One-pot cascade synthesis of α-diketones from aldehydes and ketones in water by using a bifunctional iron nanocomposite catalyst

Article abstract of DOI:10.1039/d0gc03739g

A new methodology for the synthesis of α-diketones was reportedviaa one-pot cascade process from aldehydes and ketones catalyzed by a bifunctional iron nanocomposite using H₂O₂as a green oxidant in water. The one-pot strategy showed excellent catalytic stability, comprehensive suitability of substrates and important practical utility for directly synthesizing biologically active and medicinally valuable N-heterocyclesviaan intermittent process.

Full text of DOI:10.1039/d0gc03739g

Showcasing the research of Prof. Yong Yang et al.
of Qingdao Institute of Bioenergy and Bioprocess
Technology, Chinese Academy of Sciences, China.
As featured in:
Volume 23
Number
March 2021
Pages 1885-2186
5
7
Green
a greener sustainable future
One-pot cascade synthesis of α-diketones from aldehydes
and ketones in water by using a bifunctional iron
nanocomposite catalyst
Chemistry
Cutting-edge research for
rsc.li/greenchem
A new methodology for the synthesis of α-diketones was
reported via a one-pot cascade process from aldehydes
and ketones catalyzed by a bifunctional iron nanocomposite
using H2O2 as a green oxidant in water. The one-pot strategy
showed excellent catalytic stability, comprehensive suitability
of substrates and important practical utility for directly
synthesizing biologically active and medicinally valuable
N-heterocycles via an intermittent process.
ISSN 1463-9262
PAPER
Bert U. W. Maes et al.
Efficient demethylation of aromatic methyl ethers with HCl
in water
See Yong Yang et al., Green Chem.,
2021, 23, 1955.
rsc.li/greenchem
Registered charity number: 207890

Green Chemistry
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COMMUNICATION
One-pot cascade synthesis of α-diketones from
aldehydes and ketones in water by using a
bifunctional iron nanocomposite catalyst†
Cite this: Green Chem., 2021, 23,
1955
Received 4th November 2020,
Accepted 14th January 2021
Tao Song, a,c Xin Zhou,a,c Xiaoxue Wang,a,c Jianliang Xiaob and Yong Yang
*
a,c
DOI: 10.1039/d0gc03739g
rsc.li/greenchem
Ru,7c,d,8b,c and Au,7f,9c stoichiometric Cu salts,10a virulent mer-
A new methodology for the synthesis of α-diketones was reported
via a one-pot cascade process from aldehydes and ketones cata-
lyzed by a bifunctional iron nanocomposite using H2O2 as a green
oxidant in water. The one-pot strategy showed excellent catalytic
stability, comprehensive suitability of substrates and important
practical utility for directly synthesizing biologically active and
medicinally valuable N-heterocycles via an intermittent process.
curic salts,7e or hazardous oxidants such as KMnO4,7h I2,8c,d
Ph2SO,9c HClO4
and PIFA.10d,e Even worse, these processes
10b
generally suffer from poor chemoselectivity and low tolerance
of functional groups. Thus, it is greatly desirable to develop a
facile, efficient, and highly selective approach to prepare
α-diketones, ideally, using an inexpensive and robust hetero-
geneous catalyst with high activity, excellent selectivity, and
broad substrate scope.
Very recently, we have developed a bifunctional iron nano-
composite catalyst containing two key iron species of Fe–Nx
and Fe phosphate as the oxidation and Lewis acid sites, which
α-Diketones are vital skeletons in natural products.1 As signifi-
cant precursors, they are applied to synthesize anticonvulsant
and antimicrobial drugs,2 and other heterocyclic compounds
such as quinoxaline, imidazole, and thiophene.3 Moreover,
they are also used as photoinitiators and corrosion inhibitors.4
In view of their crucial usefulness, various methods have been
developed for the preparation of α-diketones. Traditional
methods, including the substitution of oxalyl chloride or
α-keto acid chloride5 and oxidation of α-halo, α-hydroxy
ketones or their derivatives,6 always need sophisticated and
expensive substrates and harsh reaction conditions. Recently,
the introduction of two oxygen atoms across unsaturated C–C
bonds represents the most straightforward and high atom-
economical approach for the synthesis of α-diketones. In this
context, the oxidation strategies starting from various sub-
strates have been reported, including the (i) oxidation of
internal alkynes (Scheme 1a);7 (ii) oxidation of internal alkenes
(Scheme 1b);8 (iii) oxidative coupling of terminal alkynes,
alkynic acid or alkenes with aryl halide or other coupling part-
ners (Scheme 1c);9 and (iv) oxidation of α,β-unsaturated carbo-
nyls (Scheme 1d).10 Although these methods have been attrac-
tive in the synthesis aspect, they always employ high-cost, toxic
and nonrecyclable noble metal catalysts such as Pd,7a,b
aCAS Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and
Bioprocess Technology, Chinese Academy of Sciences, No. 189 Songling Road,
Qingdao 266101, China. E-mail: yangyong@qibebt.ac.cn
bDepartment of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD,
UK
cShandong Energy Institute, Qingdao 266101, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc03739g
Scheme 1 Strategies for the synthesis of α-diketones via the oxidation
of unsaturated C–C bonds.
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Table 1 Optimization of reaction conditionsa
exhibited excellent reactivity and good tolerance of functional
groups for oxidative cleavage of alkenes into ketones or their
oxidation into α-diketones using TBHP as the oxidant in water
under mild conditions (Scheme 1e).11 Mechanistic studies
reveal that one-carbon shorter ketones were produced via the
formation of an epoxide as the key intermediate from the epoxi-
dation of internal alkenes followed by subsequent Lewis acid-
catalyzed Meinwald rearrangement. As such, we envision that
the CvC double bond in α,β-unsaturated ketones could be oxi-
dized into α-diketones via the formation of their corresponding
epoxide catalyzed by the bifunctional iron nanocomposite
under appropriate reaction conditions. It is well known that
ketones and aldehydes could be converted into α,β-unsaturated
ketones via aldol or the Claisen–Schmidt condensation in the
presence of a base or an acid.12 In our continuous contributions
towards developing green and sustainable catalysts for organic
transformations,13 we herein expand the application of this
bifunctional iron nanocomposite catalyst for the synthesis of
α-diketones, and found that α-diketones could be efficiently syn-
thesized via a one-pot cascade process from easily available
aldehydes and ketones catalyzed by the bifunctional iron nano-
composite using H2O2 as the oxidant and water as a green
solvent under mild conditions (Scheme 1f). A broad substrate
scope of aldehydes and ketones with good functional group tol-
erance was converted into α-diketones in up to 90% yield. In
addition, the iron nanocomposite can be easily recovered for
successive use without obvious change in activity. To the best of
our knowledge, this is the first example of an expedient and
Entry
Additive
T/°C
t/h
Conv.(%)b
Yieldb (%)
1
2
3
4
5
6
7
8
NaOH (0.5 equiv.)
NaOH (0.3 equiv.)
NaOH (0.1 equiv.)
NaOH (0.3 equiv.)
NaOH (0.3 equiv.)
NaOH (0.3 equiv.)
NaOH (0.3 equiv.)
NaOH (0.3 equiv.)
NaOH (0.3 equiv.)
NaOH (0.3 equiv.)
NaHCO3 (0.3 equiv.)
K3PO4 (0.3 equiv.)
TEA (0.3 equiv.)
NaOH (0.3 equiv.)
—
110
110
110
90
70
50
RT
50
50
50
50
50
50
50
50
50
50
50
50
50
12
12
12
12
12
12
12
10
8
10
10
10
10
10
10
10
10
10
10
10
100
100
84
100
100
100
80
100
100
100
84
67
81
97
10
98
90
81
83
98
97
51
97
97
96
34
96 (90)c
89
57
60
47
15
0
1
3
0
5
3
8
9
10d
11
12
13
14e
15
16f
17
18
19
20
NaOH (0.3 equiv.)
NPC-800
Fe2O3
Fe3O4
FeCl3·6H2O
91
a Reaction conditions: acetophenone (0.2 mmol), benzaldehyde
(0.22 mmol), Fe@NPC-800 (10 mol% of Fe), H2O2 30 wt% in water
(5 equiv.), and H2O (2 mL). b Determined by GC. c Isolated yields.
d 3 eq. of H2O2. e Without Fe@NPC-800. f O2 atmosphere.
efficient synthesis of α-diketones from simple aldehydes and vity (Table 1, entry 6). The reaction did take place at room
ketones in a cost-effective and sustainable manner.
temperature, while a considerably lower activity and selectivity
The preparation and characterization of bifunctional iron were obtained (Table 1, entry 7). We next conducted the reac-
nanocomposite catalyst have been reported in our previous tion at 50 °C with shorter reaction times and found that 100%
study.11 The catalyst Fe@NPC-800 (where NPC represents N,P conversion of acetophenone with 90% isolated yield to benzil
co-doped carbon material and 800 represents the pyrolysis was afforded after 10 h (Table 1, entry 8), while the yields
temperature at 800 °C) has two catalytically active sites of decreased slightly after further shortening the reaction time to
Fe–Nx and Fe phosphate as the oxidation and Lewis acid sites, 8 h (Table 1, entry 9). Other types of bases such as NaHCO3,
which were simultaneously integrated into a hierarchical N,P- K3PO4, and triethylamine (TEA) were also investigated under
dual-doped porous carbon and showed the best catalytic per- otherwise identical conditions, giving relatively lower reactivity
formance for the oxidation of the internal alkenes. In this and selectivity to benzil (Table 1, entries 11–13). In addition,
study, we continued to use Fe@NPC-800 as the catalyst to test control experiments in the absence of either catalyst, or base, or
the feasibility. We initiated our investigation by choosing the in the presence of molecular oxygen as the oxidant, either gave
synthesis of benzil from benzaldehyde and acetophenone as a low reactivity or condensation to α,β-unsaturated ketone as the
model reaction in the presence of 10 mol% of Fe@NPC-800, major product (Table 1, entries 14–16), indicating the indispen-
0.5 equiv. NaOH, and 5 equiv. H2O2 at 110 °C in water sability of the catalyst and base for the success of the cascade
(Table 1, entry 1). To our surprise, benzil was obtained with reaction. Control experiments employing NPC-800 without the
100% conversion of acetophenone and 98% selectivity after introduction of iron or commercially available Fe2O3, Fe3O4,
12 h. With this satisfactory result in hand, a set of parameters, NPC-800, and FeCl3·6H2O as catalysts show inferior reactivity
including reaction temperatures, reaction times, the type and for the generation of 1,2-diketone (Table 1, entries 17–20).
addition amount of bases, and the used amount of H2O2, was
Having identified the optimal conditions, we further
subsequently screened to achieve much milder conditions. explored the generality of this protocol for the synthesis of
Decreasing the addition amount of NaOH from 0.5 to α-diketone compounds. A series of aldehydes and ketones was
0.3 equiv. had a negligible impact on reactivity (Table 1, entry 2); tested, as shown in Table 2. Halogen-substituted aromatic
however, lower conversion of acetophenone and poorer selecti- ketones (1b–1e) and aldehyde (2e) could be converted into the
vity to benzil were achieved with 0.1 equiv. of NaOH (Table 1, corresponding α-diketones (3b–3e) in more than 80% yields.
entry 3). Lowering the reaction temperature to 50 °C led to the The aromatic ketones bearing either electron-donating groups
excellent maintenance of the outstanding activity and selecti- (–Me, –OMe) or electron-withdrawing groups (–CN, –NO2, CF3)
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Table 2 Substrate scope of one-pot cascade synthesis of α-diketonesa
Fig. 1 Recycling performance of the Fe@NPC-800 catalyst.
α-Diketones as essential building blocks have been widely
applied in the synthesis of biologically active N-heterocycles.3
Therefore, to demonstrate the practical applicability of this
one-pot cascade protocol, we attempted to directly synthesize
quinoxaline and imidazole via an intermittent process without
the separation and purification of intermediates (Scheme 2).
After the formation of benzil under the optimized conditions
was completed, benzene-1,2-diamine or benzyl aldehyde/
NH4OAc was directly added into the reaction mixture for
sequential condensation cyclization. To our delight, 2,3-diphe-
nylquinoxaline (4a) and 2,4,5-triphenyl-1H-imidazole (5a) were
isolated in 83 and 67% yields, respectively. This provides an
alternative and attractive synthetic method for the preparation
of quinoxaline, imidazole or other important bioactive
N-heterocyclic compounds, further highlighting the superior-
ity of this novel bifunctional Fe nanocomposite and practical
utility of this one-pot cascade strategy.
a Reaction conditions: ketone (0.2 mmol), aldehyde (0.22 mmol),
Fe@NPC-800 (10 mol% of Fe), H2O2 30 wt% in water (5 equiv.), H2O
(2 mL), and 10 h. Yields of isolated product are reported.
b Benzaldehyde 0.44 mmol, Fe@NPC-800 (20 mol% of Fe), H2O2
30 wt% in water (10 equiv.), H2O (2 mL), and 10 h. c 0.5 equiv. of
NaOH, 90 °C, and 12 h. d 0.5 mL of acetone was used.
To gain an insight into the reaction pathway, a set of
control experiments was carried out. In the absence of the
Fe@NPC-800 catalyst, chalcone (6a) was obtained in 98% GC
yield for the reaction of benzaldehyde and acetophenone with
the assistance of 0.3 equiv. NaOH in H2O at 50 °C within 2 h
(Scheme 3, eqn. (a)). When 6a was used as the starting material
for the reaction in the absence of the Fe@PNC-800 catalyst
under otherwise identical conditions, phenyl(3-phenyloxiran-2-
yl)methanone (7a) was detected in 32% GC yield (Scheme 3,
eqn (b)), which could be achieved via the nucleophilic attack
of H2O2 on the Cv bond mediated by the base.15 In contrast,
chalcone (6a) was smoothly converted into benzil in 90% yield
were smoothly transformed into α-diketones in 61–88% yields,
while the corresponding products were obtained in relatively
higher yields with electron-donating group-substituted sub-
strates (3f–3j) than with the electron-withdrawing ones
(3k–3n). Thiophene-2-carbaldehyde (2o) and 2-naphthaldehyde
(2q) were suitable for the construction of the α-diketone
framework in 72% and 77% yields, respectively. 2,3,4,5,6-
Pentafluorobenzaldehyde (2p) also worked well to produce
1-(perfluorophenyl)-2-phenylethane-1,2-dione (3p) in 67% yield.
2,2′-(1,4-phenylene)bis(1-phenylethane-1,2-dione) (3r) could be
obtained from 1,4-diacetylbenzene (1r) containing two reaction
sites in 73% yield using a twofold amount of benzaldehyde,
catalyst, base and oxidant with the same reaction time.
Moreover, the aliphatic ketones such as 1-cyclohexylethan-1-
one (1s), 1-cyclopropylethan-1-one (1t), heptan-2-one (1u), and
acetone (1v) were also converted into the corresponding
α-diketones in 61–71% yields under slightly modified reaction
conditions.
Subsequently, we investigated the recyclability of the
Fe@NPC-800 catalyst for the benchmark reaction under the
optimized conditions. The catalyst has been used six times
consecutively with negligible changes in reactivity and selecti-
vity, indicating its durable stability (Fig. 1).
Scheme 2 Intermittent reaction for the synthesis of quinoxaline and
imidazole.
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Scheme 4 Proposed mechanism for the one-pot synthesis of
α-diketones.
Scheme 3 Control experiments for the one-pot synthesis of
α-diketones.
Conclusions
In conclusion, we developed a new synthetic method for the
synthesis of α-diketones from ketones and aldehydes catalysed
by a bifunctional iron nanocomposite. A broad range of alde-
hydes and ketones could be efficiently converted into their
corresponding α-diketones with good tolerance of functional
groups using H2O2 as a green oxidant in water under mild
conditions. In addition, the one-pot strategy was also appli-
cable to the synthesis of quinoxaline and imidazole skeletons
via an intermittent process. The catalyst could be readily
recovered for successive recycling without an obvious decay of
under the optimized conditions (Scheme 3, eqn (c)), indicating
the key role of the Fe@NPC-800 catalyst. According to our pre-
vious report,11 phenyl(3-phenyloxiran-2-yl)methanone (7a) gen-
erated by the epoxidation of chalcone could be the key inter-
mediate for benzil. Thus, phenyl(3-phenyloxiran-2-yl)metha-
none (7a) was subjected to the optimized conditions
(Scheme 3, eqn (d)), and benzil was obtained in 95% GC yields
in this case. However, phenyl(3-phenyloxiran-2-yl)methanone
(7a) was kept intact in the absence of the Fe@PNC-800 catalyst
under otherwise identical conditions (Scheme 3, eqn (e)),
further verifying that the ring-opening of epoxide was catalyzed
by Fe@NPC-800 rather than by NaOH. The O18-labeling experi-
ment was conducted using H2O18 as the solvent, and a mono-
O18-incorporated product 1,2-diketones-O18 was obtained in
81% yield, suggesting that one of the oxygen atoms in 1,2-dike-
tones originates from water (Scheme 3, eqn (f)).
catalytic performance. This study not only represents
a
simple, efficient and environmentally friendly method for the
synthesis of 1,2-diketones from simple aldehydes and ketones
but also demonstrates an excellent example of the design and
application of bifunctional catalysts for significant organic
transformations.
Taking all control experiments into account, we proposed a
plausible mechanism for the one-pot cascade synthesis of
α-diketones as presented in Scheme 4. Initially,
α,β-unsaturated ketones were formed by the condensation of
Conflicts of interest
There are no conflicts to declare.
ketones and aldehydes in the presence of
a
base.
Subsequently, the generated unsaturated carbonyls were oxi-
dized to an epoxide intermediate via the synergic processes of
Acknowledgements
Fe–Nx site-catalyzed epoxidation11,14 and the nucleophilic We gratefully acknowledge the financial support of the
attack of H2O2 mediated by the base.15 The epoxide intermedi- National Natural Science Foundation of China (No. 22002178,
ate could be converted into an α-carbonyl aldehyde in the pres- 22078350), the Natural Science Foundation and Key
ence of FePO4 as
a Lewis acid site via the Meinwald Technology R&D Program of Shandong Province
rearrangement.11,16 Subsequently, α-carbonyl carboxylic acid (ZR2020KB016, 2019GGX102075). Y. Y. also acknowledges the
was readily formed from the oxidation of α-carbonyl aldehyde Royal Society (UK) for a Newton Advanced Fellowship (NAF/R2/
under the standard reaction conditions, which underwent oxi- 180695), Open Projects of State Key Laboratory of Physical
dative decarboxylation followed by the cascade oxidation Chemistry of the Solid State Surface (Xiamen University)
process to obtain the α-diketones.
(No. 201808).
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