Photocatalyzed synthesis of unsymmetrical ureas via the oxidative decarboxylation of oxamic acids with PANI-g-C3N4-TiO2 composite under visible light

Article abstract of DOI:10.1016/j.tetlet.2020.151962

The synthesis of unsymmetrical ureas via the photocatalyzed oxidative decarboxylation of oxamic acids has been developed. The carbamoyl radicals were generated from oxamic acids in the presence of a hypervalent iodine reagent and the PANI(Polyaniline)-g-C₃N₄-TiO₂ composite under visible light irradiation. The radicals were converted in situ into the corresponding isocyanates, which were then trapped by amines to afford the corresponding products in moderate to good yields. This protocol avoided the direct use of environmentally unfriendly isocyanates and a series of substrates were tolerated. Moreover, the photocatalyst could be readily recovered by simple filtration and be reused for several runs with only a slight decrease in the catalytic activity.

Full text of DOI:10.1016/j.tetlet.2020.151962

Journal Pre-proofs
Photocatalyzed synthesis of unsymmetrical ureas via the oxidative decarboxy‐
lation of oxamic acids with PANI-g-C3N4-TiO2 composite under visible light
Liang Wang, Hao Wang, Yaoyao Wang, Minggui Shen, Shubai Li
PII:
S0040-4039(20)30405-6
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.151962
TETL 151962
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
11 March 2020
13 April 2020
19 April 2020
Please cite this article as: Wang, L., Wang, H., Wang, Y., Shen, M., Li, S., Photocatalyzed synthesis of
unsymmetrical ureas via the oxidative decarboxylation of oxamic acids with PANI-g-C3N4-TiO2 composite under
visible light, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.151962
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Graphical Abstract
Photocatalyzed synthesis of unsymmetrical ureas via
the oxidative decarboxylation of oxamic acids with
PANI-g-C3N4-TiO2 composite under visible light
Leave this area blank for abstract info.
Liang Wang,a,b,* Hao Wang,b Yaoyao Wang,b Minggui Shenb,c and Shubai Lia,*
O
O
PANI-g-C N -TiO
(1)
2, BI-OAc
3
4
R
R'
R
N
N
N
COOH
R'
(2) Et3N, -NH2
H
H
H
Visible light
R = Ar, R' = alkyl, aryl
41-77% yield

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Photocatalyzed synthesis of unsymmetrical ureas via the oxidative
decarboxylation of oxamic acids with PANI-g-C3N4-TiO2 composite under
visible light
Liang Wang,a,b,* Hao Wang,b Yaoyao Wang,b Minggui Shenb,c and Shubai Lia,*
a School of Chemical and Pharmaceutical Engineering, Changzhou Vocational Institute of Engineering, Changzhou 213164, China
b School of Petrochemical Engineering, Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology, Changzhou University, Changzhou 213164,
China
c Institute of Chemical Industry of Forest Products, National Engineering Laboratory for Biomass Chemical Utilization, Key and Open Laboratory on Forest
Chemical Engineering, Key Laboratory of Biomass Energy and Material, Jiangsu Province , Nanjing 210037 , China
*Corresponding authors. E-mail: lwcczu@126.com and sbli@czie.edu.cn
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The synthesis of unsymmetrical ureas via the photocatalyzed oxidative decarboxylation of
oxamic acids has been developed. The carbamoyl radicals were generated from oxamic acids
in the presence of a hypervalent iodine reagent and the PANI(Polyaniline)-g-C3N4-TiO2
composite under visible light irradiation. The radicals were converted in situ into the
corresponding isocyanates, which were then trapped by amines to afford the corresponding
products in moderate to good yields. This protocol avoided the direct use of environmentally
unfriendly isocyanates and a series of substrates were tolerated. Moreover, the photocatalyst
could be readily recovered by simple filtration and be reused for several runs with only a
slight decrease in the catalytic activity.
Available online
Keywords:
Photocatalysis
Oxidative decarboxylation
PANI-g-C3N4-TiO2 composite
Urea synthesis
2009 Elsevier Ltd. All rights reserved.
Urea derivatives are useful compounds that display
attractive activities in pharmaceuticals and agrochemicals.1
They have also found wide applications in molecular
recognition2 and organocatalysis3 due to their high hydrogen
bonding ability. Due to their significant importance in various
research areas, a number of synthetic approaches have been
developed. Traditionally, ureas are synthesized via the
nucleophilic addition of amines to isocyanates,4 which are
generated by the phosgenation of arylamines or the Curtius
rearrangement.5 However, the use of toxic reagents (phosgene,
triphosgene and azides), poor atom economy and limited
substrate scope are the major drawbacks. Alternatively, ureas
can be prepared via transition-metal-catalyzed coupling
reactions such as the Pd-catalyzed amidation of aryl halides,6
the Pd-catalyzed cross-coupling of aryl chlorides or triflates
with sodium cyanate,7 and Ir-8 or Ru-catalyzed9 reactions
between amines and alcohols.
The transition-metal-catalyzed oxidative carbonylation of
amines to access ureas in the presence of CO and an oxidant
has also received interest.10 However, these protocols gave
symmetrical ureas as the major products, and only aryl amines
were tolerated in some cases. To expand the substrate scope,
Zhang and co-workers reported a Pd/C catalyzed carbonylation
of azides in the presence of amines under a CO atmosphere.11
Both aryl azides and alkyl azides were suitable for this
methodology, providing unsymmetrical ureas in good yields.
Wu and co-workers synthesized unsymmetrical ureas via
sequential Pd-catalyzed carbonylation, Curtius rearrangement
and nucleophilic addition reactions.12 Similarly, our group
reported a Pd/C-catalyzed domino synthesis of urea derivatives
using chloroform as the carbon monoxide source in water.13
While acknowledging these pioneering contributions in this
field, there are still some issues that need to be addressed.
Most of these protocols required the use of homogeneous
noble metal catalysts and CO gas. Limitations such as the use
of inconvenient substrates, narrow substrate scope, and harsh
reaction conditions have also severely restricted their
applications.
Recently, visible-light photocatalysis has served as a
powerful and green synthetic technology for organic
synthesis.14 It provides environmentally benign and mild
reaction conditions with good functional group compatibility.
However, the reported photocatalyzed reactions mainly utilise
heavy metal-based photocatalysts, which were either expensive
or could not be easily recovered and reused. Further
advantages such as low cost, high stability and recyclability
make semiconductor photocatalysts more advantageous than
the homogeneous ones. To date, numerous semiconductor
photocatalysts have been prepared; however, they are mainly
used for photo-degradation and their applications in organic

2
Tetrahedron Letters
synthesis have been less reported.15 Recently, we have
In contrast, the yield was increased to 42% in the presence of
PANI-g-C3N4. These results indicated that a synergistic effect
may exist in the ternary composite. The addition of both PANI
and g-C3N4 could enhance the visible light absorption and
electron−hole separation, thereby improving the photocatalytic
activity. Control experiments also showed that no reaction
occurred in the absence of either a photocatalyst or an oxidant
(Entries 20 and 21). Finally, visible light was necessary for this
reaction and 30 mg catalyst was sufficient (Entry 22).
focused on photocatalyzed organic transformations employing
semiconductor composites as the catalysts.16 Titanium dioxide
(TiO2)-based composites such as TiO2-MoS2 and PANI-g-
C3N4-TiO2 displayed good photocatalytic activities in
thiocyanation16a and arylation reactions,16b as well as in the
synthesis of disulfides,16c α-chloro aryl ketones,16d and
oxadiazoles.16e Particularly, for the PANI-g-C3N4-TiO2 ternary
composite, the addition of PANI greatly enhanced its catalytic
activity and stability since PANI has high absorption
coefficients in the visible-light range and good electrical
conductivity. Moreover, PANI is a good electron donor and
hole conductor upon photoexcitation.
To further expand their applications in photocatalyzed
organic transformations, we herein report a PANI-g-C3N4-TiO2
composite catalyzed synthesis of unsymmetrical ureas via the
oxidative decarboxylation of oxamic acids under visible light
(Scheme 1).
Table 1. Optimization of the reaction conditions.a
photocatalyst
oxidant, 24 h
O
O
N
N
N
COOH
2a
then Et3N, benzylamine ( ), 6 h
visible light, solvent, rt
H
H
H
1a
3a
Yield
3a
Entry
Catalyst
Oxidant
Solvent
(%)b
O
O
1
2
3
4
5
6
PANI(20%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(60%)-g-C3N4-TiO2
PANI(80%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
BI-OAc
BI-OAc
DCE
DCE
DCE
DCE
DCE
DCE
35
75
64
60
<5
17
25
PANI-g-C N -TiO
(1)
2, BI-OAc
3
4
R
R'
R
N
N
N
COOH
R'
(2) Et3N, -NH2
H
H
H
Visible light
R = Ar, R' = alkyl, aryl
BI-OAc
Scheme 1. PANI-g-C3N4-TiO2 composite-catalyzed synthesis of ureas.
BI-OAc
The PANI-g-C3N4-TiO2 composite was prepared according
to our previous report (see ESI for details).16b,d,e With the
catalyst in hand, we began to explored its photocatalytic
K2S2O8
(NH4)2S2O8
(NH4)2S2O8
activity
towards
urea
synthesis
using
2-oxo-2-
DCE
(phenylamino)acetic acid (1a) and benzylamine (2a) as the
model substrates (Table 1).
7
8
/H2Oc
Initially, a photocatalyst and an oxidant were utilized in
order to achieve the radical decarboxylation of oxamic acid
(1a).17,18 The reaction was carried out in the presence of 1a (1
equiv.), PANI(20%)-g-C3N4-TiO2 (30 mg) and benziodoxole
acetate (BI-OAc, 1.5 equiv.) in DCE (1,2-dichloroethane) at
room temperature. After irradiation with a 14 W compact
fluorescent lamps (CFL) for 24 h, Et3N (3 equiv.) was added
followed by the addition of benzylamine 2a (0.5 equiv.). The
mixture was further stirred for 6 h, and a 35% yield of the
desired product 3a was obtained (Table 1, entry 1).
Encouraged by this result, different parameters were
investigated in detail. It was found that the loading of PANI
significantly affected the catalytic activity (Entries 1-4). The
composite with 40 wt% of PANI gave the best result (75%,
entry 2), which might be attributed to the fact that the addition
of PANI can promote the absorption of visible light as well as
the seperation of the electron-hole pairs. Moreover, among
these PANI-g-C3N4-TiO2 composites, the one with 40 wt% of
PANI had the largest surface area (128 m2 g−1, see ESI). The
oxidants were then examined. Switching from BI-OAc to
K2S2O8 and (NH4)2S2O8 afforded poor yields (Entries 5 and 6),
presumably due to their low solubilities in DCE. Reactions
using (NH4)2S2O8 in DCE/H2O and DMSO/H2O provided poor
yields of the desired product (Entries 7 and 8). A 62% yield of
3a was obtained when BI-OH was used, likely due to the
poorer leaving group ability of OH compared with OAc. Other
solvents were also surveyed (Entries 9-15). A comparable
yield was obtained when the reaction was performed in
dichloromethane, while the reactions in THF, 1,4-dioxane,
acetonitrile, DMF and DMSO gave much lower yields.
Different semiconductor photocatalysts were then evaluated
(Entries 16-19). No reaction was observed in the presence of
either PANI or TiO2. When g-C3N4 was employed as the
catalyst, a 22% yield of 3a was obtained. The use of g-C3N4-
TiO2 provided the desired product in a comparable yield (25%).
PANI(40%)-g-C3N4-TiO2
(NH4)2S2O8
DMSO
/H2Oc
14
9
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
BI-OH
BI-OAc
BI-OAc
BI-OAc
DCE
CH2Cl2
THF
62
70
40
36
10
11
1,4-
Dioxane
12
13
14
15
16
17
18
19
20
21
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
PANI or TiO2(Anatase)
g-C3N4
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OAc
BI-OAc
‒
MeCN
DMF
DMSO
DCE
DCE
DCE
DCE
DCE
DCE
DCE
37
15
11
0
22
25
42
0
g-C3N4-TiO2
PANI(40%)-g-C3N4
‒
PANI(40%)-g-C3N4-TiO2
PANI(40%)-g-C3N4-TiO2
0
BI-OAc
0d, 67,e
75f
22
a Reagents and conditions: 1a (0.2 mmol, 1 equiv.), oxidant (0.3 mmol, 1.5
equiv.), catalyst (30 mg), solvent (2 mL), room temperature, irradiation
with a 14W CFL for 24 h; then Et3N (3 equiv.), benzyl amine 2a (0.1
mmol, 0.5 equiv.), 6 h. bIsolated yield based on 2a. c1:1 mixture. dReaction
without light. e20 mg catalyst was used. f40 mg catalyst was used.
The substrate scope of the developed method was then
investigated and the results are summarized in Figure 1.19

3
Generally, the one-pot synthesis of unsymmetrical ureas
proceeded smoothly under the standard reaction conditions,
affording the desired products in moderate to good yields.
Initially, the synthesis of aryl/alkyl ureas were evaluated. A
series of aliphatic amines including benzylamine,
cyclohexylamine, n-hexylamine, furan-2-ylmethanamine and
thiophen-2-ylmethanamine reacted with oxamic acid 1a to give
the desired products 3a-3e in 67-77% yield. Different aryl
oxamic acids bearing functional groups such as alkyl, halo and
nitro (1f-1l) also showed good reactivities, affording the
corresponding products in moderate to good yields (3f-3l, 43-
76%). The developed protocol was also suitable for the
synthesis of aryl/aryl ureas, albeit in relatively lower yields
(3m-3u).
benziodoxole–oxamic acid complex by NMR.17 Unfortunately,
we were unable to isolate the benziodoxole–oxamic acid
complex 4 in our reaction (Scheme 2, b). Instead, when the
reaction was performed under the standard reaction conditions
for 24 h, the addition of ethanol provided compund 5 in 75%
yield (Scheme 2, c). This result might support the presence of
the cationic species (RNHC+=O).
O
O
Standard conditions
NH2
(a)
(b)
+
N
N
H
N
COOH
H
TEMPO
(3 equiv.)
H
2a
3a
, nd
1a
O
O
O
xylene
N
I
+
BI-OAc
H
N
H
COOH
O
O
O
1a
4
, trace
PANI-g-C3N4-TiO2
BI-OAc, 24 h
O
O
R
R'
R
N
COOH
N
N
(c)
O
R'
then Et3N, -NH2
2
O
(
), 6 h
H
H
H
Standard conditions
EtOH
visible light, DCE, rt
+
BI-OAc
1
3
N
H
OEt
N
COOH
H
5
, 75%
1a
O
O
O
Scheme 2. Control experiments.
N
H
N
N
N
3
N
N
H
H
H
H
H
3c
3b, 77%
, 72%
O
3a
, 75%
O
On the basis of above results and related literature,17,21
a
Cl
O
plausible mechanism is proposed in Figure 2. Initially, the
PANI-g-C3N4-TiO2 composite absorbs photons and excites the
electrons (e-) and holes (h+) under visible light. Meanwhile,
oxamic acid 1 reacts with BI-OAc to form intermediate I,
which abstracts an electron to generate radical-anion II. Then
carbamoyl radical III and o-iodobenzoic acid anion form via
the decarboxylation of radical-anion II. Afterwards, the
carbamoyl radical III is oxidized by the holes (h+) to give the
corresponding protonated isocyanate IV. Finally, intermediate
N
N
H
N
N
H
N
H
N
H
H
H
S
O
3d
, 69%
O
3f
, 65%
O
3e
, 67%
O
O
2N
O
2N
O2N
N
H
N
H
N
N
3
N
N
H
H
H
H
O
3g
3h
, 43%
O
, 48%
3i
, 44%
O
O
N
H
N
N
N
N
N
H
H
H
H
H
IV is attacked by amines 2 to deliver the corresponding ureas 3.
3k
, 68%
3j
3l
, 71%
, 76%
O
O
R
O
R
O
N
H
BI
e
N
BI
H
OMe
Br
Cl
O
O
O
O
O
I
II
N
H
N
N
N
H
N
N
H
H
H
H
O
3m
, 62%
3n
, 57%
3o
, 60%
I

O

-HOAc
COOH
CO2
NO2
I
O
COO
O
O
OAc
N
N
H
PC
N
N
H
N
H
N
H
N
H
H
O
3p
, 44%
3q
3r
, 41%
, 54%
O
R
N
OMe
Br
H
Cl
O
O
1
O
N
H
N
H
N
H
N
H
N
H
N
H
O
R
2
R
C
3
N
H
h
N
H
3s
, 57%
3t
, 55%
3u
, 59%
IV
III
Figure 1. Reagents and conditions: 1 (0.2 mmol, 1 equiv.), BI-OAc (0.3
mmol, 1.5 equiv.), PANI(40%)-g-C3N4-TiO2 (30 mg), DCE (2 mL), room
temperature, irradiation with a 14W CFL for 24 h; then Et3N (3 equiv.),
amine 2 (0.1 mmol, 0.5 equiv.), 6 h. Isolated yield.
Importantly, the heterogeneous photocatalyst can be readily
recovered and reused. A slight decrease in the catalytic activity
was observed after five consecutive runs for the model reaction
(75%, 75%, 73%, 71% and 70%, respectively).
To gain more detailed information about the reaction, some
control experiments were then performed (Scheme 2). It was
found that no desired product 3a was obtained when 3 equiv.
of the radical scavenger, 2,2,6,6-tetramethylpiperidine N-oxide
(TEMPO) was added (Scheme 2, a). This result suggested that
a radical reaction pathway might be involved in this process.
Previously, Chen and co-workers reported a hypervalent-
iodine-enabled radical decarboxylative alkenylation reaction
under visible light.20 They proposed that the benziodoxole
vinyl carboxylic acid complex, as the key intermediate, was
generated in situ from vinyl carboxylic acid and BI-OAc. The
key intermediate was also isolated and confirmed. Similarly,
Landais and co-workers also observed the formation of a
PC
Figure 2. Proposed mechanism.
In summary, a PANI-g-C3N4-TiO2 composite was prepared
and employed as an efficient heterogeneous photocatalyst in
the synthesis of unsymmetrical ureas under visible light. The
carbamoyl radicals were generated from oxamic acids in the
presence of a hypervalent iodine reagent and the PANI-g-
C3N4-TiO2 composite under visible light irradiation. The

4
Tetrahedron Letters
radicals were converted in situ into the corresponding
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corresponding products in moderate to good yields. This
protocol avoided the direct use of environmentally unfriendly
isocyanates. A broad scope of substrates were compatible with
the reaction conditions and provided the desired products in
moderate to good yields. Control experiments suggested that a
radical pathway was involved in the reaction. Moreover, the
photocatalyst could be readily recovered by simple filtration
and reused for several runs with slight decrease in the catalytic
activity.
6
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Acknowledgments
We gratefully acknowledge financial support from the
Postgraduate Research & Practice Innovation Program of
Jiangsu Province (KYCX19_1742), China Postdoctoral
Science Foundation (2018M632289), Jiangsu Key Laboratory
of Advanced Catalytic Materials and Technology
(BM2012110) and Jiangsu Key Laboratory of Biomass Energy
and Material (JSBEM201806).
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1
(a) Santella, J.B.; Gardner, D.S.; Duncia, J.V.; Wu, H.;
Dhar, M.; Cavallaro, C.; Tebben, A.J.; Carter, P.H.;
Barrish, J.C.; Yarde, M.; Briceno, S.W.; Cvijic, M.E.;
Grafstrom, R. R.; Liu, R.; Patel, S.R.; Watson, A.J.;
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19 General procedure for the synthesis of ureas and
reuse of the catalyst: A sealed tube equipped with a
magnetic stirrer bar was charged with oxamic acid 1 (0.2
mmol, 1 equiv.), BI-OAc (0.3 mmol, 1.5 equiv.),
PANI(40%)-g-C3N4-TiO2 (30 mg), DCE (2 mL). The
reaction mixture was then irradiated with a 14W CFL and
stirred at room temperature (25 °C) for 24 h. The distance
of the reaction vial from the light was approximately 5
centimeters. After reaction completion, the light was
switched off and Et3N (3 equiv.) was added to the
4
5
(a) Banthorpe, D.V. Rearrangements involving azido
groups. In The Chemistry of the Azido Group; Patai, S.,
Ed; Wiley: New York, 1971; pp 397. (b) Yagodkin, A.;

5
a School of Chemical and Pharmaceutical Engineering,
Changzhou Vocational Institute of Engineering, Changzhou
213164, P. R. China
reaction mixture slowly at room temperature and stirred
for 5 min. The amine 2 (0.1 mmol, 0.5 equiv.) was then
added and the reaction mixture stirred for 6 h. Then the
mixture was diluted with EtOAc (10 mL) and H2O (5
mL), and the solid catalyst was recovered by
centrifugation and filtration. The aqueous phase was
extracted with EtOAc (5 mL × 3). The collected organic
extracts were dried over Na2SO4, filtered and evaporated
to dryness. The crude material was purified by flash
chromatography on silica gel using a mixture of PE/EA
to give the pure product 3. The recovered catalyst was
then washed with ethanol and deionized water, dried
under vacuum, and reused for the next run.
b School of Petrochemical Engineering, Jiangsu Key
Laboratory of Advanced Catalytic Materials & Technology,
Changzhou University, Changzhou 213164, P. R. China
c Institute of Chemical Industry of Forest Products, National
Engineering Laboratory for Biomass Chemical Utilization,
Key and Open Laboratory on Forest Chemical Engineering,
Key Laboratory of Biomass Energy and Material, Jiangsu
Province , Nanjing 210037 , China
20 Huang, H.; Jia, K.; Chen, Y. Angew. Chem. Int. Ed. 2015,
54, 1881.
21 Bai, Q.-F.; Jin, C.; He, J.-Y.; Feng, G. Org. Lett. 2018,
20, 2172.
E-mail: lwcczu@126.com and sbli@czie.edu.cn
O
O
PANI-g-C N -TiO
(1)
2, BI-OAc
3
4
R
R'
R
N
N
H
N
COOH
R'
(2) Et3N, -NH2
H
H
Highlights
Visible light
R = Ar, R' = alkyl, aryl
41-77% yield
 PANI−g-C3N4−TiO2 composite-catalyzed the
radical synthesis of unsymmetrical ureas under
visible light is described.
Synthesis of unsymmetrical ureas via
photocatalyzed the oxidative decarboxylation of
oxamic acids has been developed.
 The semiconductor composite is readily prepared
and cost-effective;
 The reaction conditions are mild, green, and the
yields are satisfactory.
 The catalyst shows good stability and
photocatalytic performance, and can be reused
for several runs.
Conflict of interest statement
The authors declared that they have no
conflicts of interest to this work. We declare that
we do not have any commercial or associative
interest that represents a conflict of interest in
connection with the work submitted.
Graphical Abstract
Photocatalyzed synthesis of unsymmetrical ureas via the
oxidative decarboxylation of oxamic acids with PANI-g-
C3N4-TiO2 composite under visible light
Liang Wang,a,b,* Hao Wang,b Yaoyao Wang,b Minggui
Shenb,c and Shubai Lia,*