(1z) could be smoothly converted into 4H-thieno[2,3-
c]chromen-4-one (2z) in acetonitrile with a 98% isolated
yield.
Compound 4 is a key intermediate for the synthesis of
cannabinol, which possesses high biological activity.4,21 To
expand our newly developed method, compound 3 was
subjected to the reaction to afford a 92% isolated yield of the
target product 4 in 12 hours (Scheme 3).
Moreover, a gram-scale reaction (6 mmol of 1a) was
performed under the optimized conditions to further probe
the practicality of this system in preparative organic syn-
thesis. The corresponding product 2a was obtained in 98%
isolated yield (Scheme 4).
DDQ (5 mol%)
TBN (5 mol%)
O2 (0.1 MPa), blue LED light (18 W)
HO
O
DCE, rt, 18 h
O
O
2a, 1.16 g
1a, 6 mmol
isolated yield: 98%
Scheme 4 Gram-scale synthesis of 2a
To gain an insight into the reaction mechanism, a radi-
cal-trapping experiment was carried out. The result
showed that the photocatalytic oxidative lactonization of
1a was completely inhibited under the optimized reaction
conditions when the radical scavenger TEMPO was added
(Scheme 5). Hence, this experiment suggested that a radical
mechanism was plausible.
DDQ (5 mol%)
TBN (5 mol%)
TEMPO (2 equiv)
O2 (0.1 MPa), blue LED light (18 W)
HO
O
DCE, rt, 12 h
O
O
2a
1a
no reaction
Scheme 5 A radical-trapping experiment
According to the literature and our previous work,14c,19e
we have proposed a possible mechanism for the visible-
light-induced arene C(sp2)–H lactonization promoted by
DDQ and TBN (Scheme 6). Photolysis of TBN releases NO,
which can be oxidized to NO2 by O2. Visible light excites
DDQ to its triplet excited state (3DDQ*),17 with the reaction
of 2-phenylbenzoic acid (1a) and 3DDQ* generating the car-
boxylic radical A and [DDQH]• by hydrogen atom transfer.22
The radical A undergoes intramolecular addition to give the
intermediate radical B, which reacts with [DDQH]• to form
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

E
Y. Wang et al.
Letter
Synlett
O
TBN
OH
∗
O
O
O
hv
Cl
Cl
CN
CN
Cl
Cl
CN
CN
hv
H2O
1a
O
NO
DDQ
O
O
1/2 O2
Cl
Cl
CN
CN
O
NO2
OH
OH
OH
A
[DDQH]
Cl
Cl
CN
O
O
CN
H
DDQH2
B
O
O
2a
Scheme 6 A plausible reaction mechanism
the product 2a. Meanwhile [DDQH]• is converted into 4,5-
dichloro-3,6-dihydroxyphthalonitrile (DDQH2), which is
known to be oxidized by NO2 to regenerate DDQ.15i,19
In conclusion, we have successfully combined the
DDQ/TBN/O2 system with visible light for arene C(sp2)–H
lactonization under mild reaction conditions. The reaction
tolerates various 2-arylbenzoic acids bearing electron-do-
nating and electron-withdrawing groups, and the corre-
sponding benzocoumarin derivatives are obtained in mod-
erate to excellent yields. Additionally, this reaction can also
be performed on gram scale, which may have applications
in future research.
3763. (c) Koch, K.; Podlech, J.; Pfeiffer, E.; Metzler, M. J. Org.
Chem. 2005, 70, 3275. (d) Yang, C.; Hsia, T.; Chen, C.; Lai, C.; Liu,
R. Org. Lett. 2008, 10, 4069. (e) Demuner, A. J.; Barbosa, L. C.;
Miranda, A. C.; Geraldo, G. C.; Silva, C. M.; Giberti, S.; Bertazzini,
M.; Forlani, G. J. Nat. Prod. 2013, 76, 2234. (f) Qin, X.; Li, X.;
Huang, Q.; Liu, H.; Wu, D.; Guo, Q.; Lan, J.; Wang, R.; You, J.
Angew. Chem. Int. Ed. 2015, 54, 7167. (g) Crombie, L.; Jones, R. C.
F.; Palmer, C. J. Tetrahedron Lett. 1985, 26, 2929.
(2) Selected examples: (a) Yamaguchi, J.; Yamaguchi, A. D.; Itami, K.
Angew. Chem. Int. Ed. 2012, 51, 8960. (b) Gutekunst, W. R.;
Baran, P. S. Chem. Soc. Rev. 2011, 40, 1976. (c) Liu, Q.; Jackstell,
R.; Beller, M. Angew. Chem. Int. Ed. 2013, 52, 13871. (d) Guo, S.;
Kumar, P. S.; Yang, M. Adv. Synth. Catal. 2017, 359, 2. (e) Jin, J.; Li,
Y.; Wang, Z.; Qian, W.; Bao, W. Eur. J. Org. Chem. 2010, 1235.
(f) Li, Y.; Bao, W. Adv. Synth. Catal. 2009, 351, 865. (g) Im, H.;
Kang, D.; Choi, S.; Shin, S.; Hong, S. Org. Lett. 2018, 20, 7437.
(h) Sengoku, T.; Nagai, Y.; Inuzuka, T.; Yoda, H. Synlett 2019, 30,
199. (i) Triandafillidi, I.; Raftopoulou, M.; Savvidou, A.; Kokotos,
C. G. ChemCatChem 2017, 9, 4120.
Funding Information
This project was supported by the National Natural Science Founda-
tion of China (21776260, 21773211 and 21773210) and the Natural
(3) Selected examples: (a) Bigi, M. A.; Reed, S. A.; White, M. C. J. Am.
Chem. Soc. 2012, 134, 9721. (b) Cheng, X.; Li, Y.; Su, Y.; Yin, F.;
Wang, J.; Sheng, J.; Vora, H. U.; Wang, X.; Yu, J. J. Am. Chem. Soc.
2013, 135, 1236. (c) Liu, C.; Zhang, H.; Shi, W.; Lei, A. Chem. Rev.
2011, 111, 1780. (d) Saidi, O.; Marafie, J.; Ledger, A. E. W.; Liu, P.
M.; Mahon, M. F.; Kociok-K̈ohn, G.; Whittlesey, M. K.; Frost, C.
G. J. Am. Chem. Soc. 2011, 133, 19298. (e) Nakao, Y. Synthesis
2011, 20, 3209. (f) McMurray, L.; O’Hara, F.; Gaunt, M. J. Chem.
Soc. Rev. 2011, 40, 1885. (g) Ariyarathna, J.; Wu, F.; Colombo, S.;
Hillary, C.; Li, W. Org. Lett. 2018, 20, 6462. (h) Da, Y.; Han, S.; Du,
X.; Liu, S.; Liu, L.; Li, J. Org. Lett. 2018, 20, 5149. (i) Matsumoto,
K.; Tachikawa, S.; Hashimoto, N.; Nakano, R.; Yoshida, M.;
Shindo, M. J. Org. Chem. 2017, 82, 4305. (j) Lin, C.; Chen, Z.; Liu,
Z.; Zhang, Y. Adv. Synth. Catal. 2018, 360, 519.
Science Foundation of Zhejiang Province (LY17B060007).
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0039-1691537.
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References and Notes
(1) Selected examples: (a) Tibrewal, N.; Pahari, P.; Wang, G.; Kharel,
M.; Morris, C.; Downey, T.; Hou, Y.; Bugni, T.; Rohr, J. J. Am.
Chem. Soc. 2012, 134, 18181. (b) Alo, B.; Kandil, A.; Patil, P.;
Sharp, M.; Siddiqui, M.; Snieckus, V. J. Org. Chem. 1991, 56,
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F

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Letter
Synlett
(4) Li, Y.; Ding, Y.; Wang, J.; Su, Y.; Wang, X. Org. Lett. 2013, 15,
2574.
(5) (a) Wang, Y.; Gulevich, A. V.; Gevorgyan, V. Chem. Eur. J. 2013,
19, 15836. (b) Gallardo-Donaire, J.; Martin, R. J. Am. Chem. Soc.
2013, 135, 9350.
(6) Dai, J.; Xu, W.; Wu, Y.; Zhang, W.; Gong, Y.; He, X.; Zhang, X.; Xu,
H. J. Org. Chem. 2015, 80, 911.
(7) Wang, X.; Gallardo-Donaire, J.; Martin, R. Angew. Chem. Int. Ed.
2014, 53, 11084.
(8) (a) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev.
2013, 113, 5322. (b) Romero, N. A.; Nicewicz, D. A. Chem. Rev.
2016, 116, 10075. (c) Chen, J.; Hu, X.; Lu, L.; Xiao, W. Acc. Chem.
Res. 2016, 49, 1911. (d) Xi, Y.; Yi, H.; Lei, A. Org. Biomol. Chem.
2013, 11, 2387. (e) Ghosh, I.; Marzo, L.; Das, A.; Shaikh, R.;
Kö nig, B. Acc. Chem. Res. 2016, 49, 1566. (f) Gandeepan, P.;
Koeller, J.; Korvorapun, K.; Mohr, J.; Ackermann, L. Angew. Chem.
Int. Ed. 2019, 58, 9820. (g) Liang, X.; Niu, L.; Wang, S.; Liu, J.; Lei,
A. Org. Lett. 2019, 21, 2441. (h) Liang, Y.; Steinbock, R.; Yang, L.;
Ackermann, L. Angew. Chem. Int. Ed. 2018, 57, 10625. (i) Chen,
X.; Tan, Z.; Gui, Q.; Hu, L.; Liu, J.; Wu, J.; Wang, G. Chem. Eur. J.
2016, 22, 6218. (j) Ziegler, D. T.; Choi, J.; Muñ oz-Molina, J.;
Bissember, A. C.; Peters, J. C.; Fu, G. C. J. Am. Chem. Soc. 2013,
135, 13107. (k) Zhang, Y.; Chen, W.; Wang, L.; Li, P. Org. Chem.
Front. 2018, 5, 3562.
(9) (a) Gao, X.; Meng, Q.; Li, J.; Zhong, J.; Lei, T.; Li, X.; Tung, C.; Wu,
L. ACS Catal. 2015, 5, 2391. (b) Wang, C.; Zheng, Y.; Huo, H.;
Röse, P.; Zhang, L.; Harms, K.; Hilt, G.; Meggers, E. Chem. Eur. J.
2015, 21, 7355. (c) Narayanam, J. M. R.; Stephenson, C. R. J.
Chem. Soc. Rev. 2011, 40, 102. (d) Yu, H.; Liu, C.; Dai, X.; Wang, J.;
Qiu, J. Tetrahedron 2017, 73, 3031.
(10) Ramirez, N. P.; Bosque, I.; Gonzalez-Gomez, J. C. Org. Lett. 2015,
17, 4550.
(11) Metternich, J. B.; Gilmour, R. J. Am. Chem. Soc. 2016, 138, 1040.
(12) Morack, T.; Metternich, J. B.; Gilmour, R. Org. Lett. 2018, 20,
1316.
(13) (a) Shao, A.; Zhan, J.; Li, N.; Chiang, C.; Lei, A. J. Org. Chem. 2018,
83, 3582. (b) Yang, Q.; Jia, Z.; Li, L.; Zhang, L.; Luo, S. Org. Chem.
Front. 2018, 5, 237. (c) Zhang, M.; Ruzi, R.; Li, N.; Xie, J.; Zhu, C.
Org. Chem. Front. 2018, 5, 749.
(14) (a) Zhang, S.; Li, L.; Wang, H.; Li, Q.; Liu, W.; Xu, K.; Zeng, C. Org.
Lett. 2018, 20, 252. (b) Tao, X.; Dai, J.; Zhou, J.; Xu, J.; Xu, H.
Chem. Eur. J. 2018, 24, 6932. (c) Li, L.; Yang, Q.; Jia, Z.; Luo, S. Syn-
thesis 2018, 50, 2924.
(15) (a) Walker, D.; Hiebert, J. D. Chem. Rev. 1967, 67, 153. (b) Zhai,
L.; Shukla, R.; Rathore, R. Org. Lett. 2009, 11, 3474. (c) Zhai, L.;
Shukla, R.; Wadumethrige, S. H.; Rathore, R. J. Org. Chem. 2010,
75, 4748. (d) Zhang, Q.; Yang, F.; Wu, Y. Org. Chem. Front. 2014,
1, 694. (e) Kucklaender, U.; Bollig, R.; Frank, W.; Gratz, A.; Jose, J.
Bioorg. Med. Chem. 2011, 19, 2666. (f) Batista, V. S.; Crabtree, R.
H.; Konezny, S. J.; Luca, O. R.; Praetorius, J. M. New J. Chem. 2012,
36, 1141. (g) Zha, G.; Bare, G. A. L.; Leng, J.; Shang, Z.; Luo, Z.;
Qin, H. Adv. Synth. Catal. 2017, 359, 3237. (h) Cheng, M.; Lei, J.;
Xie, C. Synlett 2019, 30, 114. (i) Zhang, Y.; Chen, W.; Wang, L.; Li,
P. Org. Chem. Front. 2018, 5, 3562. (j) Li, C.; Li, J.; Tan, C.; Wu, W.;
Jiang, H. Org. Chem. Front. 2018, 5, 3158. (k) Das, S.; Natarajan,
P.; König, B. Chem. Eur. J. 2017, 23, 18161. (l) Chen, Q.; Wen, C.;
Wang, X.; Yu, G.; Ou, Y.; Huo, Y.; Zhang, K. Adv. Synth. Catal.
2018, 360, 3590.
(16) (a) Wendlandt, A. E.; Stahl, S. S. Angew. Chem. Int. Ed. 2015, 54,
14638. (b) Colucci, M. A.; Moody, C. J.; Couch, G. D. Org. Biomol.
Chem. 2008, 6, 637. (c) Ruchardt, C.; Gerst, M.; Ebenhoch, J.
Angew. Chem. Int. Ed. 1997, 36, 1406. (d) Zhu, X. Q.; Wang, C. H.
J. Org. Chem. 2010, 75, 5037. (e) Jung, H. H.; Floreancig, P. E. Tet-
rahedron 2009, 65, 10830.
(17) (a) Rusch, F.; Schober, J.; Brasholz, M. ChemCatChem 2016, 8,
2881. (b) Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. J. Am. Chem.
Soc. 2013, 135, 5368.
(18) Shen, Z.; Dai, J.; Xiong, J.; He, X.; Mo, W.; Hu, B.; Sun, N.; Hu, X.
Adv. Synth. Catal. 2011, 353, 3031.
(19) (a) Shen, Z.; Sheng, L.; Zhang, X.; Mo, W.; Hu, B.; Sun, N.; Hu, X.
Tetrahedron Lett. 2013, 54, 1579. (b) Ma, J. Q.; Hu, Z. M.; Li, M. C.;
Zhao, W. J.; Hu, X. Q.; Mo, W. M.; Hu, B. X.; Sun, N.; Shen, Z. L.
Tetrahedron 2015, 71, 6733. (c) Song, C.; Yi, H.; Dou, B.; Li, Y.;
Singh, A. K.; Lei, A. Chem. Commun. 2017, 53, 3689. (d) Song, C.;
Dong, X.; Yi, H.; Chiang, C.; Lei, A. ACS Catal. 2018, 8, 2195.
(e) Pan, D.; Wang, Y.; Li, M.; Hu, X.; Sun, N.; Jin, L.; Hu, B.; Shen,
Z. Synlett 2019, 30, 218. (f) Pan, D.; Pan, Z.; Hu, Z.; Li, M.; Hu, X.;
Jin, L.; Sun, N.; Hu, B.; Shen, Z. Eur. J. Org. Chem. 2019, 5650.
(20) 6H-Benzo[c]chromen-6-one (2a); Typical Procedure
To a 35-mL tube equipped with a magnetic stir bar, were added
2-phenylbenzoic acid (1a) (99 mg, 0.5 mmol), DDQ (5.7 mg, 5
mol%) and DCE (4.0 mL). After the air in the tube had been
replaced with O2, TBN (3 L, 5 mol%) was added quickly and the
tube was sealed. The tube was then placed in a dark box and
illuminated with an 18 W blue LED. The reaction mixture was
stirred until the reaction had finished (TLC monitoring). The
reaction mixture was then concentrated on a rotary evaporator
and the residue was purified by column chromatography (silica
gel, PE/EtOAc, 50:1) to afford 2a (97 mg, 99%) as a white solid;
mp 93–94 °C. 1H NMR (500 MHz, CDCl3):  = 8.38 (d, J = 7.9 Hz, 1
H), 8.10 (d, J = 7.8 Hz, 1 H), 8.04 (d, J = 8.1 Hz, 1 H), 7.81 (t, J = 7.3
Hz, 1 H), 7.56 (t, J = 7.4 Hz, 1 H), 7.46 (dt, J = 7.7 Hz, 1.1 Hz, 1 H),
7.36–7.30 (m, 2 H). 13C NMR (125 MHz, CDCl3):  = 161.2, 151.3,
134.9, 134.8, 130.6, 130.5, 128.9, 124.6, 122.8, 121.7, 121.3,
118.1, 117.8. MS (EI): m/z (%) = 196.08 (100%) [M]+.
(21) (a) Teske, J. A.; Deiters, A. Org. Lett. 2008, 10, 2195. (b) Meltzer,
P. C.; Dalzell, H. C.; Razdan, R. K. Synthesis 1981, 985.
(c) Nandaluru, P. R.; Bodwell, G. J. Org. Lett. 2012, 14, 310.
(22) Guo, X. W.; Zipse, H.; Mayr, H. J. Am. Chem. Soc. 2014, 136,
13863.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–F