Visible-light-promoted cyclodesulfurization of phenolic thioureas: An organophotoredox catalytic approach to 2-aminobenzoxazoles

Article abstract of DOI:10.1016/j.tetlet.2015.11.089

A facile and efficient one-pot operation for the photo-oxidative cyclodesulfurization of o-phenolic thioureas to afford 2-aminobenzoxazoles is reported. The protocol could be executed under visible light irradiation employing eosin Y as an organophotoredo

Full text of DOI:10.1016/j.tetlet.2015.11.089

Tetrahedron Letters 57 (2016) 155–158
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Visible-light-promoted cyclodesulfurization of phenolic thioureas:
an organophotoredox catalytic approach to 2-aminobenzoxazoles
⇑
Vinod K. Yadav, Vishnu P. Srivastava, Lal Dhar S. Yadav
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile and efficient one-pot operation for the photo-oxidative cyclodesulfurization of o-phenolic thiour-
eas to afford 2-aminobenzoxazoles is reported. The protocol could be executed under visible light irradi-
ation employing eosin Y as an organophotoredox catalyst and offers a superior alternative to the existing
methods of the cyclodesulfurization of phenolic thioureas. The salient features of the present metal-free
approach are the utilization of air and visible light as the greenest and sustainable reagents under mild
conditions.
Received 18 October 2015
Revised 26 November 2015
Accepted 27 November 2015
Available online 28 November 2015
Keywords:
Photoredox catalysis
Eosin Y
Ó 2015 Elsevier Ltd. All rights reserved.
Visible light
Aerobic oxidation
Cyclodesulfurization
2-Aminobenzoxazoles
Nature-inspired application of solar energy (visible light) for
catalytic activation of organic molecules to achieve synthetically
useful transformations has led to a flurry of activity in this
area.1–4 Consequently, visible light photoredox catalysis has
recently emerged as a powerful source for the development of
novel synthetic methodologies.5,6 The success of this strategy has
been mainly derived from the seminal work of the groups of
MacMillan,1 Yoon,2 and Stephenson,3 who employed Ru(bpy)3Cl2
(bpy = 2,20-bipyridine) and Ir(dtbbpy)3Cl2 (dtbbpy = 4,40-di-tert-
butyl-2,20-bipyridine) as efficient photoredox catalysts. Although
ruthenium and iridium transition metal complexes have proved
to be efficient visible light photoredox catalysts, they suffer from
disadvantages such as high cost, potential toxicity, and low sus-
tainability. Thus, as an alternative to the photochemistry of Ru(II)
and Ir(II) complexes, a long known dye eosin Y (20,40,50,70-tetrabro-
mofluorescein) (EY) has been widely used as an organophotoredox
catalyst.7 Advantageously, EY absorbs the green light, which is the
most abundant part of the solar light.7j
formation of the superoxide radical (OÅ2ꢀ).1e,2b,3,6b,e–o,7h The
superoxide radical (OÅ2ꢀ) formed in the photoredox cycle of Ru(II),
Ir(II) complexes or eosin Y has also been utilized in situ for
oxidative functionalization of organic compounds.6f
2-Aminobenzoxazoles are attractive synthetic scaffolds owing
to their useful biological properties, and application as important
building blocks for pharmaceutical products.8 They are currently
employed in the treatment of a wide variety of disorders, such as
HIV, neurodegeneration, and inflammatory diseases.8 Conse-
quently, 2-aminobenzoxazoles have been described as the target
of synthesis in numerous reports involving direct oxidative cou-
pling of benzoxazole with an amine or its surrogates,8a,9 and
nucleophilic displacement of 2-halogenated benzoxazole or its
precursor with amine.10 Also, 2-aminobenzoxazoles can be easily
prepared from a facile ring opening of benzoxazoles with sec-
ondary amines and oxidative ring closing process has been accom-
plished with various reagents.11 The cyclodesulfurization of
thioureas has been a popular synthetic strategy for 2-aminoben-
The use of visible light photoredox catalysis has opened up a
new route to utilize atmospheric oxygen as an oxidant in organic
synthesis. It is mostly employed as an oxidant to regenerate photo-
catalysts (PC) from their radical anion PCÅꢀ, which is formed in the
catalytic cycle via the excited state (PC⁄) by single electron transfer
(SET) from a donor. The PCÅꢀ completes the catalytic cycle by the
zoxazoles. Commonly used reagents include DDC,12a AgNO3,12b
NiO2,12c HgO,12d LiOH/H2O2
or KO2.12f However, all these
12e
methods are associated with one or more limitations such as
expensive, toxic or metallic catalysts, long reaction times, and
lower yields. The latest development of visible-light-triggered
oxidative photocatalytic preparation of 2-aminobenzoxazoles
involves ring closing reaction of o-phenolic amidines, which can
be easily prepared from a facile ring opening of benzoxazoles
with secondary amines.13
⇑
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533.
E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav).
http://dx.doi.org/10.1016/j.tetlet.2015.11.089
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

156
V. K. Yadav et al. / Tetrahedron Letters 57 (2016) 155–158
Table 1
The literature records only a few reports on photocatalytic
Optimization of reaction conditionsa
cyclizationreactionsinvolvingthesulfurradical(Scheme1aandb).14
In view of the above discussion and our continued efforts for devel-
oping efficient heterocyclization reactions,13,15 we envisaged the
present eosin Y catalyzed visible-light-mediated aerobic oxidative
cyclodesulfurization of o-phenolic thioureas to the corresponding
2-aminobenzoxazoles (Scheme 1).
H
N
H
N
photocatalyst
base, solvent
N
N
S
OH
1a
O
visible light, air
rt, time (h)
2a
In order to realize the feasibility of our envisaged preparation of
2-aminobenzoxazoles, a model reaction was carried out by stirring
a mixture of N-substituted-2-hydroxyphenylthiourea 1a (1 mmol)
with Cs2CO3 (1.2 mmol) as a base and eosin Y as a photocatalyst in
DMF under an air atmosphere (without bubbling air) and irradia-
tion with visible light (green light emitting diodes (LEDs, 4.45 W,
kmax = 535 nm) at rt. To our delight, the desired product
2-aminobenzoxazole 2a was obtained in 90% yield after 2 h of irradi-
ation. Then, a series of control experiments were carried out, which
indicated that in the absence of any one of the reagents, catalyst/
reaction parameters, the present photo-oxidative cyclodesulfuriza-
tion could not take place (Table 1). Initially, several bases, namely
Na2CO3, K2CO3, Cs2CO3, DBU (1,8-diazabicycloundec-7-ene) and
DMAP (4-dimethylaminopyridine) were tested and Cs2CO3 was
found to work most efficiently in terms of the yield and reaction
time (Table 1, entry 1 vs 3–6). Inorganic bases are more efficient
than the amine bases probably because the latter are also involved
in the single electron transfer (SET) with the catalyst eosin Y, which
reduces the yield considerably. The bases are required to promote
the formation of a more electron donor thiolate anion from thiour-
eas. It was noted that the product 2a was also formed in the
absence of a base, but relatively a longer reaction time was
required (Table 1, entry 7). A decrease in loading of the catalyst
eosin Y from 1 mol % to 0.2 mol % resulted in lower yield of the pro-
duct (Table 1, entry 1 vs 8), and on increasing the amount of eosin
Y from 1 mol % to 2 mol % there was no effect on the yield of the
product (Table 1, entry 1 vs 9). It was noted that the product 2a
was not formed in the absence of eosin Y (Table 1, entry 10). More-
over, when the reaction was performed under a nitrogen atmo-
sphere, the desired product 2a was not formed (Table 1, entry
11). This shows that the presence of O2 is essential for the forma-
tion of 2a. However, the use of an O2 balloon instead of an air
atmosphere did not enhance the yield of 2a (Table 1, entry 1 vs
12). Similarly, when the reaction was conducted in the dark, there
was no conversion of 1a to 2a (Table 1, entry 13). These results
suggest that light, eosin Y, and O2 are essential requirements for
the reaction.
Entry Solvent Air Photocatalyst
Base (1.2 mmol) Time (h) Yieldb (%)
1
2
3
4
5
6
7
8
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
+
+
+
+
+
+
+
+
+
+
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (0.2 mol %)
EosinY (2 mol %)
—
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
DBU
2
4
4
4
90
90
78
63
4
58
DMAP
—
4
30
2
n.d.
76
40
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
9
10
11
2
90
12
12
2
12
2
2
2
2
2
n.d.
n.d.
90
n.d.
69
75
67
72
36
N2 EosinY (1 mol %)
12c DMF
13d DMF
+
+
+
+
+
+
+
+
+
+
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
EosinY (1 mol %)
14
15
16
17
DCM
CH3OH
THF
CH3CN
18e DMF
19f
DMF
2
2
2
13
n.d.
88
20g DMF
21h DMF
Ru(bpy)3Cl2 (1 mol %) Cs2CO3
a
All reactions were run with 1a (1 mmol), base (1.2 mmol), and solvent (3 mL),
open to air (without bubbling air), irradiation through the flask’s bottom side using
Luxeon Rebel power green LEDs [2.50 W, kmax = 535 nm] at rt.
b
Isolated yield of 2a; n.d. = not detected.
O2 balloon was used.
Reaction was carried out in the dark.
18 W CFL.
c
d
e
f
Daylight.
g
The reaction was quenched with TEMPO (2 equiv).
Luxeon Rebel power blue LEDs [4.45 W, kmax = 447.5 nm] used for irradiation.
h
of high intensity green light. In a control study using 2 equiv of
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl), a well known rad-
ical-trapping reagent, no 2-aminobenzoxazole 2a was formed,
indicating that the reaction presumably involves a radical interme-
diate (Table 1, entry 20). Moreover, the reaction was also success-
ful on using Ru(bpy)3Cl2 as a photoredox catalyst under irradiation
with blue LEDs (Table 1, entry 21), but we opted to use eosin Y as
the organophotoredox catalyst in view of our goal to develop a
metal-free protocol.
Encouraged by the above studies, we investigated the substrate
scope under the optimized reaction conditions and results are
summarized in Table 2. o-Phenolic thioureas bearing electron-
donating or electron-withdrawing substituents generally afforded
2-aminobenzoxazoles in good to excellent yields (81–93%). How-
ever, phenolic thioureas with an electron-withdrawing group
afforded slightly higher yields (Table 2, entries 2d–2h, 2m, and
2n) as compared to those bearing an electron-donating group
(Table 2, entries 2b, 2c, and 2j). Interestingly, thioureas with vari-
ous functionalities such as CH3, C2H5, OCH3, Br, Cl, F, CF3, and NO2
were well tolerated to give aminobenzoxazoles 2 in excellent
yields and high purity.
With the aforementioned results, we screened several solvents,
DMF, DCM, CH3OH, THF, and CH3CN. DMF was found to be the best
solvent (Table 1, entry 1 vs 14–17). However, when the reaction
was carried out in 18 W CFL (Compact Fluorescent Light) the yield
was decreased to 36%. Similarly, the reaction also did not proceed
satisfactorily in daylight (Table 1, entry entries 18 and 19), which
shows the higher photocatalytic activity of eosin Y in the presence
Previous works
H
Ru(bpy)3Cl2
R
N
N
S
visible-light
open to air
(a)14a,b
R
S
R
eosin Y
S
N
visible-light
(b)14c
2
N
R
NH2
open to air
R
S
In view of an easy and high yielding conversion of o-aminophe-
nols into the corresponding phenolic thioureas with aryl isothio-
cyanates,12e we proceeded for one-pot synthesis of
2-aminobenzoxazole 2a starting directly from commercially
available phenyl isothiocynate and 2-aminophenol (Scheme 2).
Thus, we stirred phenyl isothiocynate (1 mmol) and
Present work
H
H
eosin Y
N
N
N
O
visible-light
H
N
R2
R1
R1
(c)
R2
S
open to air
OH
Scheme 1. Aerobic oxidative cyclization via sulfur radicals.

V. K. Yadav et al. / Tetrahedron Letters 57 (2016) 155–158
157
Table 2
H
N
H
N
Substrate scope for the conversion of N-substituted-2-hydroxyphenylthioureas into
N
N
SET
R2
2-aminobenzoxazolesa
R2
R1
R1
S
S
green LEDs
OH
OH
PC*
PC
535 nm
5
6
H
N
H
N
N
photocatalytic
cycle of
eosin Y (1 mol%)
R1
R2
NH
R2
R1
O2
S
Cs2CO3 (1.2 mmol)
DMF, rt, 2-4 h
open to air
vis
ible
O
eosin Y
Cs2CO3
OH
1a-n
light
ref. 12f
PC
O2
(air)
2a-nb
(81-93 % )c
H
N
H
N
N
N
O
R2
R2
R1
R1
SH
1'
H3C
N
O
N
N
SOnO
7
OH
PC = eosin Y
NH
NH
O
NH
O
OCH3
SO42
2a from 1a
2b from 1b
2c from 1c
green LEDs
535 nm
2 h, (90%)
3 h, (87%)
3 h, (86%)
H
H
N
N
N
eosin Y(1 mol%)
R2
NH
R2
R1
N
R1
N
N
S
Cs2CO3 (1.2 mmol)
DMF, rt, 2 h
F
O
NH
O
NH
O
Br
Cl
NH
F
F
OH
O
2
1
open to air
2d from 1d
2e from 1e
2f from 1f
2 h, (92%)
2 h, (91%)
3 h, (93%)
Scheme 3. Plausible pathway for the photo-oxidative cyclodesulfurization of
o-phenolic thioureas.
Cl
N
N
N
NH
O
NH
O
NH
O
H3C
Cl
PCÅꢀ to its ground state (PC). The in situ generated OÅ2ꢀ radical
combines with sulfur radical 6 to form intermediate 7. Finally,
the cyclization of 7 affords the desired products 2.
In conclusion, we have developed a new, efficient, and mild
strategy for the synthesis of 2-aminobenzoxazoles by cyclodesul-
furization of o-phenolic thioureas. The protocol utilizes visible light
and atmospheric oxygen as inexpensive and sustainable reagents
and eosin Y as an organophotoredox catalyst. The reaction could
also be rendered as one pot and afforded 2-aminobenzoxazole in
78% yield starting directly from 2-aminophenol and phenyl
isothiocyanate.
2g from 1g
2h from 1h
2i from 1i
2 h, (92%)
2 h, (92%)
2.5 h, (88%)
Et
H3C
N
N
N
NH
O
NH
O
NH
O
Cl
2j from 1j
2k from 1k
2l from 1l
4 h, (81%)
2.5 h, (89%)
3 h, (83%)
N
O
N
O
NH
Cl
NO2
NH
CF3
2m from 1m
2n from 1n
2 h, (92%)
2 h, (93%)
a
For experimental procedure, see Ref. 17.
b All compounds are known and were characterized by comparison of their
spectral data with those reported in the literature.16
Acknowledgments
c Yields of isolated pure compounds 2.
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. V.K.Y. is grateful to the CSIR,
New Delhi, for the award of a Junior Research Fellowship. V.P.S. is
grateful to the Department of Science and Technology (DST) Govt.
of India, for the award of a DST-Inspire Faculty position (Ref. IFA-
11CH-08) and financial support.
Ph
Ph
O
green LEDs
535 nm
HN
HN
Ph
N C S
HN
S
4
H2N
OH
eosin Y (1 mol%)
(1 mmol)
OH
N
Cs2CO3 (1.2 mmol)
DMF, rt, 2 h
open to air
DMF, r.t., 2 h
open to air
References and notes
3
1a
(1 mmol)
78% yield
2a
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Scheme 2. One-pot sequential reactions for the synthesis of 2-aminobenzoxazole.
2-aminophenol 3 (1 mmol) in DMF (3 mL) at room temperature for
2 h. After confirming the complete conversion of phenyl
isothiocynate to corresponding o-phenolic thioureas by TLC,
Cs2CO3 (1.2 mmol), and eosin Y (1 mol %) were added and the
reaction mixture was irradiated with visible light (green light
emitting diodes (LEDs), kmax = 535 nm) at room temperature for
2 h. The reaction resulted in the clean formation of 2a in a very
good yield (78%).
On the basis of the above observations and the literature prece-
dents,6f,12f,14 a plausible mechanism for the present visible-light-
initiated photo-oxidative cyclodesulfurization of N-substituted-2-
hydroxylphenylthioureas
1
to afford 2-aminobenzoxazole
derivatives 2 is depicted in Scheme 3. The base Cs2CO3 abstracts
thiolic H from phenolic thioureas 10 to form more oxidizable
sulfur anion 5. Eosin Y (PC) upon absorption of light goes to its
excited state (PC⁄). Single electron transfer (SET) between sulfur
anion 5 and PC⁄ affords sulfur radical 6 and PCÅꢀ. The photoredox
cycle of PC (eosin Y) is completed by the aerobic oxidation of

158
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17. General procedure for the synthesis of 2-aminobenzoxazoles 2: A mixture of N-
substituted-2-hydroxyphenylthiourea12e 1 (1 mmol), eosin Y (1 mol %), Cs2CO3
(1.2 mmol), and DMF (3 mL) was taken in a flask open to air and stirred at rt
under irradiation with visible light (green light emitting diodes (LEDs, 4.45 W,
kmax = 535 nm) for 2–3 h (Table 2). After completion of the reaction (monitored
by TLC), water (5 mL) was added and the mixture was extracted with ethyl
acetate (3 ꢁ 5 mL). The combined organic phase was dried over anhydrous
Na2SO4, filtered, and evaporated under reduced pressure. The resulting crude
product was purified by silica gel chromatography using a mixture of hexane/
ethyl acetate (4:1) as eluent to afford an analytically pure sample of product 2.
All the compounds 2 are known and were characterized by comparison of their
spectral data with those reported in the literature.16 Characterization data of
representative compounds 2 are given below:
Compound 2a:16 1H NMR (400 MHz, DMSO-d6) d: 10.54 (br s, 1H, NH), 7.76 (d,
2H, J = 8.0 Hz, ArH), 7.47 (t, 2H, J = 8.4 Hz, ArH), 7.36 (t, 2H, J = 8.0 Hz, ArH), 7.19
(t, 1H, J = 7.2 Hz, ArH), 7.09 (t, 1H, J = 7.2 Hz, ArH), 7.03 (t, 1H, J = 7.2 Hz, ArH);
13C NMR (100 MHz, DMSO-d6) d: 158.40, 147.45, 142.89, 139.19, 129.40,
124.41, 122.53, 122.05, 118.00, 117.04, 109.34. MS: m/z 211 [M+H]+. HRMS
(FAB) calcd for C13H11N2O: 211.0871 [M+H]+, found: 211.0868.
Compound 2g:16 1H NMR (400 MHz, DMSO-d6) d: 10.87 (br s, 1H, NH), 7.95 (s,
1H, ArH), 7.59 (d, 1H, J = 7.6 Hz, ArH), 7.51 (d, 2H, J = 7.6 Hz, ArH), 7.40 (t, 1H,
J = 8.0 Hz, ArH), 7.21 (t, 1H, J = 7.6 Hz, ArH), 7.15 (t, 1H, J = 7.6 Hz, ArH), 7.09 (d,
1H, J = 7.2 Hz, ArH); 13C NMR (100 MHz, DMSO-d6) d: 157.89, 147.39, 142.49,
140.65, 133.90, 131.07, 124.58, 122.49, 122.16, 117.35, 117.28, 116.55, 109.54,
MS: m/z 245 [M+H]+. HRMS (FAB) calcd for C13H10ClN2O: 245.0482 [M+H]+,
found: 245.0484.
Compound 2l:16 1H NMR (400 MHz, CDCl3) d: 7.19-7.43 (m, 8H, ArH), 7.14 (t,
1H, J = 7.6 Hz, ArH), 7.00 (t, 1H, J = 7.6 Hz, ArH), 6.81 (br s, 1H, NH), 4.70 (s, 2H,
CH2); 13C NMR (100 MHz, CDCl3) d: 162.31, 148.45, 142.75, 137.86, 128.80,
127.74, 127.59, 123.95, 120.80, 116.18, 108.82, 46.91. MS: m/z 225 [M+H]+.
HRMS (FAB) calcd for C14H13N2O: 225.1028 [M+H]+, found: 225.1025.
12. (a) Omar, A.-M. M. E.; Habib, N. S.; Aboulwafa, O. M. Synthesis 1977, 864; (b)
Simov, D.; Davidkov, K. Chem. Heterocycl. Compd. 1981, 17, 437; (c) Ogura, H.;
Mineo, S.; Nakagawa, K. Chem. Pharm. Bull. 1981, 29, 1518; (d) Qian, X.-H.; Li,
Z.-B.; Song, G.-H.; Li, Z. J. Chem. Res. (S) 2001, 4, 138; (e) Tian, Z.; Plata, D. J.;