H3PMo12O40-catalyzed coupling of diarylmethanols with epoxides/diols/aldehydes toward polyaryl-substituted aldehydes

Article abstract of DOI:10.1016/j.cclet.2020.07.018

A versatile heteropoly acid (H₃PMo₁₂O₄₀)-catalyzed coupling of diarylmethanols with epoxides was established for the synthesis of polyaryl-substituted aldehydes. Furthermore, the catalytic system was also suitable for the

Full text of DOI:10.1016/j.cclet.2020.07.018

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H3PMo12O40-catalyzed coupling of diarylmethanols with
epoxides/diols/aldehydes toward polyaryl-substituted aldehydes
Guoping Yang, Yufeng Liu, Ke Li, Wei Liu, Bing Yu, Changwen Hu
PII:
S1001-8417(20)30409-5
https://doi.org/10.1016/j.cclet.2020.07.018
CCLET 5743
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Chinese Chemical Letters
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8 June 2020
28 June 2020
8 July 2020
Please cite this article as: Yang G, Liu Y, Li K, Liu W, Yu B, Hu C, H3PMo12O40-catalyzed
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aldehydes, Chinese Chemical Letters (2020), doi: https://doi.org/10.1016/j.cclet.2020.07.018
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Chinese Chemical Letters
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Communication
H3PMo12O40-catalyzed coupling of diarylmethanols with epoxides/diols/aldehydes
toward polyaryl-substituted aldehydes
Guoping Yanga,b, Yufeng Liub, Ke Lib, Wei Liub, Bing Yuc,, Changwen Hua,*
a Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic, School of Chemistry and Chemical
Engineering, Beijing Institute of Technology, Beijing 100081, China
b Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China University of Technology, Nanchang 330013, China
c Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou 450001, China
ARTICLE INFO
Article history:
Received 8 June 2020
Received in revised form 30 June 2020
Accepted 3 July 2020
Available online
———
 Corresponding authors.
E-mail addresses: bingyu@zzu.edu.cn (B. Yu), cwhu@bit.edu.cn (C.-W. Hu)

Graphical Abstract
A versatile heteropoly acid (H3PMo12O40)-catalyzed coupling of diarylmethanols with epoxides was established for the synthesis
of polyaryl-substituted aldehydes. Furthermore, the catalytic system was also suitable for the reaction of diarylmethanols and
diols/aldehydes. The application of such an earth-abundant, readily accessible, and nontoxic catalyst provides a green approach
for the construction of polyaryl-substituted aldehydes.
ABSTRACT
A versatile heteropoly acid (H3PMo12O40)-catalyzed coupling of diarylmethanols with epoxides was established for the synthesis of polyaryl-substituted
aldehydes. Furthermore, the catalytic system was also suitable for the reaction of diarylmethanols and diols/aldehydes. The application of such an earth-abundant,
readily accessible, and nontoxic catalyst provides a green approach for the construction of polyaryl-substituted aldehydes
Keywords:
Polyoxometalates
Diarylmethanols
Epoxides
Polysubstituted aldehydes
Acid catalysis
Acid catalysis is one of the most important and attractive fields of catalysis which has been widely applied in industry for the
manufacture of various chemicals [1-3]. However, the traditional mineral acids (H2SO4, HF, etc.) and Lewis acids (AlCl3, BF3, etc.) are
generally hazardous and corrosive which will lead to toxic and corrosive wastes [4]. Therefore, the development of green and
environmentally friendly acidic catalytic systems including acidic ionic liquids and solid acids has drawn much attention in the past
decades [5-14]. Among them, heteropoly acids (HPAs) and related polyoxometalate compounds (POMs) have attracted considerable
interest from both academia and industry in the past decades due to their structural diversity and extensive applications in the fields of
material chemistry [15-17], energy sciences [18-19], CO2 chemistry [20-24], pharmaceutical chemistry [25], and catalytic chemistry
[26-36]. Particularly, the employment of simple and easily available Keggin-type HPAs (e.g., H3PW12O40, H4SiW12O40, and
H3PMo12O40) and their corresponding salts as catalysts has emerged as an efficient and versatile strategy for the synthesis of valuable
organic compounds via C-C and C-N bonds formation [37-44].
Aldehydes are an important class of building blocks for the synthesis of fine chemicals and pharmaceuticals owing to their versatile
reactivity. During the past centuries, a plethora of methods have been developed for the synthesis of aldehydes. For instance, the
selective oxidation of the parent alkanes or alcohols [45-47], photocatalysis coupling of styrenes, and vinyl ethers [48], Pd/Rh-catalyzed
reductive carbonylation of aryl halides [49]. Despite these significant achievements, the synthesis of polysubstituted aldehydes is still
very challenging due to their high reactivity [50-51]. Therefore, the development of novel and efficient catalytic methods for the direct
synthesis of polysubstituted aldehydes from commercially available starting materials is highly desired.
Inspired by the recent advances of polyoxometalate-catalyzed dehydrative coupling reactions of diarylmethanols [52-54], it is
assumed that the soft polyanions might play unique roles in the generation of carbocation 6 from the corresponding secondary alcohol
1. On the other hand, in the presence of POMs catalyst the hydrolysis of epoxide 2 to give diol 4 should be possible [55-56].
Subsequently, the diol 4 undergoes dehydration promoted by POMs to generate enol 5’. Then the coupling of the intermediate 5’ and
carbocation 6 gives the enol 7, which is further converted into the polysubstituted aldehydes 3 after tautomerization (Scheme 1a).
However, there are no such reports on the synthesis of poly-substituted aldehydes from the corresponding secondary alcohols and
epoxides/diols catalyzed by polyoxometalates so far. Herein, we report an efficient and general protocol catalyzed by the simple and
inexpensive H3PMo12O40 for the synthesis of polysubstituted aldehydes through the reaction of diarylmethanols with epoxides 2, diols
4, or aldehydes 5 under mild reaction conditions (Scheme 1b).

Scheme 1. Synthesis of polysubstituted aldehydes with polyoxometalate catalysis
We set out to realize the dehydrative coupling reaction of diarylmethanol 1a and epoxide 2a to give α,β,β-polysubstituted aldehyde
3a under polyoxometalate catalysis. As shown in Table 1, the model reaction of 1a and 2a catalyzed by 3 mol% of HPAs (i.e.,
o
H3PW12O40, H4SiW12O40, and H3PMo12O40) in DCE at 80 C for 2 h gave the desired product 3a in 54%, 66%, and 72% yields,
respectively (entries 1-3). The control experiment without any catalyst did not provide the desired product 3a, indicating the
significance of the polyoxometalate catalysis (entry 4). Further optimization of various solvents including H2O, CH3CN, PhCl, 1,4-
dioxane, THF, toluene, and CH3NO2 revealed that the polar CH3NO2 was the best solvent for this coupling reaction (entries 5-11). After
systematic investigation, we were delighted to isolate the desired product 3a in 97% yield, when using 3 mol% of H3PMo12O40 as a
catalyst, CH3NO2 as solvent at 90 oC for 2 h (entry 12). For more details, see Supporting information.
Table 1
Optimization of reaction conditions.a
Entry
1
2
3
4
Catalyst
Solvent
DCE
DCE
DCE
DCE
Yield (%)b
H3PW12O40
H4SiW12O40
H3PMo12O40
---
54
66
72
0
5
6
7
8
H3PMo12O40
H3PMo12O40
H3PMo12O40
H3PMo12O40
H3PMo12O40
H3PMo12O40
H3PMo12O40
H3PMo12O40
H2O
CH3CN
PhCl
1,4-dioxane
THF
toluene
CH3NO2
CH3NO2
7
0
72
29
23
56
88
97
9
10
11
12c
a Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), catalyst (3 mol%), solvent (1 mL), 80 oC, 2 h. DCE = 1,2-dichloroethane.
b Yields were determined by GC with biphenyl as the internal standard.
c 90 oC.
With the establishment of optimal reaction conditions, the substrate scope of this coupling reaction was then explored. As shown in
Scheme 2, 91% isolated yield of the target product 3a was obtained from the model reaction. The epoxides bearing -F, -Cl, -Br groups
on the para-position of benzene ring were converted into the corresponding polysubstituted aldehydes 3b-d in excellent yields (88%-
92%). When (4-chlorophenyl)(phenyl)methanol was applied to react with various epoxides, the desired products 3e-h were obtained in
89-93% yields. Phenyl(o-tolyl)methanol was also found to be suitable in this protocol to react with various epoxides delivering the
corresponding aldehyde products 3i-l in moderate to good yields (75%-81%). Furthermore, the reactions of bis(4-
chlorophenyl)methanol and various epoxides also worked smoothly affording the product 3m-p in good yields (87%-93%).
Unfortunately, the aryl alcohols such as benzylic alcohol and 1-phenylethanoldid not react with epoxide 2a, suggesting that this
protocol was limited in the diarylmethanols. In addition, when isopropanol and cyclohexanol were applied as the alcohol substrates
under the standard conditions, no reaction was observed. These results suggest that the current catalytic system is not suitable for the
aliphatic alcohols probably due to the inferior stability of the carbocation intermediate under the reaction conditions.

Scheme 2. Substrate scope for the reaction of diarylmethanols and epoxides. Reaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), H3PMo12O40 (3
mol%), CH3NO2 (1 mL), 90 oC, 2 h. Isolated yields were given.
According to the proposed reaction pathway in Scheme 1, the diol 4 should be one of the possible intermediates. Therefore, 1-
phenylethane-1,2-diol 4a was employed as a substrate to react with various diarylmethanols 1 catalyzed by H3PMo12O40 under the
standard conditions. As shown in Table 2, the polysubstituted aldehydes 3a, 3e, 3m, and 3i were obtained in good yields (77%-90%).
These results suggested that the coupling of diarylmethanols and diols is an efficient strategy for the construction of polysubstituted
aldehydes.
Table 2
Substrate scope for the reaction of diarylmethanols and 1,2-diol.a
[a] Reaction conditions: 1 (0.2 mmol), 4 (0.24 mmol),
H3PMo12O40 (3 mol%), CH3NO2 (1 mL), 90 oC, 2 h. Isolated
yields were given.
Table 3
Substrate scope for the reaction of diarylmethanols and 2-phenylacetaldehyde.a
[a] Reaction conditions: 1 (0.2 mmol), 5a (0.24 mmol),
H3PMo12O40 (3 mol%), CH3NO2 (1 mL), 90 oC, 2 h. Isolated
yields were given.

Additionally, the tautomerization of enol 5’ produces the corresponding aldehyde 5, suggesting that the coupling of 2-
arylacetaldehyde and diarylmethanols to synthesize the polysubstituted aldehydes should be possible in the presence of POM catalyst.
Accordingly, when 2-phenylacetaldehyde 5a was applied as a substrate to react with diarylmethanols under standard conditions, the
corresponding products 3n, 3o, 3p, and 3m were obtained in 87%-91% yields (Table 3). Compared with the previous report [50], this
procedure provides a rapid and efficient method for the preparation of polyaryl-substituted aldehydes.
Scheme 3. Gram-scale synthesis
Moreover, the gram-scale synthesis (5 mmol-scale) of the model product 3a from diarylmethanol 1a and the corresponding epoxide
2a or diol 4a was investigated under the standard conditions (Scheme 3). To our delight, this procedure was found to be practical and
scalable as the desired product 3a were obtained in 87% and 82% yields, respectively.
In order to get a deeper insight into the mechanism for the above-mentioned dehydrative reaction, some control experiments were
conducted as shown in Scheme 4. When the alcohol 1a was employed as the sole substrate under standard conditions, the corresponding
ether 8 was isolated in 81% yield (Scheme 4a). Subsequently, when ether 8 was applied as a substrate to react with epoxide 2a and diol
4 under the standard conditions, the desired aldehyde products were isolated in yields of 90% and 89%, respectively (Scheme 4b-c).
Such results indicated that ether 8 should be one of the reaction intermediates for the generation of carbocation 6, which is consistent
with the previous results [52-54]. Moreover, the above-mentioned experimental results confirm that the proposed mechanism in Scheme
1a is reasonable.
Scheme 4. Investigation of the reaction mechanism.
In conclusion, the coupling of diarylmethanols with epoxides (or diols, aldehydes) catalyzed by 3 mol% of heteropoly acid, i.e. H3PMo12O40,
was established for the synthesis of aromatic polysubstituted aldehydes. Our route employs earth-abundant, readily accessible, and nontoxic
heteropoly acid as catalyst, and it can be operated under mild reaction conditions providing various polysubstituted aldehydes in good to
excellent yields. Importantly, this reaction could be scaled up to gram-scale. Such findings provide a new and green approach for the
construction of aromatic polysubstituted aldehydes.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.
Acknowledgments
We thank the financial support from the National Natural Science Foundation of China (Nos. 21871026, 21971224), Research Found
of East China University of Technology (Nos. DHBK2019265, DHBK2019267, DHBK2019264).

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