A metal-free heterogeneous photocatalyst for the selective oxidative cleavage of CC bonds in aryl olefins: via harvesting direct solar energy

Article abstract of DOI:10.1039/d0gc01187h

Selective cleavage of CC bonds is highly important for the synthesis of carbonyl containing fine chemicals and pharmaceuticals. Novel methodologies such as ozonolysis reactions, Lemieux-Johnson oxidation reaction etc. already exist. Parallel to these, catalytic methods using homogeneous catalysts also have been discovered. Considering the various advantages of heterogeneous catalysts such as recyclability and stability, couple of transition metal-based heterogeneous catalysts have been applied for this reaction. However, the pharmaceutical industries prefer to use metal-free catalysts (especially transition metal-free) to avoid further leaching in the final products. This is for sure a big challenge to an organic chemist and to the pharmaceutical industries. To make this feasible, a mild and efficient protocol has been developed using polymeric carbon nitrides (PCN) as metal-free heterogeneous photocatalysts to convert various olefins into the corresponding carbonyls. Later, this catalyst has been applied in the gram scale synthesis of pharmaceutical drugs using direct solar energy. Detailed mechanistic studies revealed the actual role of oxygen, the catalyst, and the light source.

Full text of DOI:10.1039/d0gc01187h

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DOI: 10.1039/D0GC01187H
ARTICLE
A Metal-Free Heterogeneous Photocatalyst for the Selective
Oxidative Cleavage of C=C Bonds in Aryl olefins via Harvesting
Direct Solar Energy
Received 00th January 20xx,
Accepted 00th January 20xx
Yu Zhanga, b, Niklas Simon Langeb, Nareh Hatamib, Emanuel Rongec, Waldemar Schillingb, Christian
Joossc, Shoubhik Dasa*
DOI: 10.1039/x0xx00000x
Selective cleavage of C=C bonds is highly important for the synthesis of carbonyl containing fine chemicals and
pharmaceuticals. Novel methodologies such as ozonolysis reactions, Lemieux–Johnson oxidation reaction etc. already exist.
Parallel to these, catalytic methods using homogeneous catalysts also have been discovered. However, on the verge of
recyclability, heterogeneous catalysts are in the forefront and couple of transition metal-based heterogeneous catalysts
already exist in the market. However, pharmaceutical industries prefer to use metal-free catalysts (especially transition
metal-free) to avoid further leaching in the final products. This is for sure a big challenge to an organic chemist and to the
pharmaceutical industries! To make it feasible, a mild and an efficient protocol has been developed using polymeric carbon
nitrides (PCN) as the metal-free heterogeneous photocatalyst to convert various olefins into the corresponding carbonyls.
Later, this catalyst has been applied in the g-scale synthesis of pharmaceutical drug using direct solar energy. The detailed
mechanistic studies revealed the actual role of oxygen, catalyst, and the light source.
Figure 1 Methodologies for the oxidative cleavage of olefins.
Introduction
Oxidative cleavage of olefins is highly attractive since it
generates high-valued oxygen-containing functional groups
from the cheap and widely available starting materials
(olefins).1-4 However, the most popular protocols for the C=C
bond cleavages are still the old-fashioned ozonolysis reaction
and Lemieux–Johnson oxidation reaction, which require either
an ozone generator or involve toxic and expensive reagents
such as O3, osmium tetroxide etc. (Figure 1).5-6 To replace these,
cheaper and safer oxidants such as KMnO4, PhIO/HBF4 etc. have
been developed. However, generating over stoichiometric
amounts of by-products is pushing them far away from the
installation of atom economy in organic synthesis.7-9
Ozonolysis,
metal/non-metal oxidants
O
Further
reduction
O
O
O3
O
O
Ozone
generator
KMnO4, PhIO/HBF4, OsO4
Limitations: Poor atom-economy, toxic oxidants, poor selectivity
(References: 7-9)
Molecular oxygen
as the oxidant
Fe-based homogeneous catalysts
Disulfide based photocatalyst
O
O
Electrochemistry using Pt electrodes
Heterogeneous catalysts
Limitations: Expensive ligands, poor substrates scope, expensive
electrodes, poor recyclability and selectivity of the catalysts.
(References: 11-14 & 16)
Compared to the over-stoichiometric oxidants, molecular
oxygen can be considered as a clean, cheap oxidant and will be
ideal for the oxidative cleavage of olefins.10-14 For example, Xiao
and co-workers developed an excellent method for the
selective cleavage using iron catalyst and bisimidazoline ligand
under O2 atmosphere.15 Later, Feng group have developed a
This work: metal-free
heterogeneous catalyst
O
O
O
O
O
O
O2 (Balloon)
g-C3N4 as a photocatalyst
NHS (20 mol%)
O
H2C
commercially available iron catalyst for this reaction.16
A
Acetonitrile, sunlight
g-scale, 66%
Fenofibrate
Treatment for
abnormal lipid level
Cl
binuclear copper complex catalysed system was also described
by the Group of Wang which generated the carbonyl
Cl
H
O
H
O
O
O
O
Detected
Cl
aDepartment of Chemistry, Campus Groenenborger, Groenenborgerlaan 171, 2020
Antwerpen, Belgium
Harvesting solar energy
g-Scale reactions
Metal-free heterogeneous catalyst
Cleavage of complex molecules
bInstitut für Biomolekulare und Organische Chemie, Georg-August-Universität
Göttingen, Tammannstraße 2, 37077 Göttingen, Germany
cInstitut für Materialphysik, Georg-August-Universität Göttingen, Friedrich-Hund-
Platz 1, 37077 Göttingen, Germany
compounds from terminal alkenes.17 The first photocatalytic
system was reported by the group of Inoue using
dimethoxybenzenes as a
Electronic Supplementary Information (ESI) available: experimental procedures and
details, mechanistic and optimization results and NMR spectra. See
DOI: 10.1039/x0xx00000x
sensitizer.18 Fukuzumi et al. further developed this reaction
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using 10-methylacridinium ion.19 Furthermore, various
developed an excellent cleavage protocol using homogeneous
photocatalysts (electron-rich aromatic disulfide, Eosin Y) and
catalytic amount of iodine were displayed, respectively.20 Not
only by photocatalysis, Chiba and co-workers developed an
excellent electrochemical pathway but it required expensive
platinum electrodes to achieve the target conversion.21
ascorbic acid (AA) and 3-mercaptopropionic acidVi(eMw APrAtic)lewOnelirnee
investigated (entries 5–7). Among them, addition of NHS
DOI: 10.1039/D0GC01187H
increased the formation of the desired product to 61%. Finally,
addition of 20 mol% of NHS, increased the yield to 80% of 1b.
However, further loading of NHS did not perform any further
improvement (entries 8–9). Trace amount of product formed
without the catalyst (entries 10). Furthermore, investigation of
different solvents also did not improve the reaction yield since
acetonitrile has higher oxygen solubility than others (entries
11–13).45
Compared to the homogeneous catalysts,
a selective
heterogeneous catalyst is in high demand due to their post-
reaction stability and recyclability.22 Recently, Cho et al.
reported
a heterogeneous method using BiVO4 as a
Table
1 Optimization of the C=C bond cleavage using a
photocatalyst.23 In fact, iron-based heterogeneous catalysts
have been also developed, however, they required high O2
pressure, exhibited limited substrates scope and above all, lack
of selectivity in the product distribution.24-25 So far, only heavy
metal based catalytic systems have been discovered. However,
pharmaceutical industries prefer to avoid heavy metal based
systems to avoid leaching in the final products which could be
detrimental for the final use.
heterogeneous photocatalyst.a-e
O
O2 (balloon)
PCN
additive
CH3CN (1 mL)
blue LED
F
F
1a
1b
We were particularly interested for a metal-free heterogeneous
photocatalyst which is currently an interesting avenue in
organic synthesis.26-28 For this purpose, we focused on the
polymeric carbon nitrides (PCN) which is easy to synthesize and
even commercially available. This catalyst has also exhibited
wide applications ranging from water splitting to CO2 reductions
and others.29-34 Advantageously, PCN have band gap of 2.7 eV
(enough to harvest solar energy), where valence band (VB)
resides at +1.6 V and conduction band (CB) at -1.10 V.35 This
high CB should be sufficient to generate reactive oxygen species
from oxygen (superoxide radical anion (O2/O2•-) has the
reduction potential of -0.56 V vs SCE) and high positive VB
position can be utilized for the oxidation of aryl olefins.36-38
Additionally, development of solar energy mediated organic
transformation is also attractive for the augmentation of
sustainable approach in the organic synthesis.39-40 Indeed,
sunlight is renewable and available throughout the entire
world. The average intensity of the total solar irradiance is
about 1366.1 W m-2 which provides roughly 4.3x1020 J energy
only in 1 h.41 Therefore, if it is harvested properly and is utilized
in the organic synthesis, can solve many of the sustainable
issues.42-43 Owing to the different absorption of organic
molecules and also non-uniform irradiation of sunlight, it can be
challenging to adopt into the organic synthesis.39-40 However, if
solar energy mediated organic synthesis can be applied via
storage of solar energy, it can solve many of the energy
intensive synthetic procedures.42-43 Based on all the above
Entry
1
Cat. Amount
3 mg
additive
No
Solvent
CH3CN
Yield [%]
18
No
No
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
DMF
31
40
43
0
5 mg
8 mg
12 mg
8 mg
8 mg
8 mg
8 mg
0 mg
8 mg
8 mg
8 mg
8 mg
2
3
No
4
AA
5
MPA
NHSc
NHSd
NHS
NHSe
NHSd
NHSd
NHSd
6
6
61
80
5
7
8
9
79
46
30
48
10
11
12
13
DMA
information, herein we report an efficient, metal-free and [a] General reaction conditions: O2 balloon, 12 W blue LED, 0.25 mmol of 1a,
recyclable heterogeneous photocatalyst which harvested solar PCN, 1 mL of acetonitrile, additive, room temperature, 24 h. [b] Yields were
energy to cleave the C=C bonds.44
determined by GC calibration using dodecane as internal standard. [c] N-
hydroxysuccinimide (10 mol%). [d] N-hydroxysuccinimide (20 mol%). [e] N-
hydroxysuccinimide (30 mol%).
Results and discussion
At the outset of our reaction, 4-fluoro--methylstyrene (1a)
With these optimized reaction conditions in hand, the scope of
the photocatalytic system to other a-substituted styrenes was
extended (Scheme 1; entries 2b–17b). To our delight, other
was chosen as a model substrate to optimize the reaction
conditions (Table 1). At first, different amount of PCN were
applied using acetonitrile as the solvent under the irradiation of
12 W blue LED for 24 h. Gratifyingly, 43% of 4-
fluoroacetophenone (1b) was achieved using 8 mg of PCN
(entries 1–4). However, the increasing amount of catalyst did
not improve the yield of the reaction. Furthermore, different
additives (10 mol%) such as N-hydroxysuccinimide (NHS),
substituted
-methylstyrenes
such
as
4-chloro--
methylstyrene, 2-methoxy-a-methylstyrene and 4-methyl--
methylstyrene generated the corresponding carbonyl
compounds in excellent yields (Scheme 1; entries 2b–6b).
Furthermore, 1,1'-diphenylethylenes, substituted 1,1'-
2 | J. Name., 2012, 00, 1-3
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diphenylethylenes also worked excellently to afford the 3,4-dimethylstyrene,
ARTICLE
3,4-dimethoxystyrene,
2,4,5-
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corresponding benzophenone derivatives (Scheme 1; entries trimethoxystyrene and 4-vinylbiphenyDlOI:t1o0.10g3e9n/De0rGatCe0118th7He
7b–9b). Meanwhile, other a-substituted styrene derivatives corresponding aldehydes (Scheme 2; entries 18b–21b).
such as -n-propylstyrene, -n-butylstyrene and -iso-
butylstyrene etc. exhibited moderate to high yields of the
corresponding ketones (Scheme 1; entries 10b–14b). Further
investigations towards the hetero-aromatic ring substituted
styrenes were also carried out. Indeed, these substrates reacted
smoothly to form the 4-benzoylpyridine, 2-benzoylpyridine and
2-(Prop-1-en-2-yl)thiophene with excellent isolated yields
(Scheme 1; entries 15b–17b). It should be also noted that in
none of the cases corresponding epoxides, diols or further allylic
oxygenated products were observed and thereby
demonstrated high selectivity of the photocatalyst.
Scheme 2 Scope of the C=C bond cleavage of terminal styrenes.a-b
O2 (balloon)
PCN (8 mg)
NHS (20 mol%)
CH3CN (1 mL)
blue LED, 9-24h
O
R
R
Substrate
Product and yield
Substrate
Product and yield
O
O
18a
18b, 60%
20a
20b, 56%
O
O
O
O
Scheme 1 Scope of the C=C bond cleavage of a-substituted
styrenes.a-b
O
O
O
O
O
O
O
O
19a
19b, 52%
21a
21b, 41%
O2 (balloon)
PCN (8 mg)
NHS (20 mol%)
O
[a] O2 balloon, 12 W blue LED, 0.25 mmol of starting material, 8 mg of PCN, 1
mL of acetonitrile, 20 mol% of N-hydroxysuccinimide, room temperature. [b]
isolated yields.
R2
R2
R1
R1
CH3CN (1 mL)
blue LED, 24-40 h
Substrate
Product and yield
Substrate
Product and yield
O
O
Scheme 3 Scope of the C=C bond cleavage of -substituted styrenes
and -bromo styrenes.a-b
2a
10a
11a
12a
13a
2b, 78%
10b, 75%
O
O
O
air
Br
PCN (8 mg)
O
Br
Cl
Cl
R
or
R
or
R
R
CH3CN (1 mL)
blue LED, 9-16h
3a
3b, 90%
11b, 72%
O
O
Substrate
Product and yield
Substrate
Product and yield
Br
Br
4a
4b, 68%
12b, 61%
O
O
O
O
O
O
O
O
O
O
O
O
O
O
22a
22b, 62%
26a
26b, 42%
13b, 75%
O
5a
O
5b, 71%
O
O
Br
Br
23b, 65%
27a
27b, 70%
23a
6a
7a
8a
6b, 78%
O
14a
14b, 53%
O
O
Br
O
Br
O
O
O
O
S
S
15a
15b, 70%
7b, 90%
24a
24b, 52%
28a
29a
28b, 64%
O
O
O
O
Cl
Cl
Br
Br
N
HO
HO
O
N
O
O
O
O
O
8b, 75%
16a
16b, 71%
25a
25b, 39%
29b, 61%
O
O
[a] Reaction conditions: air, 12 W blue LED, 0.25 mmol of starting material, 8
mg of PCN, 1 mL of acetonitrile. [b] isolated yields.
N
N
17a
17b, 75%
9b, 81%
9a
[a] O2 balloon, 12 W blue LED, 0.25 mmol of starting material, 8 mg of PCN, 1
mL of acetonitrile, 20 mol% of N-hydroxysuccinimide, room temperature. [b]
isolated yields.
Inspired by the excellent reactivity, we were interested to
expand the substrates scope only using the heterogeneous
photocatalyst (Scheme 3; entries 22b–26b). Besides the model
substrate (1a) (see Table 1, entry 3), anethole, trans--
methylstyrene, etc. also exhibited moderate activities even
under air (22b–26b). To our surprise, -bromostyrene
derivatives underwent the migration of the bromide to the β-
carbon and formed the phenacyl bromide derivatives (27b-
29b).15 It should be noted that some of the complex molecules
such as triacetylresveratrol (Scheme 4, 30a),47 trans-
Next, we became interested to apply the photocatalytic system
to mono-substituted olefins since they could afford high-valued
aldehydes. For example, veratraldehyde (21b) is
a key
intermediate for the synthesis of many of the pharmaceuticals
such as verazide, amiquinsin, hoqquizil, and so on.46 To our
delight, this heterogeneous photocatalyst exhibited medium to
good reactivity towards the mono-substituted styrenes such as
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chlorprothixene (31a),48 are highly challenging molecules to molecules generated the corresponding oxygenated products in
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DOI: 10.1039/D0GC01187H
cleave under the normal circumstances. Notably, with this high yields.
robust photocatalyst, selective cleavage of these complex
Scheme 4 Scope of the C=C bond cleavage of complex molecules.a-b
O2
PCN (8 mg)
R3
R4
R3
R4
R1
R2
R1
R2
NHS (20 mol%)
O O
CH3CN (1 mL)
blue LED, 16-40 h
Substrate
Substrate
Product and yield
Product and yield
O
O
O
O
O
O
O
O
O
O
O
+
O
O
O
O
O
30a
30b-1, 57%
30b-2, 57%
O
N
O
O
33b, 57%
Cl
Cl
33a
S
S
31a
31b, 62%
O
N
N
O
O
32a
32b, 71%
34b, 68%
34a
[a] O2 balloon, 12 W blue LED, 0.25 mmol of starting materials, 8 mg of PCN, 1 mL of acetonitrile, 20 mol% of N-hydroxysuccinimide, room temperature. [b]
isolated yields.
Scheme 5 Direct solar energy mediated C=C bond cleavage in g-scale. infrared spectroscopy (FTIR), UV/Vis, scanning electron microscopy
(SEM), high resolution transmission electron microscopy (HRTEM)
were measured and compared the difference between the freshly
O
O2 (balloon)
PCN (80 mg)
NHS (20 mol%)
prepared catalyst and the recycled catalyst. Analysis of the catalyst
CH3CN (12 mL)
Sunlight, 17h
71%
68%
66%
5a, 1.0 g
before and after
a reaction using XRD showed the same
O2 (balloon)
PCN (50 mg)
NHS (20 mol%)
CH3CN (10 mL)
Sunlight, 17h
O2 (balloon)
PCN (50 mg)
NHS (20 mol%)
CH3CN (10 mL)
Sunlight, 25h
N
N
characteristic peaks at 27.3° and 13.2° which is also matched with
literature report (Figure S6).50-51 From the UV-Vis and FTIR, no
difference in the absorbance was observed. TEM and SEM revealed
the morphology of the PCN particles before and after the recycling.
They formed rounded plates with an average diameter of (30±10) nm
and a thickness of the order of a few angstrom. Due to the
agglomeration, bigger particles were composed by the stacks of the
plates. No significant changes in the morphology was observed with
TEM (Figure 2 [d–e]) and SEM (Figure S6) after recovering the
catalyst. To get more insight into the crystal structure, selected area
electron diffraction measurements were carried out. The diffuse
rings did not exhibit any sign of crystallinity. A more detailed analysis
of the intensity profile did not provide any evidence of a change
during catalysis. In addition, the HRTEM images demonstrated clearly
the amorphous nature of the catalyst.
O
O
32a, 1.0 g
O
O
O
O
O
Cl
O
Cl
O
35a, 1.0 g
Fenofibrate
As we mentioned in the introduction, natural sunlight is an
inexpensive and widely available energy source. Therefore, we
sought to harvest direct solar energy for the selective cleavage of C=C
bonds. Not only the small molecule (5a) was cleaved into the
corresponding ketone, but the complex molecule such as 32a, was
also cleaved selectively under the sunlight. These reactions were
easily scaled up to g-scale. Additionally, fenofibrate,
a
pharmaceutical drug used for the treatment of abnormal lipid levels,
was synthesized in g-scale from the corresponding olefin (Scheme 5;
entry 35b).49
To ensure the stability and reusability of the photocatalyst, PCN was
recycled multiple times under the optimized reaction conditions. In
fact, catalyst was recycled 5 times and can be reused further (Figure
S5). To further investigate the stability of PCN catalyst, elemental
analysis (Table S1), X-ray powder diffraction (XRD) fourier-transform
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Figure 2 Analysis of new and recovered PCN by FTIR [a], UV/Vis [b], intensity profile of the selected area electron diffraction pattern (SAED)
[c], brightfield TEM [d-e] as well as HRTEM with SAED as an inset [f-g].
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Inspired by the generality of the substrates scope, we sought to
gather mechanistic information about the role of the light, oxygen,
catalyst and additive in our reactions. According to the control
experiments, no product was observed in the absence of light,
photocatalyst and oxygen. Only trace amount of the product was
observed in the presence of the additive which clearly
demonstrated the necessity of the photocatalyst and oxygen in
the reaction (Table S2). Further, on/off light experiments were
also performed to prove that the yield of the product was
completely suppressed in the absence of light (Figure 3).
Table 2 Quenching experiments for the C=C bond cleavage.a-b
O2 (balloon)
PCN (8 mg)
O
NHS (20 mol%)
CH3CN (1 mL)
quencher
F
F
blue LED, 24 h
Quenchers
BHT
Equivalents
0.5
Yield [%]
Conclusions
radical
Furthermore, the effect of different quenchers was investigated
to figure out the reactive oxygen species and possible
intermediates in our reactions (Table 2).52 When 2,6-di-tert-butyl-
4-methylphenol (BHT) or 2,2,6,6-tetramethyl-1-piperidinyloxyl
(TEMPO) were added, the yield was dramatically decreased which
proved that the reaction was undergoing a radical pathway.
Further addition of benzoquinone in the reaction displayed the
presence of superoxide radical anion. Furthermore, addition of
CuCl2 and catalase to the reaction mixture showed lower yields
which proved that there could be involvement of single electron
processes and the presence of peroxide species in this reaction.
0
0
TEMPO
0.5
radical
tert-butanol
CuCl2
1.0
42
46
9
hydroxide radical
single electron
peroxide radical
1.0
Catalase
100 mg
Benzoquinone
Soduim azide
1.0
2.0
2
superoxide radical
singlet oxygen
43
[a] O2 balloon, 12 W blue LED, 0.25 mmol of starting material, 8 mg of PCN,
1
mL of acetonitrile, 20 mol% of N-hydroxysuccinimide, room
temperature, different quenchers. [b] Yields were determined by GC
analysis using n-dodecane as an internal standard.
Figure
3 On/off light experiments of 4-(1-Propenyl)-1,2-
dimethoxybenzene.a-b
To get further information about the reaction mechanism, Stern-
Volmer quenching experiments were carried out (Figure 4) which
is clear that the excited state of the photocatalyst was quenched
by the olefin and not by the oxygen or additive.52 To show the
actual source of oxygen in the product, reaction was investigated
under 18O-labelled oxygen atmosphere. To our delight, only 18O-
labelled product (37) was observed and isolated from the reaction.
Interestingly, we also detected the 18O-labelled intermediate (36)
in this reaction, and in this case both of the oxygen atoms in
dioxetane came from the oxygen atmosphere (Figure 5).
[a] O2 balloon, 12 W blue LED, 0.25 mmol of starting material, 8 mg of PCN,
Figure 4 Stern-Volmer plot for the different concentrations of
olefins, O2 and additive.
1
mL of acetonitrile, 20 mol% of N-hydroxysuccinimide, room
temperature. [b] Yields were determined by NMR using iodoform as an
internal standard.
20
Substrate 1a
Saturated O2
15
Additive
10
5
Linear (Substrate 1a)
Linear (Saturated O2)
Linear (Additive)
0
0
0.1
0.2
0.3
C (quencher) [M]
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Combing all the mechanistic information, mechanism of this
reaction was proposed (Figure 5). At first, visible-light irradiation
created charge separation in PCN, produced oxidative valence
band and reductive conduction-band.40 Then the conduction-band
electron reduced the oxygen to produce superoxide radicals. The
valence band hole oxidized 1a to the corresponding radical cation
1c. This radical cation 1c combined with the superoxide radical
anion and 1f was formed via [2+2] cycloaddition. However, there
was another pathway (pathway B) which involved the additive N-
hydroxysuccinimide (NHS). Firstly, 1h was generated through NHS
(1g) via hydrogen atom transfer (HAT).53 Meanwhile, the
hydrosuperoxide anion was generated. Afterwards, 1h attacked
1a to form the cation 1d which further reacted with
hydrosuperoxide anion to afford 1e which later formed the
intermediate 1f by releasing NHS. At last, the cleavage of
dioxetane 1f generated the final product (1b).
in bulk chemistry, for the syntheses of pharmaceuticals and
natural product.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank Fonds der Chemischen Industrie (FCI, Liebig-
Fellowship to S.D.) and Chinese Scholarship Council (CSC
funding to Y.Z.) for the financial support.
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TOC:
O2 (balloon)
g-C3N4 (photocatalyst)
NHS (20 mol%)
O
O
CH3CN, Sunlight
Wide substrates scope
Solar energy mediated
Synthesis of pharmaceuticals
Cleavage of complex molecules
Metal-free catalyst
Recyclable catalyst