PPh3/I2-catalyzed one-pot synthesis of 4,6-diarylpyrimidin-2(1H)-ones

Article abstract of DOI:10.1080/00397911.2020.1815784

A method using PPh₃/I₂-catalyzed three-component Biginelli reaction of aromatic ketone, aromatic aldehyde and urea to synthesize4,6-diarylpyrimidin-2-(1H)-ones is described. Under the experimental conditions, the main reaction produc

Full text of DOI:10.1080/00397911.2020.1815784

Synthetic Communications
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PPh3/I2-catalyzed one-pot synthesis of 4,6-
diarylpyrimidin-2(1H)-ones
Yanzhi Zhang , Min Lei , Kangsheng Xue , Guoxiang Sun , Yang Zhou , Jinjun
Hou , Huali Long , Zijia Zhang , Xubing Chen & Wanying Wu
To cite this article: Yanzhi Zhang , Min Lei , Kangsheng Xue , Guoxiang Sun , Yang Zhou ,
Jinjun Hou , Huali Long , Zijia Zhang , Xubing Chen & Wanying Wu (2020): PPh3/I2-catalyzed
one-pot synthesis of 4,6-diarylpyrimidin-2(1H)-ones, Synthetic Communications, DOI:
10.1080/00397911.2020.1815784
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1815784
PPh3/I2-catalyzed one-pot synthesis of 4,6-diarylpyrimidin-
2(1H)-ones
a
d,e
b
Yanzhi Zhang , Min Lei , Kangsheng Xue , Guoxiang Sunb,c, Yang Zhoud,e
,
ꢀ
ꢀ
Jinjun Houd,e, Huali Longd,e, Zijia Zhangd,e, Xubing Chena, and Wanying Wud,e
b
aCollege of Pharmacy, Dali University, Dali, P.R. China; College of Pharmacy, Changchun University of
c
Chinese Medicine, Changchun, P.R. China; School of Chemistry and Chemical Engineering, Yancheng
d
Institute of Technology, Yancheng, P.R. China; Shanghai Research Center for Modernization of
Traditional Chinese Medicine, National Engineering Laboratory for TCM Standardization Technology,
e
Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, P.R. China; University of
Chinese Academy of Sciences, Beijing, P.R. China
ABSTRACT
ARTICLE HISTORY
Received 9 February 2020
A method using PPh3/I2-catalyzed three-component Biginelli reaction
of aromatic ketone, aromatic aldehyde and urea to synthesize4,6-dia-
rylpyrimidin-2-(1H)-ones is described. Under the experimental condi-
tions, the main reaction products are 4,6-diarylpyrimidin-2-(1H)-ones
not 4,6-diaryl-3,4-dihydropyrimidin-2(1H)-ones. The advantage of this
method is to achieve improved product yields with an inexpensive
and readily available catalyst, and easy experimental set-up, which
provides a good method to synthesize 4,6-diarylpyrimidin-2-(1H)-
ones.
KEYWORDS
46-diarylpyrimidin-2-(1H)-
ones; PPh3/I2; Aldehyde;
Ketone; Biginelli reaction
GRAPHICAL ABSTRACT
Introduction
The synthesis of pyrimidin-2(1H)-ones or 3,4-dihydropyrimidin-2(1H)-ones has been
valued by pharmaceutical and organic synthetic chemists because these compounds
exhibit various biological activities, such as anti-hypertension,[1] antioxidant,[2] antifun-
gal,[3] anticancer,[4] HIV gp-120-CD4 inhibitors,[5] and et al.[6] Biginelli-type three-com-
ponent reaction is an important and effective method to construct these heterocyclic
compounds. It is because of the extensive biological activities of pyrimidin-2(1H)-ones
CONTACT Xubing Chen
Wu wanyingwu@simm.ac.cn
abing780113@163.com
College of Pharmacy, Dali University, Dali, P.R. China; Wanying
Shanghai Research Center for Modernization of Traditional Chinese Medicine,
National Engineering Laboratory for TCM Standardization Technology, Shanghai Institute of Materia Medica, Chinese
Academy of Sciences, Shanghai, P.R. China; University of Chinese Academy of Sciences, Beijing, P.R. China
ꢀ
These two authors attributed equally to this work.
Supplemental data for this article can be accessed on the publisher’s website
ß 2020 Taylor & Francis Group, LLC

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Y. ZHANG ET AL.
Scheme 1. Reported Biginelli-type reaction catalyzed by I2.
or 3,4-dihydropyrimidin-2(1H)-ones that organic chemists have been paying attention
to Biginelli-type reaction.
In the past few decades, many methods for Biginelli-type three-component reaction
of ketone, aldehyde and urea have been reported, including: FeCl3ꢁ6H2O,[7] ZrCl4,[8]
ZnCl2,[9] Fe(NO3)3ꢁ9H2O,[10] p-TSA,[11] H2SO4,[12] Cu(OTf)2,[13] LiClO4,[14]
NiCl2ꢁ6H2O,[15] VCl3,[16] LaCl3ꢁ7H2O,[17] Mn(OAc)3ꢁ2H2O,[18] InBr3,[19] SSi-GO,[20]
Cu(ClO4)2ꢁ6H2O,[21] MW,[22] Sc(OTf)2,[23] and etc[24]. Among these optimized condi-
tions, two results reported by Pasha M. A. et al.[25] and Cai C. et al.[26], respectively,
have attracted our attention because both studies used iodine as the catalyst.
As shown in Scheme 1, Pasha M. A. et al. reported that the three-component reaction
of acetophenone (1a), benzaldehyde (2a) and urea (3a) was carried out in the presence
of I2 (5 mol%) under neat conditions at 80 ꢂC for 6 min to obtain 4,6-diphenylpyrimi-
din-2(1H)-one (4a) in 95% yield.[25] The same reaction was reported by Group Cai C.
using I2 (50 mol%) as catalyst under neat conditions at 140 ꢂC for 30 min to obtain 4,6-
diphenyl-3,4-dihydropyrimidin-2(1H)-one (5a).[26]
Under normal conditions, raising the reaction temperature and extending the reaction
time facilitates the oxidation of 5a to 4a. Therefore, it is reasonable to hypothesize that
at a lower temperature, a shorter reaction time to produce 5a, and at a higher tempera-
ture, a longer reaction time to produce 4a, not vice versa. To test our hypothesis, we
repeated the reaction in strict accordance with the conditions described in the literature,
and the results were summarized in Figure 1.
At first, the reaction was carried out in the presence of I2 (5 mol%) under 80 ꢂC for
6 min.[25] Figure 1(a) showed that the reaction was almost inactive, and no products
were detected. The reaction was repeated three times under the same conditions, and all
came with the same results. The results of our repeated did not match that reported in
the literature, and it indicated that the method was not feasible to repeat under the con-
ditions provided in the study.[25] Then, the reaction proceeded in the presence of I2
(50 mol%) under 140 ꢂC for 30 min.[26] As shown in Figure 1(b), 4a was the main reac-
tion product, not 5a reported in the literature. 4a was isolated, and the structure was

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3
Figure 1. (a) The repeated reaction based on ref 25; (b) The repeated reaction based on ref 26.
validated by LC-MS and NMR. Such results will cause confusion and mistake among
peer researchers. Therefore, we intend to re-investigate the three-component Biginelli
reaction catalyzed by I2.
During the repetition of the experimental procedures described above, we found two
critical shortcomings of the reaction conditions: (a) the difficulty of stirring the reaction
mixture under neat conditions; (b) intolerance to high temperatures of the groups.
Therefore, the goal of the optimization is to make the reaction take place in an appro-
priate solvent at a relatively low temperature. Initially, the reaction was started with 1a,
2a, and 3a in the presence of I2 (20 mol%) in MeCN at 80 ꢂC for 2 h. Both 4a and 5a
were detected in these conditions, with the yield of the target product 4a at 45%.
Additionally, it showed that the product yield of 4a did not improve further by increas-
ing the amount of I2 or prolonging the reaction time.
In our previous study, we reported that PPh3/I2 could be used as deoxygenative car-
bonylation reagent to promote organic reaction.[27] Since the three-component Biginelli
reaction is accompanied by dehydrating, we proposed that PPh3/I2 may promote the
reaction. To test our assumption, we carried out the model reaction of 1a, 2a, and 3a
in the presence of PPh3 (20 mol%) and I2 (20 mol%) in MeCN at 80 ꢂC for 2 h.
Surprisingly, the desired product 4a was obtained at 76% yield, which showed that our
original assumption was feasible. In order to further optimize the reaction conditions,
we carried out a systematic study on the Biginelli reaction in the presence of PPh3/I2.
Results and discussion
First, we optimized the solvents for the model reaction, and the results summarized in
Table 1. Solvents commonly used in the laboratory, such as MeCN, DCM, THF, EtOAc,
toluene, 1,4-dioxane, DMF, and DMSO, were examined for this model reaction (Table
1, Entries 1–8), and 1,4-dioxane showed the best solvent effect with the 89% yield of 4a.
Then, 1,4-dioxane was chosen as the reaction medium to optimization the reaction tem-
perature. As shown in Table 1, desired product 4a was not observed under rt or 40 ꢂC,
and a meager yield of 4a (15%) was obtained under 60 ꢂC (Table 1, entries 9–11). There

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Y. ZHANG ET AL.
Table 1. Screening of solvent and reaction temperaturea .
Entry
Solvent
Temperature (oC)
Yields of 4a (%)b
1
2
3
4
5
6
7
8
MeCN
DCM
THF
80
80
80
80
80
80
80
80
rt
76
83
83
EtOAc
71
Toluene
1,4-dioxane
DMF
82
89 (82)c
62
DMSO
64
0
0
15
9
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
10
11
12
40
60
100
85
aReaction conditions: 1a (600 mg, 5 mmol), 2a (636 mg, 6 mmol), 3a (450 mg, 7.5 mmol), PPh3 (26.2 mg, 1 mmol), I2
(25.4 mg, 1 mmol), solvent (0.5 mL), 2 h.
bYields were determined by LC-MS.
cThe number in parentheses refers to the yield of isolated 4a.
was no significant difference under 80 ꢂC or reflux temperature, with the yields 89%
and 85%, respectively. Therefore, we chose 1,4-dioxane as the reaction medium and
80 ꢂC for all the following experiments.
Next, the amount of PPh3 and I2 was also optimized, and the results were summar-
ized in Table 2. We fixed the PPh3 to 20 mol% and then examined the effect of the
amounts of I2 on the yield of 4a. When the amount of I2 was increased from 0 to
15 mol%, the yields of 4a (0–88%) were gradually increased (Table 2, Entries 1–4).
However, when the amount of I2 exceeded 15 mol%, the yields of 4a (88–89%) were no
longer significantly improved (Table 2, entries 4–6). These results showed that 15% of
I2 was sufficient. Then, we fixed the amount of I2 at 15 mol% to optimization the PPh3.
The results presented in Table 2 indicated that 20 mol% of PPh3 was optimal according
to the yields of desired product 4a. By optimizing the model reaction, we established
the optimal reaction conditions. In summary, the reaction carried out in the presence of
PPh3 (20 mol%) and I2 (15 mol%) in 1,4-dioxane as solvent at 80 ꢂC.
To further explore the potential of this protocol for synthesizing 4,6-diarylpyrimidin-
2(1H)-ones (4), the three-component reaction of aromatic ketone (1), aromatic aldehyde
(2) and urea (3a) proceeded under the optimized conditions (Table 3).
As shown in Table 3, a series of ketone and aldehyde were selected for this Biginelli
type reaction. For benzaldehyde, the yields of o-substitution were lower than those of
m- and p-substituted substrates. The electronic effect showed that the electron-donating
group was favorable to the reaction, while the electron-withdraw group was unfavorable
to the reaction. The substitution effect of aromatic ketone was similar to that of aro-
matic aldehyde. The yields of the desired product 4 were related both to the substitution
position and to the electron effect.

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5
a
Table 2. Optimization of the amount of PPh3 and I2 .
Entry
PPh3 (mol%)
I2 (mol%)
Yields of 4a (%)b
1
2
3
4
5
6
7
8
20
20
20
20
20
20
0
0
5
0
16
10
15
20
30
15
15
15
15
15
46
88 (82)c
89
88
45
53
61
82
82
5
9
10
11
10
15
30
aReaction conditions: 1a (600 mg, 5 mmol), 2a (636 mg, 6 mmol), 3a (450 mg, 7.5 mmol), PPh3, I2, 1,4-dioxane (0.5 mL),
80 ꢂC, 2 h.
bYields were determined by LC-MS.
cThe number in parentheses refers to the yield of isolated 4a.
Table 3. Synthesis of 4,6-diarylpyrimidin-2(1H)-ones (4)a.
Entry
Ar1
Ar1
Time (h)
Product 4
Yield (%)b
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
3-HOC6H4
4-CH3OC6H4
4-CH3OC6H4
4-CH3OC6H4
4-ClC6H4
C6H5
2
4
4
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
82
72
82
77
45
70
70
78
20
71
62
77
72
50
68
13
2-CH3OC6H4
3-CH3OC6H4
4-CH3OC6H4
2-ClC6H4
3-ClC6H4
4-ClC6H4
4
13
13
16
10
10
4
4
4
4
8
4-FC6H4
9
4-NO2C6H4
3-HOC6H4
4-CH3OC6H4
4-HOC6H4
4-ClC6H4
4-ClC6H4
4-HOC6H4
4-CH3OC6H4
10
11
12
13
14
15
16
4-ClC6H4
2-HOC6H4
2
4
aReaction conditions: 1 (5 mmol), 2 (6 mmol), 3a (450 mg, 7.5 mmol), PPh3 (26.2 mg, 1 mmol), I2 (19.1 mg, 0,75 mmol),
1,4-dioxane (0.5 mL), 80 ꢂC.
bIsolated yields.
We propose a mechanism of the PPh3/I2-catalyzed Biginelli-type reaction, as shown
in Scheme 2. Initially, PPh3 reacts with I2 to form A is an active dehydration agent.
Aldehyde reacts with urea to form an acyl imine intermediate C by two paths: (a) at the
action of A; (b) at the action HI. Since the reaction does not require the addition of an
equivalent amount of PPh3/I2, we believe that HI also has a catalytic effect. Then,
Michael addition of 1 onto C to form intermediate D, which takes place intramolecular
cyclization reaction to form E under the action of HI. At last, E is oxidized by air to
obtain the desired product 4.

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Y. ZHANG ET AL.
Scheme 2. A plausible mechanism for the formation of compound 4.
In conclusion, we have demonstrated a simple and efficient procedure for the synthesis of
4,6-diarylpyrimidin-2-(1H)-one derivatives via three-component Biginelli reaction of aro-
matic ketone, aromatic aldehyde, and urea using PPh3/I2 as the catalyst. This procedure
offers several advantages, including mild reaction conditions, moderate to good yields of
products, as well as a simple experimental and set-up procedure, which makes it a useful
and attractive process for the synthesis of these compounds. Besides, under this reaction
condition, no matter how the substrates change, the product is 4,6-diarylpyrimidin-2-(1H)-
ones without the interference of 4,6-diaryl-3,4-dihydropyrimidin-2(1H)-ones.
Experimental
NMR spectra were recorded on a Bruker AMX 500 instrument using solvent peaks as DMSO-
d6 solutions. Yields of condition screen experiments were determined by LC-MS (Waters
LCMS/SQD þ UPLC H-class spectrometer). HRESIMS were determined on a Micromass Q-
Tof Global mass spectrometer, and ESIMS were run on a Bruker Esquire 3000 Plus
Spectrometer. TLC was performed on GF254 silica gel plates (Yantai Huiyou Inc., China).

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General procedures for the synthesis of compounds 4
A mixture of PPh3 (26.2 mg, 1 mmol), I2 (19.1 mg, 0.75 mmol) in 1,4-dioxane (0.5 mL)
was stirred at room temperature for 15 min. Then, 1 (5 mmol), 2 (6 mmol), 3a (450 mg,
7.5 mmol) was added to the mixture, and heated to 80 ꢂC for an appropriate time
(Table 3). After completion of the reaction (TLC), DCM or EtOAc (20 mL) was added,
and the solution was washed with NaS2O3 (20 mL, 10%). Then, the organic layer was
washed with water (2 ꢃ 20 mL) and brine (2 ꢃ 20 mL) and was dried over anhyd.
MgSO4. The organic layer was concentrated under reduced pressure, and the crude
product was purified by column chromatography to afford the corresponding prod-
uct 4.
4a: The product was obtained as a white solid in 82% yield, mp: 229.6–231.8 ꢂC, Rf
1
¼ 0.44 (DCM/MeOH, 30: 1, v/v). H NMR (500 MHz, DMSO-d6) d 12.08 (s, 1H), 8.17
(d, J ¼ 7.2 Hz, 4H), 7.60–7.54 (m, 7H). 13C NMR (126 MHz, DMSO-d6) d 164.48 (2c),
160.01, 134.73 (2c), 131.51 (2c), 128.85 (4c), 127.61 (4c). ESI-MS: m/z 249.3 [M þ H]þ;
HRMS (ESI): [M þ H]þ calculated for C16H13N2O, 249.1022; found 249.1017.
Funding
This work was supported by Special Fund for Strategic Pilot Technology Chinese Academy of
Sciences [XDA12020379], the National Natural Science Foundation of China [grants 81973455],
National Key R&D Program of China [2018YFC1707900].
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