Visible-Light-Irradiated Graphitic Carbon Nitride Photocatalyzed Diels–Alder Reactions with Dioxygen as Sustainable Mediator for Photoinduced Electrons

Article abstract of DOI:10.1002/anie.201703438

Photocatalytic Diels–Alder (D–A) reactions with electron rich olefins are realized by graphitic carbon nitride (g-C₃N₄) under visible-light irradiation and aerobic conditions. This heterogeneous photoredox reaction system is highly e

Full text of DOI:10.1002/anie.201703438

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Accepted Article
Title: Visible Light Irradiated Graphitic Carbon Nitride (g-C3N4)
Photocatalyzed Cation Radical Diels-Alder Reactions with
Dioxygen as Sustainable Mediator for Photoinduced Electrons
Authors: Yubao Zhao and Markus Antonietti
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703438
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10.1002/anie.201703438
Angewandte Chemie International Edition
COMMUNICATION
Visible Light Irradiated Graphitic Carbon Nitride (g-C3N4)
Photocatalyzed Cation Radical Diels-Alder Reactions with
Dioxygen as Sustainable Mediator for Photoinduced Electrons
Yubao Zhao* and Markus Antonietti
Abstract: Photocatalytic Diels-Alder (D-A) reaction with electron rich
olefins is realized by graphitic carbon nitride (g-C3N4) under visible
light irradiation and aerobic condition. This heterogeneous photo-
redox reaction system is highly efficient, and apparent quantum yield
reaches a remarkable value of 47 % for the model reaction.
Dioxygen plays a critical role as electron mediator, which is distinct
from the previous reports in the homogeneous Ru(II) complex photo-
redox system. Moreover, the reaction intermediate vinylcyclobutane
is captured and monitored during the reaction, serving as a direct
evidence for the proposed reaction mechanism. The cycloaddition
process is thereby determined to be the combination of direct [4 + 2]
cycloaddition and [2 + 2] cycloaddition followed by photocatalytic
rearrangement of the vinylcyclobutane intermediate.
initiating the radical chain reactions.[4] Recently, novel photo-
redox systems for D-A reaction were developed, and the cation
radical D-A reactions with electron-rich dienophiles were
described, realized by homogeneous photo-redox systems
catalyzed by Ru(II) complexes under visible light and Cr (III)
complexes under UV light irradiation.[5] Interestingly, the reaction
mechanism in Cr (III) complex system is distinct from that in
Ru(II) photo-redox system. The radical chain propagation is
dominating in the Ru(II) system, while the dioxygen mediated
photocatalytic process is favorable in Cr(III) complex system.[5a,c]
From the perspectives of both basic science and practical
applications, a heterogeneous photo-redox system with a low
cost visible light-driven photocatalyst is highly desirable. g-C3N4,
with band gap of 2.7 eV and valence band level of 1.4 V vs.
NHE at pH = 7,[6] is a promising candidate to achieve the goal of
cation radical D-A reaction with electron rich dienophiles
employing visible light as energy input.
Photo-redox catalysis, which could potentially employ the
renewable solar energy to realize diverse chemicals conversions,
has been resurged in the recent decade, not only because of the
current global energy and environmental crisis, but also due to
the attractive novel reaction mechanisms enriching the
potentialities in organic synthesis.[1] Visible light accounts for
44% (versus 3% for UV light) of the solar spectrum, setting the
motivation for the development of the visible light driven
photocatalytic systems for synthesis of value-added or complex
molecules.[2] Additionally, UV light harms many sensitive
molecules, and visible light photocatalysis promises wider
applicability for realizing increasingly ambitious photo-redox
reactions.[2a]
Diels-Alder (D-A) reactions are an exceptionally versatile tool
in organic synthesis for C-C bond formation.[3] According to the
Frontier Molecular Orbital (FMO) theory, the reactions between
electron-rich dienes and electron-poor dienophiles are
dominated by HOMO(diene) - LUMO(dienophile) interactions and are
facile thermal reactions. If electron density of dienes and
dienophiles do not favor such an interaction, harsh reaction
conditions are required, or some orbital energy tuning
methodologies can be employed with restrictions.[3c] Fortunately,
it was found that oxidation to the cation radical of the dienophile
Herein, we report a highly efficient visible light driven
heterogeneous photo-redox system for D-A reaction on g-C3N4.
It exhibits high apparent quantum yield and broad substrate
scope. Moreover, two critical points in the reaction mechanism
are validated: 1) dioxygen serves as an active mediator for
photo-induced conduction band electrons; 2) vinylcyclobutane is
determined to be an intermediate in the reaction.
Using 0.5 mmol trans-anethole (1) and 1.5 mmol 2,3-
dimethyl butadiene (2) as the substrates, the photocatalytic D-A
reaction was successfully realized on g-C3N4 with 97 % yield of
cycloadduct (3) after 1 hour visible light irradiation. A series of
control reactions and optimization experiments were conducted
to figure out the roles of each part in this photocatalytic system
(Table 1). Visible light irradiation and g-C3N4 are the requisites
for the system as energy source and photocatalyst, respectively
(Table 1, entries 2 and 3). Flushing the system with nitrogen
lowers the yield to only 5 %; and the addition of 14 mol % of
benzoquinone, which is an efficient (superoxide) radical
scavenger,[7] the reaction is inhibited to trace amount of
conversion (Table 1, entries 4 and 5). Aerobic condition is
obviously necessary to this system, and the detailed contribution
of dioxygen to this reaction system is discussed later in the
reaction mechanism. This efficient cycloaddition reaction was
further scaled up to 1 mL trans-anethole, and a remarkable yield
of 95 % was achieved within 2.5 h visible light irradiation,
indicating the applicability of this methodology (Table 1, entry 6).
The photocatalyst possesses good stability, showing a high yield
of 94% at the 5th cycle.
drives
cycloaddition. One-electron chemical oxidation (commonly by
aminium salts) or photoinduced electron transfer to
a significantly enhanced reaction rate of the D-A
a
photosensitizer (such as 9,10-dicyanoanthracene) is required for
Dr. Y, Zhao, Prof. Dr. M. Antonietti
Department of Colloid Chemistry
Max Planck Institute of Colloids and Interfaces
Am Mühlenberg 1, 14476 Potsdam, Germany
E-mail: yubao.zhao@mpikg.mpg.de
Table 1. Control and optimization experiments of the photocatalytic cation
radical [4 + 2] cycloaddition of trans-anethole with 2,3-dimethyl-1,3-butadiene.
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.201703438
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COMMUNICATION
vinylcyclobutane intermediate reaches
a peak, and then,
gradually decreases to zero; while for 3, the concentration keeps
increasing until the dienophile is completely converted. These
curves are characteristic for
a consecutive reaction with
vinylcyclobutane as the intermediate and 3 as the final product.
Due to the appropriate photocatalytic activity of g-C3N4 towards
the conversion of 1 to vinylcyclobutane (reaction rate constant,
k1) and vinylcyclobutane to 3 (reaction rate constant, k2), the
balance of k1 and k2 engenders the temporary building up of the
vinylcyclobutane intermediate, providing important hint to a valid
reaction mechanism.
Entry
Photocatalyst
Light
Irradiation
Atmosphere
Yield[b]
(%)
1[a]
2
g-C3N4
-
Yes
Yes
No
Air
97
Air
0
3
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
Air
0
60
50
40
30
20
100
80
60
40
20
0
(a)
ON
(b)
4
Yes
Yes
Yes
Yes
Yes
Yes
Nitrogen
Air
5
OFF
5[c]
6[d]
7[e]
8[f]
9[g]
Trace
95
Air
Air
22
0
10 20 30 40 50 60
Reaction Time (min)
3
6
9
12
15
Irradiation Time (min)
(d)
Dioxygen loss
Air
8
20
16
12
8
0.5
0.4
0.3
0.2
0.1
0.0
(c)
320
240
160
80
O
Light On
Air
Trace
Anethole conversion
O
[a] Standard reaction conditions: 0.5 mmol 1, 1.5 mmol 2, 25 mg g-C3N4, 30
mg MgSO4, 5 mL CH3NO2 was added in a photoreactor with 15 mL volume,
followed by 1 hour visible light irridiation. [b] Yield is based on the 1H-NMR
analysis with an internal standard. [c] 37 μmol Benzoquinone was added in the
reaction mixture for capturing the superoxide radical. [d] Gram scale reaction:
1 mL (6.7 mmol) of 1, 2.3 mL 2, 100 mg MgSO4, and 200 mg g-C3N4 was
added in a flask with 40 mL CH3NO2, followed by 2.5 h visible light irradiation.
[e] CH3CN as solvent. [f] CH2Cl2 as solvent. [g] Cyclohexane as solvent.
0.0 0.1 0.2 0.3 0.4 0.5
Amount (mmol)
4
0
0
0
10 20 30 40 50 60
Irradiation Time (min)
0
5
10 15 20 25 30 35
Irradiation Time (min)
Figure 1. (a) The yield of 3 in the reaction with alternating light irradiation and
dark intervals. (b) Apparent quantum yield (AQY) of the reaction. (c) The
variation of the amount of D-A cycloadduct (3) and vinylcyclobutane
intermediate during the reaction. (d) Oxygen partial pressure of the headspace
(10 mL) in the photoreactor.
In lieu of the polar CH3NO2, some less polar solvents were
tested in this system. A dramatic slowing down of the reaction
rate is observed: 21 % and 8 % yields for reactions in CH3CN
and CH2Cl2, respectively. The reaction in the non-polar
cyclohexane gives no conversion (Table 1, entries 7, 8, and 9).
The dependence of the photocatalytic performance on the
polarity of the solvent strongly demonstrates that the D-A
reaction in this system occurs via a pathway involving cation
radical.[8]
To further study the effect of visible light irradiation on the
reaction, an experiment with alternating light and dark phases
was conducted. As shown in Figure 1a, the initial 5 min
irradiation leads to 37% yield. The reaction stops immediately
when the light is blocked, and it restarts in the following light
irradiation interval, reflecting the possibility of temporal control of
the reaction. After total 35 min light irradiation segments, the
reaction with 0.5 mmol 1 reaches complete conversion. To
quantitatively describe the efficiency of this photocatalytic
reaction system, the apparent quantum yield (AQY) was
measured. The AQY of the reactions with 5, 10, and 15 min
visible light irradiation is 47.6 ± 3.0 %, 43.9 ± 1.4 %, and 38.9 ±
1.4 %, respectively, demonstrating the remarkable photo-
conversion efficiency of this photocatalytic system (Figure 1b).
By GC-MS and NMR analysis, the intermediate in the
photocatalytic D-A cycloaddition reaction is identified to be the
As mentioned above, the presence of dioxygen is a
prerequisite to the effectiveness of this photo-redox system. As
such, a quantitative analysis of the dioxygen variation during the
reaction could lead to a fresh insight into the role of dioxygen as
well as the reaction mechanism. The dioxygen concentration of
the head space in the photo-reactor was thereby monitored by
an oxygen sensor throughout the photocatalytic reaction
process.[9] As shown in Figure 1d, the dioxygen partial pressure
drops rapidly in the initial 5 min, and decreases gently in the
following 25 min, until the end of the reaction. The inset in Figure
1d shows that the dioxygen loss is only 14 (mol) % of the trans-
anethole conversion. Meanwhile, the excess diene 2 undergoes
oxidative cleavage of the C=C bond to produce 3-Methyl-3-
buten-2-one, which accounts for 70 (mol) % of the dioxygen loss.
Accordingly, less than 0.05 equiv. of dioxygen services the D-A
reaction cycle, converting 1 equiv. of the trans-anethole into D-A
cycloadduct. The dioxygen loss data herein is suggesting two
possible facts: 1) dioxygen could be regenerated during the
reaction (working as mediator for photoinduced electrons); or 2)
radical chain propagation dominates the reaction. However, the
latter is less significant than the former according to the
experimental result that the absence of dioxygen leads to
minimal reactivity of the system. Additionally, ESR (electron spin
resonance) spectra show the characteristic peaks of the DMPO-
vinylcyclobutane derivative, which originates from the
photocatalytic [2 + 2] cycloaddition of 1 and 2.[5b,
The
8, 10]
variation of vinylcyclobutane intermediate concentration and the
product amount during the photocatalytic reaction is shown in
Figure 1c. After a transient raise, the concentration of the
−
O2 (DMPO, 5,5-Dimethyl-1-pyrroline N-oxide), demonstrating
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10.1002/anie.201703438
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the formation of the superoxide radical under visible light
irradiation (Figure S3).
thermodynamically feasible,[5] according to the previous
discussion, the chain propagation pathways are not very
persistent, as indicated by the inactivity of the system without
dioxygen.
The combination of all the above experimental facts strongly
suggest that dioxygen plays a critical role of electron mediator
−
(O2/O2
,
− 1.42 vs. Fc+/Fc in CH3CN),[10] accepting the
It is interesting to compare the data with the Ru(II) and Cr (III)
complex systems. The Ru(II) complex system without air only
loses 50 % photocatalytic efficiency.[5a] In the Cr(III) systems, the
absence of air causes ~ 90% of activity loss, and the presence
of the 10 mol% superoxide radical scavenger (benzoquinone)
lead to 60 % yield loss (from 88 % to 34 % yield).[5e] In the
present g-C3N4 photocatalytic system, substitution of air by
nitrogen causes > 95% activity loss, and the addition of the
superoxide radical scavenger leads to completely inactivity. The
D-A reactions in the photo-redox systems proceed via multiple
pathways. The radical chain propagation is dominating in the
Ru(II) complex system; and the dioxygen mediated pathway
accounts for higher percentage in the Cr(III) complex systems.
In the heterogeneous g-C3N4 system, the dioxygen mediated
pathway make a main contribution to the reaction.
photoinduced conduction band electrons and transporting them
to the cationic adduct intermediates to neutralize the charge and
close the photocatalytic cycle.
Figure 2. Proposed mechanism of the photocatalytic [4+2] cycloaddition of 1
and 2 on g-C3N4 under visible light irradiation. SET, single electron transfer.
According to the above solid analysis, a detailed reaction
mechanism is proposed (Figure 2). Visible light irradiation drives
charge separation in g-C3N4, producing oxidative valence band
holes and reductive conduction band electrons.[6b,c] The
conduction band electrons reduce the dioxygen and produce
superoxide radicals. The valence band hole oxidize 1 to the
corresponding cation radical 1+ (1/1+, 1.0 V vs. Fc+/Fc in
CH3NO2).[5e] If there is no diene available (Pathway A), this
cation radical will react with another molecular 1 to produce a [2
+ 2] cycloadduct (5), being consistent with the solvent polarity
dependence feature of the system.[11b,12]
Chart 1. The scope of the g-C3N4 photocatalyzed cation radical Diels-Alder
reactions under visible light irradiation.
Via pathway B, 1+ reacts with
2
to produce the
vinylcyclobutane cation radical, which then extracts electron
from superoxide radical and converts to molecular 4. As
analyzed above, 4 is an intermediate, and will continue a
photocatalytic cation radical sigmatropic shift to the final [4 + 2]
cycloadduct 3. This kind of rearrangement was also
independently observed in Cr(III) complex photo-redox system,
under thermal conditions, as well as in the cation radical reaction
initiated by PET or aminium salt.[5b,13] In addition, pathway C
describes the direct [4 + 2] cycloaddition of 1+ and 2, producing
g-C3N4 photocatalyzed system exhibits broad substrate
scope. A series of dienes and dienophiles are successfully
converted to their respective D-A cycloaddition products with
high yield and excellent stereoselectivity (Chart 1). With 1 as the
dienophile, isoprene shows slightly lower activity than 2 in the
cycloaddition reaction, giving 90 % yield (6) after 1 h visible light
irradiation. With 1-acetoxy-1,3-butadiene and 2,4-dimethyl-1,3-
pentadiene, the reactions produce 7 and 8 with 52 % and 54 %
yield, respectively. With more electron-rich 1,2-dimethoxy-4-
propenylbenzene as the dienophile, cycloaddition reactions with
a series of dienes give 9, 10, and 11 with 54 %, 98 % and 92%
yield, respectively. The N-Vinylcarbazole (enamine) is capable
of being dienophile in the cycloaddition reaction in this system,
resulting 12 with 91 % yield in 2 h photocatalytic reaction.
a
cycloadduct cation radical 3+. After a further one electron
transfer from the superoxide radical to 3+, the final cycloaddition
product 3 is produced.
Pathways D and E describe the possible radical chain
propagation, in which the cycloadduct cation radicals (3+ and 4+)
extract electrons from olefin 1, producing neutral molecules and
regenerating
1+.
Although
these
reactions
are
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a
heterogeneous
photocatalytic system with low cost, metal-free g-C3N4 sheets for
visible light harvesting and conversion. D-A reactions with
electron-rich dienophiles could be efficiently realized, and the
apparent quantum yield reaches a remarkable value of 47%.
Dioxygen plays a critical role as an active electron mediator
(0.05 equiv. O2 for 1 equiv. of olefin) in this photo-redox system;
and the reaction is overwhelmingly dominated by dioxygen
mediated reaction pathway. In addition, the reaction
intermediate of vinylcyclobutane is captured and monitored
during the reaction, serving as a direct evidence for the
proposed reaction mechanism. The cycloaddition process is
thereby presumably a combination of direct [4 + 2] cycloaddition
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a [2 + 2] cycloaddition followed by photocatalytic
rearrangement of the vinylcyclobutane. There are some unique
advantages from this system, such as the heterogeneous metal-
free photocatalyst being low cost and environmental benign,
visible light being the energy input, and the aerobic reaction
condition, which endow this protocol with significance in
potential heterogeneous photocatalytic synthetic utility.
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YZ acknowledges the scholarship from Max Planck Society. The
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Keywords: Photocatalysis • Graphitic Carbon Nitride • Visible
Light Irradiation • Electron Mediator • Diels-Alder Reaction
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[2]
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COMMUNICATION
COMMUNICATION
Photocatalytic Diels-Alder (D-A)
reaction with electron rich olefins is
realized by graphitic carbon nitride (g-
C3N4) under visible light irradiation,
with an apparent quantum yield of
47 % for the model reaction. Dioxygen
plays a critical role as electron
mediator. The cycloaddition process is
validated to be the combination of
direct [4 + 2] cycloaddition and [2 + 2]
cycloaddition followed by
Yubao Zhao* and Markus Antonietti
Page No. – Page No.
Visible Light Irradiated Graphitic
Carbon Nitride (g-C3N4)
Photocatalyzed Cation Radical Diels-
Alder Reactions with Dioxygen as
Sustainable Mediator for
Photoinduced Electrons
photocatalytic rearrangement of the
vinylcyclobutane intermediate.
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