A Bifunctional Iron Nanocomposite Catalyst for Efficient Oxidation of Alkenes to Ketones and 1,2-Diketones

Article abstract of DOI:10.1021/acscatal.9b05197

We herein report the fabrication of a bifunctional iron nanocomposite catalyst, in which two catalytically active sites of Fe-N_x and Fe phosphate, as oxidation and Lewis acid sites, were simultaneously integrated into a hierarchical N,P-dual doped porous carbon. As a bifunctional catalyst, it exhibited high efficiency for direct oxidative cleavage of alkenes into ketones or their oxidation into 1,2-diketones with a broad substrate scope and high functional group tolerance using TBHP as the oxidant in water under mild reaction conditions. Furthermore, it could be easily recovered for successive recycling without appreciable loss of activity. Mechanistic studies disclose that the direct oxidation of alkenes proceeds via the formation of an epoxide as intermediate followed by either acid-catalyzed Meinwald rearrangement to give ketones with one carbon shorter or nucleophilic ring-opening to generate 1,2-diketones in a cascade manner. This study not only opens up a fancy pathway in the rational design of Fe-N-C catalysts but also offers a simple and efficient method for accessing industrially important ketones and 1,2-diketones from alkenes in a cost-effective and environmentally benign fashion.

Full text of DOI:10.1021/acscatal.9b05197

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Article
A Bifunctional Iron Nanocomposite Catalyst for Efficient
Oxidation of Alkenes to Ketones and 1,2-Diketones
Tao Song, Zhiming Ma, Youzhu Yuan, Jianliang Xiao, Yong Yang, and Peng Ren
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b05197 • Publication Date (Web): 25 Mar 2020
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A Bifunctional Iron Nanocomposite Catalyst for Efficient Oxidation of Alkenes to
Ketones and 1,2-Diketones
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Tao Song,a Zhiming Ma, a,b Peng Ren,a,b Youzhu Yuanc Jianliang Xiao,d* and Yong Yang a*
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a CAS Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and Bioprocess
Technology, Chinese Academy of Sciences, No. 189 Songling Road, Laoshan District, Qingdao
266101, China.
b University of Chinese Academy of Sciences, Beijing 100049, China
c State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory
for Green Chemical Production of Alcohols-Ethers-Esters, College of Chemistry and Chemical
Engineering, Xiamen 361005, P R China.
d Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD United
Kingdom.
Keywords: Bifunctional catalysis, iron nanocomposites, alkenes oxidation, ketones, 1,2-diketones.
Abstract
We herein report the fabrication of a bifunctional iron nanocomposite catalyst, in which two
catalytically active sites of Fe-Nx and Fe phosphate, as oxidation and Lewis acid sites, were
simultaneously integrated into a hierarchical N,P-dual doped porous carbon. As a bifunctional
catalyst, it exhibited high efficiency for direct oxidative cleavage of alkenes into ketones or their
oxidation into 1,2-diketones with a broad substrate scope and high functional group tolerance using
TBHP as the oxidant in water under mild reaction conditions. Furthermore, it could be easily
recovered for successive recycles without appreciable loss of activity. Mechanistic studies disclose
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that the direct oxidation of alkenes proceeds via the formation of an epoxide as intermediate
followed by either acid-catalyzed Meinwald rearrangement to give ketones with one carbon shorter
or nucleophilic ring-opening to generate 1,2-diketones in a cascade manner. This study not only
opens up a fancy pathway in the rational design of Fe-N-C catalysts, but also offers a simple and
efficient method for accessing industrially important ketones and 1,2-diketones from alkenes in a
cost-effective and environmentally benign fashion.
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1. Introduction
Ketones, including 1,2-diketones, are very important and useful intermediates or building
block in synthetic organic chemistry, pharmaceuticals, agrochemicals, and bulk/fine chemicals.[1]
Whilst many methods have been developed for the preparation of ketones, those that employ
alkenes as the substrate are particularly appealing, as they are one of the most fundamental and
most easily accessible chemicals. Indeed, alkenes have been used as starting materials for the
synthesis of ketones over the past decades. However, only a handful of examples have been
reported to date of preparing ketones and 1,2-diketones from trans-disubstituted alkenes via
catalytic oxidation (Scheme 1).[2,3] Furthermore, the methods developed generally suffer from such
disadvantages as limited alkene scope, cost and toxicity of the metal catalysts and/or oxidants, and
difficult product/catalyst separation, and environmental compatibility. Therefore, it is highly
desirable to develop new approaches for the oxidation of alkenes into ketones and 1,2-diketones
in a cost-effective and environmentally benign manner.
It has been well-documented that epoxides can be transformed into ketones, 1,2-diols, 1,2-
diketones, carboxylic acids, or other novel molecules via either facile C-O cleavage or challenging
C-C bond cleavage under various conditions, such as the nucleophilic ring-opening to generate 2-
hydroxyalkylated derivatives,[4] the acid-catalyzed Meinwald rearrangements to yield carbonyl
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compounds,[5] or the strong base-promoted isomerization to give allylic alcohols.[6] Considering
epoxides are readily available form alkenes via a one-step oxidation process,[7] we thought that
direct oxidation of alkenes to afford ketones and/or 1,2-diketones might be realized in a cascade
manner via the formation of epoxides as an intermediate followed by acid/base catalysis or
nucleophilic ring-opening (Scheme 1).
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Previous works:
[Ti], [Mn]
[Ru], [Pd]
(Ref. 3a-e)
TBHP, H2O2 (Ref. 2a, 2b)
tBuOOH, O2
O
O
1) CTAB, NIS
Cr(IV)
(Ref. 3f)
R1
R1
2) PDC, AcOH,
MW (Ref. 2c)
R2
R2
R1
R2
O
(PhCOSe)2O
TBAI
(Ref. 2d)
NaIO4
I2, IBX, KMNO4
(Ref. 3g-i)
Cost and unreuseable noble metal catalysts
Eco-unfriendly oxidants
Limited alkene scope
This works: a sustainable synergistic cascade alkene oxidation
O
R1
R2
00
Fe@NPC-8
O
H2
TBHP,
O
Fe@NPC-800
Meinwald rearrangment pathway
R1
R1
R2
R2
TBHP
Nucleophilic ring-opening pathway
Fe@NPC-8
00
O
TBHP,
TBAI
CH
3CN/H
R1
2O
R2
Novel bifunctional reusable iron nanocatalyst
O
High efficiency with broad alkene scope
Mild reaction conditions
Switchable reaction pathways
Scheme 1. Synthesis of ketones and 1,2-diketones from oxidation of alkenes.
The key to this strategy is the rational design and preparation of a bifunctional catalyst, in
which active redox and acid/basic sites are integrated into a single catalyst to cooperatively enable
the cascade epoxidation and ring-opening reactions in a sequential manner. Two such proof-of-
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concept catalysts have been reported and demonstrated the potential for the reaction.[2a,2b]
However, they suffer from the issues of restricted substrate scope, demanding reaction conditions
and low catalyst stability, which significantly impede their practicability in chemical synthesis.
Therefore, a more efficient bifunctional catalyst, ideally heterogeneous one, which allows for the
oxidation of alkenes into ketones with high activity and productivity, expanded scope, easy
catalyst/product separation, and milder reaction conditions, is urgently needed.
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Herein, we report a facile and easily scalable method to prepare a bifunctional iron
nanocomposite, in which two catalytically active iron species, i.e. Fe-Nx and FePO4, as redox and
Lewis acid sites co-exist on the surface of hierarchical N,P-dual doped porous carbon. With
inexpensive and biocompatible iron as the active sites, the nanocomposite enables synergistically
cooperative catalysis for the oxidation of various alkenes into ketones and 1,2-diketones in high
yields with tert-butyl hydroperoxide (TBHP) as the oxidant in water or a CH3CN-water mixture
under mild reaction conditions. Furthermore, the catalyst can be readily recovered for reuse
without appreciable loss of catalytic performance.
2. Catalyst Preparation and Characterization.
2.1 Catalyst Preparation. The iron nanocomposite catalysts were prepared in a facile and
environmentally benign manner, following our recently developed method that starts from
naturally available and renewable biomass as nitrogen and carbon sources (see Supporting
Information for the details).[8] Typically, fresh bamboo shoots were first cut into slices, dried, and
grounded into powder, followed by a hydrothermal process. The resulting solids were
homogeneously mixed with Fe(NO3)3 and PPh3, as Fe and P sources, respectively, followed by
pyrolysis under a N2 atmosphere at varying temperatures to obtain the iron nanocomposites
catalysts, hereinafter denoted as Fe@NPC-T (where T represents the pyrolysis temperature, e.g.,
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700, 800, and 900 C). The iron contents were determined to be 5.0 wt% for Fe@NPC-700, 5.8
wt% for Fe@NPC-800, and 7.0 wt% for Fe@NPC-900, respectively, by the coupled plasma
optical emission spectrometry (ICP-OES) (Table S2).
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2.2 Catalyst Characterization. The transmission electron microscope (TEM) and high-
resolution images (Figure 1A-I) for the catalysts Fe@NPC-T show that metallic iron nanoparticles
(NPs) containing Fe and/or Fe3C crystalline phases with uniform size were evenly dispersed on
graphitic carbon. The high-angle annular dark-field scanning transmission electron microscopy
(HAADF-STEM) images and energy-dispersive X-ray (EDX) maps (Figure 1J) reveal that Fe, N,
P, O and C atoms are homogeneously distributed as well. X-ray diffraction (XRD) patterns (Figure
S1A) and magnetic measurements (Figure S2) verify that the metallic Fe and/or Fe3C NPs were
supported on graphitic carbon. Two characteristic D and G bands at 1590 and 1350 cm-1 in Raman
spectra (Figure S1B) provided solid evidence for the formation of graphitic carbon with a certain
degree of graphitization and varying defected sites.[8] N2 adsorption/desorption measurements
demonstrate that the catalysts Fe@NPC-T possess hierarchically micro-, meso-, and macropores
with high specific surface areas and large pore volumes (Figure S3 and Table S1, S2).
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Figure 1. (A, B) TEM images of Fe@NPC-700, (C) HR-TEM image of Fe@NPC-700; (D, E)
TEM images of Fe@NPC-800, (F) HR-TEM image of Fe@NPC-800; (G, H) TEM images of
Fe@NPC-900, (I) HR-TEM image of Fe@NPC-900; (J) HAADF STEM image and corresponding
EDX mappings of Fe, N, O, and P of Fe@NPC-800.
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The nature of N, P, and Fe in Fe@NPC-T was studied by X-ray photoelectron spectroscopy
(XPS; Figure 2a and 2b, Figure S4). The N 1s spectrum contains 5 peaks at 398.02, 399.6, 400.4,
401.3, and 402.5 eV, which are assigned to pyridinic, Fe-Nx, pyrrolic, quaternary, and oxidized N,
respectively (Figure 2a). The content of each type of nitrogen varies as the pyrolysis temperature
increases (Table S3). Of the peaks, the one at 399.6 eV is in good consistence with that of
ironphthalocyanine (FePc), indicating that Fe coordinates with N atoms in aromatics to form
catalytically active Fe-Nx sites.[9] This observation also clearly suggests the N atoms are
incorporated into the carbon lattice. Three types of P species could be identified from the P 2p
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spectrum, that is, Fe-P (zero valence state, 129.2 and 130.4 eV),[10] P-C (132.8 eV),[8d, 11] PO3 and
PO43- (5+, 133.7 and 134.4 eV)[11] (Figure S4). The formation of FePO4 was further supported by
FT-IR experiment with appearance of a characteristic band at 1045 cm-1, assignable to the
stretching of the P-O bond present in PO43- (Figure S7).[12] The Fe 2p XPS spectrum (Figure 2b)
were fitted into three peaks at 706.8, 710.4, and 712.0 eV, respectively. The peak at 706.8 eV can
be attributed to zero valence Fe (metallic iron or carbide).[9b, 13] The peak at 710.4 eV can be
assigned to Fe in Fe-Nx configuration, while the peak at 712.0 eV can be indexed to Fe3+ in
FePO4.[14] A notable positive shift in binding energy assignable to Fe-Nx configuration in
Fe@NPC-T compared with that of FePc implies interactions of Fe-Nx with metallic iron NPs.[9b]
Noticeably, the content for each iron species in the Fe@NPC-T varies with pyrolysis temperatures.
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Figure 2. High-resolution XPS spectra of N 1s (a) and Fe 2p (b) for Fe@NPC-700, 800 and 900;
Fe K-edge XANES (c) and Fourier transform (FT) EXAFS (d) spectra of Fe@NPC-700, 800 and
900, with FePc, Fe foil, and FePO4 as references.
X-ray absorption measurements were further performed to analyze the coordination
environment of Fe at the atomic level. The near-edge absorption energy position of Fe@NPC-T
was located between that of Fe foil and FePO4 in Fe K-edge absorption near-edge structure
(XANES) suggests that the valence state of Fe is in between 0 and +3 (Figure 2c). This is in
agreement with the XPS results. The Fourier transform (FT) k3-weighted EXAFS spectra of
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Fe@NPC-T show one main peak at around 1.5 Å, which belongs to the first coordination shell of
Fe-N and is notably similar to that of FePc, suggesting that they are more likely to form Fe-Nx
coordination structures (Figure 2d).[9b, 15] Besides, the peak at around 2.5 Å, ascribed to the Fe-C
distance of the second neighbor shell was also observed.[9b] However, the spectrum of Fe@NPC-
700 is more similar to Fe foil with two strong peaks at around 2.1 and 4.5 Å, which correspond to
the Fe-Fe distance,[9b] revealing the presence of metallic iron crystalline structures. The peak of
the Fe-Fe bond could also be observed in Fe@NPC-800 and Fe@NPC-900, corroborating that
metallic iron exists in both samples, which are consistent with the HR-TEM and XRD results.
Noticeably, the EXAFS spectrum of Fe@NPC-800 or Fe@NPC-900 shows a higher peak intensity
for Fe-N bonding and a lower intensity for Fe-Fe bonding compared to that of Fe@NPC-700.
Nonetheless, these data suggest that Fe-Nx configurations are incorporated in the graphitic carbon
frameworks in the catalysts and their intensity increases with the increase of pyrolysis
temperatures.
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3. Results and discussion
3.1 Oxidation of Alkenes to Ketones. To examine the catalytic activity of the iron
nanocomposite Fe@NPC-T for the oxidative cleavage of alkenes, trans-stilbene 1a was chosen as
a model substrate (Table 1). The reaction was performed in the presence of Fe-@NPC-800 as a
catalyst and 5 equivalents of TBHP as an oxidant at 100oC in water. To our delight, full conversion
of 1a with formation of one-carbon shorter benzophenone 2a as major product in 86% selectivity
was observed (entry 1). The selectivity to 2a as high as 92% could be achieved with 98%
o
conversion of 1a at 90 C under otherwise identical conditions (entry 2). However, a further
decrease of reaction temperature to 80 oC led to a rather lower reactivity (entry 3). Lowering the
amount of TBHP (e.g., 3 equivalents with respect to 1a) drastically decreased the reactivity too
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(entry 4). Using hydrogen peroxide, air, or molecular oxygen instead of TBHP as terminal oxidant,
no reaction was observed with full recovery of 1a under identical conditions (entries 5-7). Further
screening of the as-prepared catalysts, e.g., Fe@NPC-700 and Fe@NPC-900, reveal that the
pyrolysis temperature for preparation of the iron nanocomposite has an important influence on the
reactivity (entries 8 and 9), with Fe@NPC-800 affording the best result. Control experiments
employing commercially available Fe2O3, Fe3O4, Fe(NO3)3, FePO4, iron phthalocyanine (FePc),
and nano Fe powder as catalysts for the reaction all show inferior reactivity with formation of a
mixture of products (entries 11-16). In addition, when NPC-800 without the introduction of iron
was employed as a catalyst, no reaction was observed under the same conditions, indicating the
essential role of the Fe@NPC-T in catalysis (entry 17). Therefore, the optimized conditions are as
follows: Fe@NPC-800 as a catalyst, TBHP (5 equivalents with respect to trans-stilbene) as an
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oxidant, 90 C in water. Of note, the carbon balance closure is nearly 90% for the benchmark
reaction under different conditions, as shown in Table S9.
Table 1. Optimization of reaction conditions for synthesis of ketones.a
O
O
O
O
[Fe] (x mol% Fe)
[O], H2O
+
+
+
O
1a
3a
2a
4a
Selectivity [%]b
5a
5a
Entry
[Cat.]
[O]
T [oC]
Conv.[%]b
2a
3a
4a
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Fe@NPC-800
Fe@NPC-800
Fe@NPC-800
Fe@NPC-800
Fe@NPC-800
Fe@NPC-800
Fe@NPC-800
Fe@NPC-700
Fe@NPC-900
Fe@NC-800
Fe2O3
TBHP
TBHP
TBHP
TBHP
H2O2
100
90
80
90
90
90
90
90
90
90
90
90
90
90
90
90
90
100
98
57
43
0
86
92
65
91
-
0
0
10
4
0
2
9
0
-
2
3
12
0
14
9
4c
5
-
-
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6
Air
0
-
-
-
-
7d
8
O2
0
-
-
-
-
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
79
100
90
19
23
11
7
42
71
59
32
30
45
71
41
77
-
47
10
24
68
48
28
29
37
7
11
11
17
0
0
0
0
0
0
0
0
0
0
-
9
10
11
12
13
14
15
16
17
Fe3O4
22
27
0
Fe powder
Fe(NO3)3
FePc
31
11
0
22
16
-
FePO4
NPC-800
-
aReaction conditions: trans-stilbene (0.2 mmol), catalyst (10 mol% of Fe), oxidant (5 eq.), H2O (4
mL), 90 oC, 12 h. bDetermined by GC-MS. c3 eq. TBHP. dOxygen balloon.
After identifying the optimal reaction conditions, we next investigated the generality of this
protocol as shown in Table 2. Overall, a variety of alkenes could efficiently undergo the selective
cleavage reaction to afford their corresponding one-carbon shorter ketones in high yields with
good tolerance of various functional groups. Electron donating group-substituted 1,2-diarylalkenes
gave higher yields than those substituted by electron-withdrawing groups, e.g., -NO2 (1i), -CN (1j,
1k), and -CF3 (1l). Halogen-substituted 1,2-diarylalkenes (1b-e) tolerate the present conditions
and the respective diaryl ketones were achieved in 80-87% yields with no undesired
dehalogenation taking place. Other functional groups, e.g., PhthN- (1m), -NH2 (1n), -COOH (1o),
and MeO2C- (1p), were also compatible with the present conditions. Delightfully, 2,3,4,5,6-
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pentafluoro-substituted 1,2-diarylalkene (1r) also worked well to produce 2,3,4,5,6-
pentafluorobenzophenone with a decent yield, which is frequently used in the synthesis of
OLED[16] and F-containing polymers.[17] More importantly, heterocycle-substituted internal
alkenes, such as 2-styrylthiophene (1t) and 2-styrylpyridine (1s), are compatible with this catalytic
system, delivering their respective ketones in 71% and 77% yield, respectively. These results
indicate that this method is highly useful for the preparation of pharmaceuticals and functional
materials. Furthermore, aryl and alkyl-substituted unsymmetric alkenes (1u-z) could undergo the
oxidative cleavage reaction to selectively produce their corresponding ketones in good yields
under the optimized conditions. In addition, various substituted styrenes (1ab-ae) work well to
produce its corresponding aldehyde as the final product. For those 1,1-disubstituted terminal
alkenes, e.g., ethene-1,1,2-triyltribenzene (1af) , (E)-prop-1-ene-1,2-diyldibenzene (1ag), and
1,1,2-trisubstituted alkenes, e.g., ethene-1,1-diyldibenzene (1ah), prop-1-en-2-ylbenzene (1ai),
they were converted into their corresponding one-carbon shorter ketones and tertiary alcohols (2af-
ag) in satisfactory yields, respectively. However, for dialkyl substituted alkene, e.g., 2-octene
(1aa), a mixture of its corresponding epoxide, aldehyde and acid was observed with no formation
of the desired ketone, mainly due to the higher oxidation potential of aliphatic acids than benzyl
carboxylic acids, which is unfavourable for the oxidative decarboxylation of aliphatic acids to the
corresponding carbon radical and subsequent cascade oxidation process.[18]
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Table 2. Substrate scope of alkenes for synthesis of ketones.a
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O
Fe@NPC-800 (10 mol% Fe)
R2
R1
R1
R2
TBHP (5 eq.)
o
O
2
H2 , 90 C, 12h
1
7
8
O
O
O
O
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F
Cl
Br
2a 84% (86%)c
O
2b 81%
O
2c 87%
O
2d 80%
O
MeO
MeO
MeO
Cl
2e 81%
O
2f 74%
O
2g 91%
O
2h 86%
O
O2N
NC
F3C
CN
2k 85%
2i 57%
O
2j 75%
O
2l 83%
O
O
PhthN
H2N
HO2C
MeO2C
2n 55%
2o 80%
O
2m 82%
O
2p 85%
O
O
F
F
N
S
F
F
F
2t 77%
O
2r 65%
2s 71%
2q 90%
O
O
O
MeO
NC
2x 84%
2w 86%
2v 74%
O
2u 81%
O
Styrenes
O
R
2ab R = 4-Br 70%
2ac R = 4-OMe 87%
2ad R = 3-CN 81%
2ae R = 2-Cl 77%
2aa NDb
2z 84%
2y 80%
1,1-Disubstituted termianl alkenes
O
1,1,2-trisubstituted alkenes
OH
O
Ph
OH
2u 87%
2a 81%
2af 71%
2ag 67%
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aReaction conditions: alkene (0.2 mmol), catalyst (10 mol% of Fe), TBHP 70 wt% in water (5 eq.),
H2O (4 mL), 90 oC, 12 h. bNo detected. ccis-stilbene was used instead. Yields of isolated product
are reported.
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3.2 Oxidation of Alkenes to 1,2-Diketones. In the course of the condition optimization for
the reaction of trans-stilbene 1a catalyzed by Fe@NPC-800, benzil (5a) was detected, albeit with
considerably lower yield (Table 1, entries 2 and 3). This finding indicates that 1,2-diketones could
be synthesized from alkenes if appropriate conditions are in place. Therefore, we attempted to
synthesize 1,2-diketones from alkenes under the catalysis of Fe@NPC-800 by modification of the
reaction conditions.
With 1a still as the model substrate, a set of parameters including solvent, additive,
temperature, and oxidant were intensively screened, and the results are compiled in Table S6 and
Figure S11. To our pleasing, in the presence of 30 mol% of TBAI, 83% of isolated yield to benzil
5a was achieved using Fe@NPC-800 (10 mol% of Fe) as a catalyst, 5 equivalents of TBHP as an
o
oxidant (with respect to 1a), CH3CN/H2O (15/1, v/v) as solvent at 90 C. Note that TBAI was
employed as an additive because of the well-known capacity of iodide as a good nucleophilic and
leaving group.[ 3e,3f,19]
With the optimized conditions in hand, we subsequently explored the generality of this
method to selectively convert various alkenes to into 1,2-diketones. As shown in Table 3, all the
stilbenes examined could be efficiently converted to their corresponding 1,2-diketones in high
yields. A variety of functional groups, such as halides (1b-d), Me- (1e), MeO- (1g and 1h), -CN(1j
and 1k), CF3- (1l), and NO2- (1i), MeO2C- (1p) are all tolerated. 2,3,4,5,6-pentafluoro-substituted
1,2-diarylalkene (1r) also worked well to produce 5m in satisfactory yield. (E)-2-
styrylnaphthalene (1q) and heteroatom-containing alkene, 2-styrylthiophene (1t), were efficiently
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converted to their corresponding 1,2-diketones in high yields. Notably, 1-((E)-2-(cyclohexa-1,5-
dien-1-yl)vinyl)-4-((E)-styryl)benzene (1o) containing two C=C bonds was smoothly oxidized
into 5o in 74% yield with 10 equivalent of TBHP and 24 h. Aryl and alkyl-substituted unsymmetric
alkenes (1q’, 1r’, 1u, 1x-z) also worked well to deliver their respective 1,2-diketones in 57-81%
yields. More surprisingly, styrene (1w’) was directly oxidized into phenylglyoxylic acid (PGA)
5w in 57% yield, a key building block for wide applications in synthesis of ɑ-amino acids,
pharmaceuticals, and agrochemicals,[20] which still remains a challenge in organic synthesis from
readily available starting materials under facile and sustainable conditions. 1,1-disubstituted
alkenes (1x’ and 1y’) could be converted into the corresponding α-hydroxyl-acid (5x and 5y) in
51% and 45% yields, respectively. Likewise, cycloalkenes and aliphatic alkenes, such as,
cyclohexene (1z’), and 2-octene (1aa’), are also suitable as substrates for the oxidation to 1,2-
diketones. For the terminal aliphatic alkene, vinylcyclohexane (1ab’), was also oxidized into ɑ-
keto acid 5ab in decent yield. Note that, 2-hydroxycyclohex-2-en-1-one (5z) was obtained via the
isomerization of cyclohexane-1,2-dione. The α-hydroxyl-ketones (5ac and 5ad) were obtained in
moderate yields when using 1,1,2-trisunstituted alkenes (1ac’ and 1ad’) as substrates instead. In
addition, several biologically active natural derivatives containing C=C bond, e.g., Camphor
(1ae’), Resveratrol (1af’), Pterostilbene (1ag’), and Polydatin (1ah’), can be efficiently oxidized
into their respective 1,2-diketones in high yields. Overall, this method demonstrates a great
substrate scope, highlighting its practical utility for direct synthesis of benzils from alkenes under
mild and sustainable reaction conditions.
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Table 3. Substrate scopes of alkenes for synthesis of 1,2-diketones.
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Fe@NPC-800 (10 mol% Fe)
TBHP (5 eq.)
O
R2
R2
R1
R1
TBAI (30 mol%), 90oC, 12 h
O
O
CH3CN/H2 (15/1, v/v)
5
1
7
8
1,2-diaryl-alkenes
O
9
O
O
O
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O
O
O
O
5a 83%
O
Cl
F
Br
5d 82%
5b 81%
5c 87%
O
O
O
MeO
MeO
O
O
O
O
MeO
CN
5g 72%
5e 77%
5f 81%
5h 80%
O
O
O
O
O
O
O
O
MeO2C
F3C
O2N
NC
5l 82%
5k 53%
O
5i 77%
5j 74%
F
O
O
O
F
F
O
O
O
O
O
F
S
O
F
5o 74%b
5p 77%
5n 85%
5m 70%
1-aryl-2-alkyl-alkenes
O
O
O
O
O
O
5q 81%
O
O
O
N
5s 67%
5t 62%
5r 68%
terminal alkenes
O
1,1-disubstituted alkenes
CO2H
O
CO2H
Ph
HO
HO
OH
Me
O
O
O
5v 72%
5y 45%
5w 57%
5x 51%
5u 75%
1,2-dialkyl alkenes
terminal alkenes
1,1,2-trisubstituted alkenes
O
O
c
O
O
Ph
Me
O
Ph
Ph
OH
O
OH
O
OH
5ad 52%
O
O
OH
5aa 57%
5ac 43%
5ab 53%
5z 51%
OAc
nature product derivates
OAc
O
OAc
O
O
H
MeO
O
AcO
O
O
AcO
OAc
OAc
O
AcO
O
OMe
O
OAc
OAc
from Camphor
from Resveratrol
from Pterostilbene
from Polydatin
5ae 62%
5af 84%
5ag 87%
5ah 81%
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aReaction conditions: alkene (0.2 mmol), Fe @NPC-800 (10 mol% of Fe), TBHP 70 wt% in water
(5 eq.), TBAI (22.1 mg, 30 mol%), CH3CN/H2O (2 mL, v/v = 15/1), 90 oC, 12 h. bTBHP 70 wt%
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c
in water (10 eq.), 24 h. Isomerization product was obtained. Yields of isolated product are
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reported.
To further explore the applicability of this method, we carried out a one-pot cascade reaction
starting from alkenes for synthesis of biologically active heterocyclic compounds, such as 2,3-
diphenylquinoxaline (6a) and 2,4,5-triphenyl-1H-imidazole (6b) (Scheme 2), because 1,2-
diketones are well-known building blocks for such constructions.[21] After the full conversion of
trans-stilbene under the optimized conditions, benzene-1,2-diamine or benzyl aldehyde/NH4OAc
was directly added into the reaction mixture for sequential condensation cyclization. To our
delight, 6a and 6b were isolated in 87 and 67% yields, respectively. Comparatively, a considerable
lower reactivity was achieved when the condensation was performed directly starting from benzil
5a in the absence of the catalyst Fe@NPC-800 under otherwise identical conditions (Scheme S2).
Such a finding is in line with previous reports [21c-f] and clearly indicates that the catalyst Fe@NPC-
800 with Lewis acid sites could greatly facilitate the condensation, further highlighting its
expanded potential.
NH2
N
Ph
Ph
(1 eq.)
NH2
N
6a
50oC, 5 h
Fe@NPC-800 (10 mol% Fe)
TBHP (5 eq.)
O
100% conversion
87% yield
Ph
Ph
Ph
TBAI (30 mol%), 90oC, 12 h
CH3CN/H2O (15/1, v/v)
Ph
O
Ph
N
5a
(1 eq.)
Ph
O
Ph
N
3 eq. NH4OAc,
100oC, 8 h
Ph
H
6b
100% conversion
67% yield
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Scheme 2. Applicability of one-pot cascade reaction for synthesis of heterocycles starting from
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alkene.
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3.3 Stability of the Catalyst Fe@NPC-800. The stability and reusability of the catalyst
Fe@NPC-800 was also investigated. Upon completion of the benchmark oxidation reaction, the
Fe@NPC-800 catalyst was easily separated from the product and recollected via centrifugation,
and after washing and drying, it was used in subsequent cycles. As shown in Figure S13 in the
Supporting Information, the catalyst exhibited good stability and could be easily separated and
reused at least 6 times without significant loss of both the activity and selectivity, strongly
indicating its robust stability.
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3.4 Relationship between Catalytic Performance and Properties of the Catalyst. To
better understand the reaction pathway of the oxidative cleavage of alkenes into a class of
important ketones with one-carbon shorter, the benchmark reaction under the optimized conditions
was further investigated. The product distribution as a function of reaction time as shown in Figure
3 discloses that the epoxidation of trans-stilbene 1a with TBHP to produce epoxide 3a dominates
the reaction at the early stage in the presence of Fe@NPC-800 (within 1 h), accompanied with
concomitant formation of diphenylacetaldehyde (7a) and benzophenone (2a). This may not be
surprising because the Fe-Nx sites are key species in Fe@NPC-800, which structurally mimics Fe-
porphyrin complexes and have been well-documented to efficiently enable alkene epoxidation.[22]
With an elapse of reaction times, 1a was consumed quickly, and the formation of benzophenone
2a prevailed accompanied with the gradual conversion of both 3a and 7a. We subsequently
correlated the content of Fe-Nx sites in the as-prepared catalysts Fe@NPC-T (T = 700, 800, and
900oC) and the catalytic activities as shown in Table S8, and found that the conversion of trans-
stilbene 1a highly depends on the content of Fe-Nx sites under otherwise identical conditions,
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indicating that Fe-Nx is indeed catalytically active sites for the oxidation of alkenes in the present
catalytic system.
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Oxidation -
Decarboxylation
Meinwald
Rearrangement
CHO
O
Epoxidation
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
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1a
7a
2a
3a
Figure 3. Product distribution of oxidation of trans-stilbene as a function of reaction times under
the optimized conditions.
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Figure 4. Pyridine-adsorption FT-IR spectra on the catalyst Fe@NPC-800 observed at varying
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temperatures.
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Interestingly, diphenylacetaldehyde 7a as an intermediate was detected during the reaction
course. This is reminiscent of the well-known Meinwald rearrangement of epoxide in the presence
of acid.[5] As such, pyridine-adsorption FT-IR experiment for the catalyst Fe@NPC-800 was
conducted. As shown in Figure 4, the spectrum discloses the presence of Lewis acid sites on the
catalyst, with appearance of a characteristic peak at 1450 cm-1 and its concentration as high as
267.6 μmol per gram (Table S5), which most likely stem from the high valence of iron in FePO4.
Pyridine-poisoning experiment (Table S7) shows a considerable decrease of reactivity with 52%
conversion of 1a, with maintaining 3a remaining as the major product due to the loss of the
concentration of acid sites on the catalyst. Investigation on the catalytic activity and product
selectivity (or distribution) over the catalysts Fe@NPC-700 and Fe@NPC-900 with different
concentration of Lewis acid sites also gave similar results (Table 1, Table S8, and Figure S9), that
is, higher conversion of 1a with higher content of Fe-Nx sites, while better selectivity to 2a with
higher concentration of acid sites. This finding not only supports the presence of acid sites in the
catalyst; but more importantly it indicates that the acid sites exert a critical effect on reaction
efficiency and product selectivity. For comparison, the catalyst Fe@NC-800 without introduction
of P was prepared according to the same preparation method, which gave a relatively lower
reactivity, and especially an inferior selectivity to 2a under otherwise identical reaction conditions
(Figure S9). The pyridine-adsorption FT-IR experiment also discloses that the Fe@NC-800
catalyst processes a relatively stronger concentration of Lewis acid sites (Figure S8 and Table S5).
Such observations confirm that the addition of PPh3 could facilitate the formation of FePO4, giving
rise to the Lewis acid sites. Further studies show that the reaction activity and selectivity to
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benzophenone follows a volcanic trend when varying the added amount of PPh3 for catalyst
preparation, with the 20 wt% loading giving the best catalytic activity (Figure S10).
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[O]
O
OH
R2
H2O
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R1
R2
R1
R2
R1
R2
R1
CO2
Decarboxylation
[O]
[Fe]
R2
[Fe]
R2
O
O
CHO
R2
COOH
R2
R1
R1
R1
R1
Meinwald
Rearrangement
Path I
Epoxidation of Olefins
R2
R2
O
tBuOOH
R1
R1
R2
O
[Fe]
R1
O
[Fe]
[Fe]
O
Path II
R2
R1
Nucleophilic
Substitution
OH
[O]
[Fe]
R1
H2O
R2
[Fe]
O
O
O
[Fe], [O]
R2
R2
R1
R1
I
I
O
Scheme 3. Proposed mechanism for oxidation of alkenes to ketones and 1,2-diketones.
3.5 Mechanistic Studies. Based on the results discussed above, we proposed a plausible
reaction pathway as shown in Scheme 3. Firstly, an alkene is epoxidized by the Fe@NPC-800
catalyst with TBHP to generate an epoxide. Subsequently, the epoxide is converted to ketone with
one-carbon shorter, in which sequential reactions take place, involving (i) a Lewis acid-catalyzed
Meinwald rearrangement of the epoxide to an ɑ-disubstituted acetaldehyde, and (ii) oxidation of
the aldehyde to a disubstituted acetic acid, (iii) decarboxylation of the acid with loss of one carbon
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atom followed by one-electron oxidation of the resulting radical, and (iv) attach at the carbon
cation by water as nucleophile and further oxidation to produce a ketone.
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To gain more evidence for this mechanism, a set of control experiments were subsequently
carried out (Scheme 4). Stilbene epoxide 3a could be efficiently converted towards benzophenone
2a in 92% GC yield (84% isolated yield) under the optimized conditions (Scheme 4 eq. a); but in
the absence of TBHP under otherwise identical conditions, only 53% conversion of 3a was
achieved, affording Meinwald rearranged product diphenylacetaldehyde 7a in 49% GC yield
(Scheme 4 eq. b). This observation indicates that TBHP as oxidant could accelerate selective
conversion of epoxide to the desired ketones. Next, diphenylacetic acid 6a, which is readily formed
from oxidation of diphenylacetaldehyde 7a, could also be effectively transferred into 2a in 89%
GC yield together with 11% of 1,2-diphenylmethanol and evolution of a CO2 molecule (confirmed
by GC analysis) (Scheme 4 eq. c). As is known, diphenylacetic acid can easily undergo oxidative
decarboxylation into benzyl radical with release of CO2.[18] In the presence of an oxidant, the
generated benzyl radical can be oxidized to carbocation, which can be easily attacked by a
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nucleophile like H2O to form an alcohol and further oxidized into ketone. To prove this, H2 O
labelling experiment was performed (Scheme 4 eq. d and Figure S12). No appreciable difference
in reactivity could be observed, and 96% of isolated yield of benzophenone was obtained with
100% 18O labeling. Besides, 4% of diphenylmethanol with 100% 18O labeling was also detected.
This finding not only demonstrates that the origin of oxygen atom in benzophenone and 1,2-
diphenylmethanol stems from water rather than TBHP, but further proves that water indeed
participates in the reaction as a nucleophile to attack the in-situ generated decarboxylated
intermediate (Scheme 3). Taking into account all these control experimental results, we may safely
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conclude that the proposed reaction pathway is most likely to be operative in our catalytic system
(Scheme 3, path I).
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O
O
Fe/NPC-800 (10 mol% Fe)
Ph
a)
Ph
+
Ph
Ph
Ph
H
O
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TBHP (5 eq.)
H2O, 90oC, 12 h
2a
3a
92% GC yield 8% GC yield
100% Conv.
CHO
O
Fe/NPC-800 (10 mol% Fe)
Ph
b)
c)
+
Ph
Ph
Ph
Ph
H
O
3a
H2O, 90oC, 12 h
Without TBHP
7a
49% GC yield
4% GC yield
53% Conv.
COOH
O
OH
Fe/NPC-800 (10 mol% Fe)
+
Ph
Ph
Ph
Ph
Ph
Ph
11% GC yield
O18H
TBHP (5 eq.)
H2O, 90oC, 12 h
2a
6a
100% Conv.
89% GC yield
O18
Fe/NPC-800 (10 mol% Fe)
d)
Ph
Ph
Ph
Ph
Ph
Ph
1a
TBHP (5 eq.)
H2O18, 90oC, 12 h
96% GC yield
4% GC yield
100% Conv.
Scheme 4. Control experiments for the synthesis of ketones.
On the other hand, a competitive pathway of nucleophilic substitution prior to the Meinwald
rearrangment is also possible if a strong nucleophile presents during the ring-opening of epoxide
catalyzed by Lewis acid. If this is the case, the reaction would proceed following the pathway as
shown in Scheme 3, path II. To investigate the possibility, new control experiments were
conducted (Scheme 5). Firstly, using 2-iodo-1,2-diphenylethan-1-one A as the substrate for the
reaction under the optimized conditions (Scheme 5 eq. a), nearly quantitative yield to benzil 5a
was observed. The iodide in A, as a good leaving group, can be readily substituted by water to
produce 2-hydroxy-1,2-diphenylethan-1-one B in CH3CN/H2O mixture at 90oC (Scheme 5 eq. b),
which can be quantitatively converted into 5a under the optimized conditions (Scheme 5 eq. c).
In contrast, a considerably lower reactivity was observed when A was subjected to the optimized
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conditions in the absence of water (Scheme 5 eq. d), indicating that H2O as a nucleophile is
favorable for the reaction process. Taken all the results together, we consider that the proposed
reaction pathway as illustrated in Scheme 3, path II is feasible.
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O
O
Fe/NPC-800 (10 mol% Fe)
Ph
Ph
a)
b)
c)
Ph
Ph
TBHP (5 eq.)
CH3CN/H2O (15/1, v/v)
90oC, 12 h
O
I
A
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100% Conv.
97% GC yield
O
O
CH3CN/H2O (15/1, v/v)
90oC, 12 h
Ph
Ph
Ph
Ph
OH
I
B
A
100% Conv.
87% GC yield
O
O
Fe/NPC-800 (10 mol% Fe)
Ph
Ph
Ph
Ph
O
TBHP (5 eq.)
CH3CN/H2O (15/1, v/v)
90oC, 12 h
OH
B
5a
100% Conv.
100% GC yield
O
O
Fe/NPC-800 (10 mol% Fe)
d)
Ph
Ph
Ph
Ph
Anhydrous TBHP (5 eq.)
CH3CN
I
O
5a
21% GC yield
A
90oC, 12 h
37% Conv.
Scheme 5. Control experiments for the synthesis of 1,2-diketones.
4. Conclusions
In conclusion, we have developed a novel, inexpensive bifunctional iron nanocomposite
catalyst in a facile and environmentally friendly manner. Two key iron species, Fe-Nx and FePO4
as the oxidation and acid sites, respectively, were simultaneously integrated on the hierarchical
porous carbon, which are primarily responsible for the direct oxidative cleavage of alkenes via the
formation of epoxide as the key intermediate. A broad range of alkenes could be efficiently
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converted into industrially important ketones and 1,2-diketones with good tolerance of functional
groups using TBHP as the oxidant under mild reaction conditions. In addition, the catalyst can be
readily recovered for successive recycles. This study provides a simple and efficient method for
the synthesis of synthetically significant ketones and 1,2-diketones from the direct oxidation of
alkenes in a cost-effective and environmentally friendly manner, and we believe the method should
be very useful in the synthesis of fine chemicals, pharmaceuticals, agrochemicals and materials.
AUTHOR INFORMATION
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Corresponding Author
*E-mail: yangyong@qibebt.ac.cn
*E-mail: j.xiao@liverpool.ac.uk
ORCID
Yong Yang: 0000-0002-6118-5200
Jianliang Xiao: 0000-0003-2010-247X
Tao Song: 0000-0003-2088-8177
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
The Supporting Information is available free of charge at https://pubs.acs.org/doi/xx.xxxx/xxxxxx.
Raman spectra; N2 sorption isotherms and pore size distribution; XRD pattern; Ni 2p, P 2p, N 1s,
and C 1s XPS spectra for Fe@NPC-700, Fe @NPC-800, and Fe@NPC-900; chemical composition
1
13
and texture properties of Fe@NPC-700, Fe @NPC-800, and Fe@NPC-900. H and C NMR
spectroscopy data for all obtained products.
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ACKNOWLEDGMENT
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The authors would like to acknowledge the financial support from the Key Technology R&D
Program of Shandong Province (2019GGX102075), DICP & QIBEBT (Grant No. DICP &
QIBEBT UN201704) and the Dalian National Laboratory for Clean Energy (DNL) Cooperation
Fund, CAS (Grant No. DNL201904), Open Projects of State Key Laboratory of Physical
Chemistry of the Solid State Surface (Xiamen University) (No. 201808). Y.Y also acknowledges
the Royal Society (UK) for a Newton Advanced Fellowship (NAF\R2\180695)
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