Organic Photoredox Catalysis for Pschorr Reaction: A Metal-Free and Mild Approach to 6 H-Benzo[ c[chromenes

Article abstract of DOI:10.1055/s-0037-1610279

An expedient organic photoredox Pschorr reaction has been developed that opens up a synthetic route to 6 H-benzo[ c[chromenes. The process can be performed under mild conditions by using eosin Y as a photoredox catalyst and acetonitrile as the solvent. Th

Full text of DOI:10.1055/s-0037-1610279

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–E
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J.-Y. He et al.
Letter
Synlett
Organic Photoredox Catalysis for Pschorr Reaction: A Metal-Free
and Mild Approach to 6H-Benzo[c]chromenes
Jing-Yao He
Qi-Fan Bai
N2BF4
R2
H
eosin Y (1 mol%)
MeCN (2 mL)
R1
O
Chengan Jin
Gaofeng Feng*
R1
R2
36 W green LEDs
rt, N2, 16 h
O
R1 = H, Me, Cl, F, NO2
R2 = H, Me, OMe
15 examples
29–98% yield
Zhejiang Key Laboratory of Alternative Technologies for Fine
Chemicals Process, Shaoxing University, Shaoxing, 312000,
P. R. of China
chfeng@usx.edu.cn
Received: 30.07.2018
Accepted after revision: 22.08.2018
Published online: 12.09.2018
of their lower price, wider structural variation, and better
environmental profile, organic photoredox catalysts,4
which are metal-free alternatives, have attracted significant
attention from synthetic chemists.
DOI: 10.1055/s-0037-1610279; Art ID: st-2018-r0486-l
Abstract An expedient organic photoredox Pschorr reaction has been
developed that opens up a synthetic route to 6H-benzo[c]chromenes.
The process can be performed under mild conditions by using eosin Y
as a photoredox catalyst and acetonitrile as the solvent. The diazonium
salts can be either preformed or generated in situ from the correspond-
ing amines with t-BuONO. The process is amenable to gram-sale syn-
thesis of 6H-benzo[c]chromenes, which can be further transformed into
both 6H-benzo[c]chromen-6-ones through oxidation or to 6H-ben-
zo[c]chromen-6-amine through sp3 C–H bond amination. The protocol
provides an attractive route for the synthesis of a library of 6H-ben-
zo[c]chromes.
The Pschorr reaction5 is a seminal protocol for accessing
tricyclic heteroaromatics through intramolecular arylation
of arenediazonium salts in the presence of metal catalysts
such as palladium, iron, or copper (Scheme 1a). Although
Pschorr cyclization is a versatile protocol for the synthesis
heteroaromatic arenes, it usually suffers from high reaction
temperatures, high metal catalyst loadings, and low yields.
Possessing a high reduction potential, aryl diazonium salts
are well-known oxidative quenchers of excited photoredox
catalyst to generate aryl radicals, and their application has
recently been being extensively studied.6 The first visible-
light-mediated photoredox Pschorr cyclization for accessing
phenanthrene derivatives from a stilbene diazonium salt
was reported by Cano-Yelo and Deronzier, who used
Ru(bpy)32+ as a photoredox catalyst.7 An organic photoredox
Pschorr reaction for accessing 6H-benzo[c]chromenes has,
to the best of our knowledge, not been previously reported.
Here, we report an organic photoredox Pschorr reaction to
give 6H-benzo[c]chromene8 that uses the organic dye eosin
Y as a catalyst under irradiation by green light-emitting
diodes (LEDs) (Scheme 1b).
Key words organic photoredox catalysis, Pschorr reaction, diazonium
salts, benzochromenes, C–H bond functionalization
Visible-light photoredox catalysis has emerged as a
powerful synthetic tool for organic transformations.1 The
catalysts most commonly applied to photoredox reactions
are ruthenium2 or iridium3 polypyridyl complexes or or-
ganic dyes. Although it has been demonstrated that ruthe-
nium and iridium polypyridyl complexes are extremely
powerful photocatalysts that permit various transforma-
tions to be carried out under mild conditions, they are
much more expensive than organic dyes. Recently, because
(a) Pschorr reaction
+
N2
metal catalyst
R2
R1
R1
R2
high temperature
Y
Y
(b) This work: metal-free and room temperature synthesis
+
NH2
Y
N2
Y
eosin Y
R2
R1
R1
R2
R2
R1
green LEDs, rt
Y
Scheme 1 The Pschorr reaction
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

B
J.-Y. He et al.
Letter
Synlett
We started our investigation with 2-(benzyloxy)ben-
zenediazonium tetrafluoroborate as a model substrate. The
reaction conditions in terms of solvent, catalyst, illumina-
tion, and concentration were extensively studied (Table 1).
It was found that the desired 6H-benzo[c]chromene was
isolated in 90% yield when a solution of substrate in MeCN
(2 mL) was irradiated with 36 W green LEDs for 16 hours at
room temperature in the presence of 1 mol% of eosin Y (in
the protonated form). Several other solvents, such as DMSO,
DMF, MeOH, EtOAc, and acetone were screened (entries 2–
6); however, these were not as effective as MeCN (entries
2–6). Interestingly, the concentration of substrate was
found to be crucial to the intramolecular cyclization, as
lower yields of 50% and 55% were observed when larger (4
mL) or smaller (1 mL) amounts of MeCN were used (entries
7 and 8). Rhodamine B was found to be less productive than
eosin Y under the same conditions. Meanwhile, it was con-
firmed by control experiments that both light and eosin Y
play central roles in the present transformation (entries 10
and 11).
benzyl motifs (Me, OMe, or Cl) were compatible with this
transformation and gave good to excellent yields under the
optimized conditions. It was noteworthy that a substrate
Table 1 Optimization of the Reaction Conditions.a
N2BF4
eosin Y (1 mol%)
MeCN (2 mL), rt, 16 h
H
O
36 W green LEDs
O
1a
2a
Entry
Conditions
none
Yieldb (%)
1
2
90
DMSO (2 mL)
DMF (2 mL)
61
3
33
4
MeOH (2 mL)
EtOAc (2 mL)
trace
69
5
6
acetone (2 mL)
MeCN (1 mL)
72
7
55
Having identifying the optimal conditions for the intra-
molecular arylation of the diazonium salt, we then ex-
plored the substrate scope with various substituted 2-(ben-
zyloxy)benzenediazonium tetrafluoroborates, synthesized
through a sequence involving nucleophilic substitution of a
2-nitrophenols and a benzyl bromide, reduction of the ni-
tro group, and diazotization of the resulting amine.5d The
results are shown in Scheme 2. A variety of functional
groups on the phenyl ring (Me, Cl, F, or NO2) and various
8
MeCN (4 mL)
50
9
rhodamine B (1 mol%)
no catalyst, 48 h
no light, 48 h
46
10
11
NRc
NRc
a Reaction conditions: 2-(benzyloxy)benzenediazonium tetrafluoroborate
(0.2 mmol), photoredox catalyst (0.002 mmol, 1mol%), solvent (1–4 mL),
36 W green LEDs, r.t., 16 h.
b Isolated yield.
c No reaction.
R2
N2BF4
H
eosin Y (1 mol%)
R1
MeCN (2 mL)
R1
O
R2
1
36 W green LEDs
rt, N2, 16 h
O
2
Me
OMe
MeO
Me
Me
Me
O
O
O
O
O
O
O
2a (90%)
2b (95%)
2c (48%)
2d and 2d' (49:51) (98%)
2e and 2e' (22:78) (73%)
Cl
Me
Me
Me
Me
Me
Me
Me
Me
Me
O
2i (64%)
Me
O
O
O
O
O
2g (88%)
2f (30%)
2h (90%)
2j and 2j' (50:50) (77%)
MeO
OMe
O2N
Me
Me
Cl
Cl
F
O
O
O
O
O
O
2l (72%)
2m (57%)
2n (80%)
2o (29%)
2k and 2k' (24:76) (73%)
Scheme 2 Substrate scope investigation
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

C
J.-Y. He et al.
Letter
Synlett
with a Cl substituent provided the corresponding products
in relative lower yields. This was probably due to a protode-
halogenation reaction taking place during the photoredox
process. As expected, two regioisomers with various ratios
were observed when substrates with substituents at meta-
position of the benzyl motif were employed.
fluxing CH2Cl2 for 36 hours gave 6H-benzo[c]chromen-6-
one (4) in 91% yield (Scheme 5a).9 Moreover, the sp3 C–H
bond adjacent to the oxygen atom of 6H-benzo[c]chromene
could be readily aminated, affording N-phenyl-6H-ben-
zo[c]chromen-6-amine (5) in 80% yield (Scheme 5b).10
An alternative pathway for accessing 6H-ben-
zo[c]chromene through organic photoredox Pschorr reac-
tion was tried by using (phenoxymethyl)benzenediazoni-
um tetrafluoroborates as starting materials (Scheme 3),
synthesized through a sequence involving nucleophilic sub-
stitution of a substituted phenol with 2-nitrobenzyl bro-
mide, reduction of the nitro group, and diazotization of the
resulting amino group.5d To our delight, when 2-[(4-fluoro-
phenoxy)methyl]benzenediazonium tetrafluoroborate (1p)
or 2-[(2-chlorophenoxy)methyl]benzenediazonium tetra-
fluoroborate (1q) was subjected to the optimized condi-
tions, the desired 2-fluoro-6H-benzo[c]chromene (2p) and
4-chloro-6H-benzo[c]chromene (2q) were obtained in 57%
and 52% yield, respectively. Interestingly, the reduction of
the diazonium salt was observed in each case, which might
be responsible for the moderate yields of the desired products.
PCC (3.0 equiv)
(a)
(b)
DCM (25 mL)
reflux, 36 h, 91%
O
O
O
4
1a
NH2
FeCl2 4H2O (10 mol%)
TBHP (2.0 equiv)
+
toluene (2 mL)
N2, 75 °C, 24 h, 80%
O
N
O
H
5
1a
Scheme 5 Further transformations of 6H-benzo[c]chromene
In an attempt to improve the efficiency of the synthesis,
we examined the in situ generation of the diazonium salt
from the corresponding aniline11 (Scheme 6). Indeed, when
2-(benzyloxy)aniline (7) was used as a substrate, 6H-ben-
zo[c]chromene (2a) was obtained in 85% yield by simply
adding 1.5 equivalents of t-BuONO to the reaction mixture,
although the yield was a little lower than that obtained
from the corresponding diazonium salt.
F
H
F
N2BF4
F
eosin Y (1 mol%)
MeCN (2 mL)
+
O
O
O
2p (57%)
36 W green LEDs
rt, N2, 16 h
1p
3p (13%)
NH2
eosin Y (1 mol%)
t-BuONO, MeCN
O
36 W green LEDs
rt, N2, 16 h, 85%
O
2a
7
H
O
N2BF4
eosin Y (1 mol%)
MeCN (2 mL)
+
Scheme 6 Synthesis of 6H-benzo[c]chromene from 2-(benzyloxy)ani-
line
O
O
36 W green LEDs
rt, N2, 16 h
Cl
Cl
1q
2q (52%)
Cl
3q (23%)
A plausible mechanism for the intramolecular reaction
pathway is shown in Scheme 7. First, 2-(benzyloxy)ben-
zenediazonium tetrafluoroborate (or the t-butoxide salt
formed through in situ diazotization) is reduced to the cor-
responding benzene radical A by the excited photoredox
catalyst eosin Y* through single-electron transfer. Mean-
while, eosin Y is oxidized to eosin Y+. Intramolecular addi-
tion of the resulting radical aryl A to the neighboring aryl
ring then takes place, leading to the formation of radical in-
termediate B, which is then oxidized by eosin Y+ to regener-
ate the original eosin Y to complete the catalytic cycle and
provide the aryl cation C. Finally, aryl cation C is deproton-
ated with X– to afford the desired 6H-benzo[c]chromene.
In conclusion, we have successfully developed an organ-
ic photoredox Pschorr reaction for the synthesis of 6H-ben-
zo[c]chromes by using eosin Y as the catalyst and MeCN as
the solvent.12,13 The reaction proceeds under mild conditions
and tolerates various functional groups. Moreover, the pro-
cess is amenable to gram-sale synthesis of 6H-ben-
zo[c]chromenes, and the diazonium salts can be generated
Scheme 3 Synthesis of 6H-benzo[c]chromenes from (phenoxymethyl)
benzenediazonium tetrafluoroborates
A gram-scale (4 mmol) reaction of 2-(benzyloxy)ben-
zenediazonium tetrafluoroborate (1a) was carried out un-
der the standard conditions to test the practicality of the
method. The desired 6H-benzo[c]chromene (2a) was isolat-
ed in 83% yield, which was slightly lower than that of its 0.2
mmol scale counterpart (Scheme 4).
Post-manipulations
of
the
obtained
6H-ben-
zo[c]chromene were carried out (see Supporting Informa-
tion). Treatment of 6H-benzo[c]chromene with PCC in re-
N2BF4
H
eosin Y (1 mol%)
MeCN (40 mL)
O
36 W green LEDs
rt, N2, 16 h, 83%
O
1a
4 mmol, 1.192 g
2a
Scheme 4 Gram-scale synthesis of 6H-benzo[c]chromene
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

D
J.-Y. He et al.
Letter
Synlett
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H
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A
X– = BF4– or t-BuO–
eosin Y+
R2
H
*eosin Y
R1
photoredox
cycle
O
B
visible light
eosin Y
X–
R2
H
R2
R1
R1
C
O
O
HX
Scheme 7 Plausible mechanism.
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in situ with t-BuONO from the corresponding amines to im-
prove the efficiency of the synthesis. Synthesis of other het-
erocycles by using this method is under investigation.
Funding Information
Financial support from the National Natural Science Foundation of
China (NO 21302130 and 21676166) and the Science Technology De-
partment of Zhejiang Province (NO 2014C31141) are acknowledged
with thanks.
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Acknowledgment
We thank Professor Jian Jin of the Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences for discussion and help.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610279.
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References and Notes
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(b) Zhang, M.; Zhu, C.; Ye, L.-W. Synthesis 2017, 49, 1150. (c) Xie,
J.; Jin, H.; Hashmi, A. S. K. Chem. Soc. Rev. 2017, 46, 5193.
(d) Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016, 116, 10075.
(e) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. J. Org. Chem.
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Acc. Chem. Res. 2016, 49, 1911. (g) Ghosh, I.; Marzo, L.; Das, A.;
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Org. Chem. 2016, 81, 6945. (j) Koike, T.; Akita, M. Inorg. Chem.
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Deronzier, A. J. Photochem. 1987, 37, 315.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

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Letter
Synlett
(8) For
a
recent report on photoredox approach to 6H-
mixture was then bubbled with N2 for 10 min. After irradiating
with 36 W green LEDs for 16 h, the mixture was purified
directly by column chromatography (silica gel).
benzo[c]chromenes, see: Deng, Q.; Tan, L.; Xu, Y.; Liu, P.; Sun, P.
J. Org. Chem. 2018, 83, 6151.
(9) Sun, C.-L.; Gu, Y.-F.; Huang, W.-P.; Shi, Z.-J. Chem. Commun.
2011, 47, 9813.
(10) Chen, D.; Pan, F.; Gao, J.; Yang, J. Synlett 2013, 24, 2085.
(11) Natarajan, R.; Kumar, N.; Sharma, M. Org. Chem. Front. 2016, 3,
1265.
(12) 6H-Benzo[c]chromenes 2a–o: Typical Procedure
A 10 mL reaction vial was charged with a magnetic stirrer bar,
the appropriate diazonium salt (0.20 mmol), eosin Y (1.3 mg, 1
mol%), and anhyd MeCN (2 mL). The vial was sealed and the
(13) 2,7-Dimethyl-6H-benzo[c]chromene (2i)
White crystalline solid; yield: 26.9 mg (64%; 0.2 mmol scale);
mp 72–73 °C. IR (film): 2923, 1501, 1459, 1250, 1028, 816, 736
cm–1 1H NMR (400 MHz, CDCl3): δ = 7.58 (d, J = 8.0 Hz, 1 H),
.
7.55 (d, J = 1.6 Hz, 1 H), 7.29 (dd, J = 8.0, 7.6 Hz, 1 H), 7.14 (d, J =
7.6 Hz, 1 H), 7.06 (dd, J = 8.4, 1.6 Hz, 1 H), 6.91 (d, J = 8.0 Hz, 1
H), 5.16 (s, 2 H), 2.39 (s, 3 H), 2.33 (s, 3 H).
13C NMR (100 MHz, CDCl3): δ = 152.3, 133.1, 131.3, 130.2, 130.0,
129.9, 129.4, 127.8, 123.9, 122.8, 119.8, 116.8, 65.6, 20.9, 18.5.
GC/MS: m/z (%) = 209 (100) [M – H]+, 210 (72) [M]+. HRMS (CI):
m/z [M]+ Calcd for C15H14O: 210.1045; found: 210.1048.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E