Metal-free Semiconductor Photocatalysis for sp2 C?H Functionalization with Molecular Oxygen

Article abstract of DOI:10.1002/cctc.201801948

Designing metal-free catalysts for solar energy conversion is a long-standing challenge in semiconductor photoredox catalysis (SPC). With visible-light-responsive hexagonal boron carbon nitride (h-BCN) as a non-metal photocatalyst, this system affords C?H/N?H coupling products with broad substitution tolerance and high efficiency with molecular oxygen as the terminal oxidant. The catalyst exhibits remarkable performance for the selective C?H functionalization of electron-rich arenes to C?N products (yields up to 95 %) and good stability (6 recycles). Both nitrogen heteroarenes and amine salts are competent coupling nucleophiles. Mechanically, the reactive oxygen species are superoxide anion radical (O₂^?.) and H₂O₂, which are proved by electron spin resonance (ESR) data, KI-starch, and control experiments. In addition, kinetic isotope effect (KIE) experiments indicate that C?H bond cleavage is not involved in the rate limiting step. This semiconductor-based photoredox system allows for C?H amination free of any metals, ligands, strong oxidants, and additives. It provides a complementary avenue to C?H functionalizations and enables synthetic applications efficiently in a sustainable manner.

Full text of DOI:10.1002/cctc.201801948

Accepted Article
Title: Metal-free Semiconductor Photocatalysis for sp2 C-H
Functionalization with Molecular Oxygen
Authors: Meifang Zheng, Indrajit Ghosh, Burkhard König, and Xinchen
Wang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
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To be cited as: ChemCatChem 10.1002/cctc.201801948
Link to VoR: http://dx.doi.org/10.1002/cctc.201801948
A Journal of
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ChemCatChem
10.1002/cctc.201801948
CCOMMUNICATION
2
Metal-free Semiconductor Photocatalysis for sp C-H
Functionalization with Molecular Oxygen
1
2
2*
1
Meifang Zheng, Indrajit Ghosh, Burkhard König, and Xinchen Wang *
Abstract: Designing metal-free catalysts for solar energy conversion
exhibits good catalytic performance and high stability under
[4]
is a long-standing challenge in semiconductor photoredox catalysis
visible light photoredox conditions.
(
SPC). With visible-light-responsive hexagonal boron carbon nitride
Direct transformation of ubiquitous, but chemically inert,
aryl C–H bonds can provide efficient and expedient access
to arenes with diverse structural properties. Carbon–carbon
(h-BCN) as a non-metal photocatalyst, this system affords C–H/N–H
coupling products with broad substitution tolerance and high
efficiency with molecular oxygen as the terminal oxidant. The
catalyst exhibits remarkable performance for the selective C-H
functionalization of electron-rich arenes to C-N products (yields up to
(
C–C), carbon–oxygen (C–O), or carbon–nitrogen (C–N)
bonds are formed without prior functionalization of the
arenes as halides, trifluoroates, or boronic acids or their
[
5]
derivatives. In recent years, certain homogeneous efforts
have been made to C-H functionalizations with high
9
5%) and good stability (6 recycles). Both nitrogen heteroarenes and
amine salts are competent coupling nucleophiles. Mechanically, the
[6]
efficiency. Despite the indisputable advances and high


reactive oxygen species are superoxide anion radical (O
, which are proved by electron spin resonance (ESR) data, KI-
starch, and control experiments. In addition, kinetic isotope effect
KIE) experiments indicate that C–H bond cleavage is not involved in
the rate limiting step. This semiconductor-based photoredox system
allows for C–H amination free of any metals, ligands, strong oxidants, may allow to address most of the aforementioned challenges.
Herein, for the first time, semiconductor photoredox catalysis
SPC) is applied to oxidative photoredox manifolds with
2
) and
efficiency, reported methods often require elevated reaction
temperatures, stoichiometric amounts of strong oxidants,
precious metals with ligands, and the installation and
removal of directing groups. Therefore, a heterogeneous
2 2
H O
(
2
system with O as the only oxidant for C–H functionalization
and additives. It provides
a complementary avenue to C–H
(
functionalizations and enables synthetic applications efficiently in a
sustainable manner.
oxygen and visible light driven C–H activation.
Semiconductors have shown great potential in driving
several important reactions (e.g., water splitting, CO
reduction, and environmental remediation) over the last
2
[
1]
decades. The photocatalytic ability is thermodynamically
determined by the redox window caused by photoinduced
−
electrons (e ) (produced at conduction band, CB) and holes
+
(
h ) (produced at valence band, VB), that is, the reaction can
proceed only if the redox potential of the photocatalyst is
[
2]
greater than that of the reactants. Most metal-based
semiconductors are thermodynamically capable of triggering
those reactions, however, wider bandgap means shorter
wavelength absorption. Thus, recent efforts focus on the
development of metal-free materials for relevant chemical
reactions because electronic transition 3]in conjugated
[
systems could be stimulated by visible light. It also has the
benefits of containing earth-abundant elements and meeting
environmental-friendly principles. Among the reported metal-
free semiconductor photocatalysts, h-BCN has drawn
interdisciplinary attention because its balance in suitable
bandgap and efficient visible-light responsiveness. Besides,
it is an inexpensive and easily-available material, and
Figure 1. Structure and optical characterization of h-BCN. (a) TEM of h-BCN.
(b) UV-vis DRS of h-BCN and inset is the picture of h-BCN . (c) The UPS of h-
BCN. (d) The valence band and conductiopn band of h-BCN.
[
a]
Dr. M. F. Zheng, Dr. I. Ghosh, Prof. B. König, Prof. X. C. Wang
State Key Laboratory of Photocatalysis on Energy and Environment,
College of Chemistry, Fuzhou University, Fuzhou 350116 (China)
1
Institute of Organic Chemistry, Faculty of Chemistry and Pharmacy,
University of Regensburg, Regensburg 93040, Germany Institute of
Organic Chemistry, Faculty of Chemistry and Pharmacy, University
of Regensburg, Regensburg 93040, Germany
E-mail: Burkhard.Koenig@ur.de; xcwang@fzu.edu.cn
Website: https://wanglab.fzu.edu.cn
Transmission electron microscope (TEM) spectra of h-
BCN are recorded and shown in Figure 1 a. In designing
organic photosynthesis, it is crucial that the catalytic material
is visible-light responsive. Based on the previous reports,
carbon-free h-BN (hexagonal boron nitride) material is a
typical insulator with a wide bandgap of about 5.5 eV, and
the doping of carbon narrows the band gap (see the
synthesis of h-BCN in the SI), and thus turning the insulator
2
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201801948
CCOMMUNICATION
4
b
[a] Unless otherwise noted, the reaction was carried out with 12 mg of catalyst,
into a visible light responsive semiconductor. As shown in
Figure 1b, the bandgap was estimated to be 2.75 eV by
ultraviolet-visible diffuse reflectance spectroscopy (UV-vis
DRS). The valence band was determined via ultraviolet
photoelectron spectrometer (UPS) and the valence band
level of h-BCN is 6.18 eV vs. vacuum according to UPS
spectra (Figure 1c). The VB level is +1.78 V vs. NHE at pH =
0
O
.2 mmol of 1a, 0.8 mmol of 2a, and 1 mL solvent at r.t. with 1 atmosphere of
2
balloon for 48 h under 3 w blue LEDs illumination. Yield was determined by
gas chromatography. DMSO is short for dimethyl sulfoxide. DCE is short for
1,2-dichloroethane. “n.d.” means “not detected”. [b] The reaction was carried
out under N atmosphere. [c] The reaction was carried out without light.
2
Table 2. Scope of C-H amination
7
, and the CB level is –0.97 V vs. NHE at pH = 7 (Figure 1d).
Besides, photoluminescence lifetimes dynamically play an
important role in photocatalysis, the average PL lifetime of
as-obtained h-BCN material was 2.73 ns (Figure S2).
First, we optimized the reaction conditions using 1,3,5-
trimethoxybenzene (1a) and pyrazole (2a) as model
substrates (Table 1). After investigating the solvents, we
obtained a satisfactory yield of the amination product 3a
using DMSO as the solvent (SI, Table S1; Table 1, entry 1).
The AQY (apparent quantum yield) is ca. 1.0% at 450 nm
(see the details in the SI). Additionally, the control
experiments indicated that oxygen, visible light, and
photocatalyst were indispensable for this transformation
(Table 1, entries 2–4). The general homogeneous organo-
metallic photosensitizers, such as Ru(bpy) Cl and Ir(bpy)
3
2
3
,
are unable to activate the arene C–H bond due to their lower
oxidation potential (Table 1, entries 5–6). Homogeneous
dyes, such as Eosin Y and Rh-6G, failed to oxidize the arene
2
sp C–H bond (Table 1, entries 7-8). Previous accomplishes
with DDQ and the acridinium salt (9-mesityl-3,6-di-tert-butyl-
1
0-phenylacridiniumtetrafluoroborate) were tested with 1a
and 2a as the model substrates under O atmosphere, the
yields were inferior comparing with that of using BCN as the
2
[5c, 5e]
photocatalyst (entries 9–10).
We also compared the
performance of h-BCN with other heterogeneous
photocatalysts, and results show the activity of h-BCN is
better than that of TiO (P25) and CdS under visible light
2
irradiation (Table 1, entries 11-12) in our reaction conditions.
Table 1. Optimization of reaction conditions.
[
a] Standard reaction conditions: 0.2 mmol 1, 4 or 5 equivalents of 2, 12 mg h-
BCN, and 1-1.5 mL DMSO were added in a photoreactor, followed by 48-72 h
visible light irradiation.
[
a]
entry
1
cat.
solvent
DMSO
yield
With optimized reaction conditions in hand (Table 1, entry
1), we investigated different electron-rich arenes in the next
set of experiments. As shown in Table 2, this methodology
can be applied to different aromatic C–H bonds. With anisole
as substrate, the corresponding C–N coupling products were
obtained in good yields with a 2:1 ratio of the para- and
ortho-products 3b. Furthermore, naphthalene and its
derivative could couple to pyrazoles with good isolated yields
h-BCN
93% (91%)
trace
n.d.
[
b]
2
3
h-BCN
h-BCN
--
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
[
c]
4
5
6
7
8
9
n.d.
Ru(bpy)
Ir(bpy)
3
Cl
2
n.d.
(
3c–3e). In particular, the reaction with trimethyl benzene did
3
n.d.
not show benzylic oxidation and gave 48% yield of the
corresponding C-H N-arylation product 3f. The acetyl group
on naphthalene remains unchanged and a 95% yield of 3g
was obtained. With 1,4-dimethylpyrazole as the substrate,
Eosin Y
Rh-6G
DDQ
n.d.
n.d.
2
1% yield of 3h was isolated. A diverse range of N-
CH
3
CN
21%
75%
n.d.
containing nucleophiles could be directly coupled with arenes
in this heterogeneous photoredox system (Table 3).
Imidazoles and trizoles were compatible with this SPC
system with good to excellent yields (3i–3m). Besides,
anilines could be obtained from this catalytic sequence by
using an ammonium salt as the nitrogen source. Primary
1
0
1
2
Acridinium
TiO
CdS
DCE
1
1
2
DMSO
DMSO
5%
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ChemCatChem
10.1002/cctc.201801948
CCOMMUNICATION
o
amines are important building blocks due to the presence of
can not oxidize OH to hydroxyl radicals (E , (OH/OH) =
a synthetically versatile NH
2
group. Traditionally, anilines are
2.02 V vs. NHE at pH = 7). So it is thermodynamically
unfeasible to produce hydroxyl radicals with BCN materials
under visible light. This is beneficial for the formation of the
target products, because hydroxyl radicals may cause photo-
prepared via a nitration/hydrogenation sequence and the
process requires harsh conditions. Wu’s group reported a
one-step synthesis of anilines from benzene with ammonia
as the amine source using a combination of an acridinium
[
10]
degradation and thus probably decrease the yield.
[7]
salt and cobalt catalysis. Starting from these observations,
we screen a variety of commercially available ammonium
Therefore, the h-BCN catalyst produces light induced
electrons and holes under the illumination of blue LEDs. The
holes could accept electrons from the arenes to generate the
aryl cation radical intermediate. And the photo-exited
electrons of h-BCN would be accepted by oxygen generating
+
–
+
–
+
2–
salts, such as H
4 4 3 4 2 3
N OAc , H N HCO , and (H N ) CO .
+ –
Among them, ammonium carbamate (H
4
2 2
N H NCO ) was
best suited for the reaction (Table S2 in the SI). Using 5.0
equivalents of ammonium carbamate with anisole, it resulted
in the formation of a 5:1 ratio mixture of para- and ortho-
anisidine yield of 41% (3q). Delightfully, a 53% yield of
naphthalen-1-amine 3p was obtained under catalytic
conditions nearly identical to those applied to azoles.

O2
.
The recyclability of the catalyst was studied. The catalyst
was recovered by centrifugation and reused for at least five
more times without losing catalytic efficiency in C–N arylation
(Figure 2). This demonstrates the high stability of h-BCN
during the prolonged photochemical operation. Raman
spectra were identical before and after the reaction, which
indicate the stability of the catalysts as well (Figure S3).
Dark
3
6
1
min
min
2 min
3480
3500
3520
3540
3560
Field (G)
Figure 3. Investigation for mechanistic insights. (a) The intermolecular KIE
−3
experiment. (b) ESR spectra of a solution of h-BCN (3.0 × 10 M), 1a (0.10 M),
and DMPO (0.50 M) in DMSO.
Figure 2. Evaluation of catalytic recycling.
An intermolecular KIE experiment was carried out to study
the heterogeneous radical pathway of this photochemical
system (Figure S4). The KIE of 1.30 was observed from
nuclear magnetic resonance (NMR) data (Figure 3a), which
revealed that the C–H bond cleavage of the arene might not
be involved in the rate-determining step. Then, the
mechanism was studied by control experiments and ESR.
The coupling reaction totally stopped with the addition of
TEMPO or 1,1-diphenylethene under the standard conditions
(
see details in the SI). The results indicate a radical process.
An ESR experiment was conducted employing 5,5-dimethyl-
-pyrroline-N-oxide (DMPO) as a spin trap. The spectra
1
−
Figure 4. The proposed mechanism for the photocatalytic C-H amination by h-
BCN with visible light.
show the characteristic peaks of DMPOO , demonstrating
the formation of the superoxide radical (Figure 3b). The
observations strongly suggest that oxygen plays a critical
2
[8]
role as electron acceptor. Namely, O
from the conduction band to form O
electrons in h-BCN possess a strong thermodynamical
2
accepts the electrons
[6c,
Based on the above experiments and previous reports,
–

. The lightexcited
10b, 11]
2
we propose a possible mechanism (Figure 4). Initially,
h-BCN is excited to produce electrons and holes by blue
LEDs. Then, an arene carbon cation radical A is generated
upon photoinduced electron transfer (PET) from the arene to
the catalyst (valence band level of +1.78 V vs. NHE at pH =
). The radical cation A then acts as electrophilic partner for
nitrogen nucleophiles. That is, the nucleophile 2a forms a σ-
o

driving force to reduce O
pH = 7) to superoxide O
state potential that is unable to seize electrons from arenes
2
(E , (O
2
/O
2
)= 0.16 V vs. NHE at

 [9]
2
.
Molecular O
2
has a ground
o
+
(
E , (1b/1b = 1.63 V vs. NHE at pH = 7). Besides, the
7
potential of the valence band (+1.78 V vs. NHE at pH = 7)
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ChemCatChem
10.1002/cctc.201801948
CCOMMUNICATION
adduct with cation radical A via direct C–H aryl amination to
generate intermediate B, neutralizing the charge. The
Macromolecules 2012, 45, 607-632; d) Y. Zheng, H. H. Lin, B. Wang
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B
forms
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In summary, we have developed a simple metal-free
pathway for the C–H amination of arenes with a SPC system
at ambient conditions. This approach exploits a recyclable
catalyst that is capable of oxidizing arenes through a single
electron transfer (SET) to generate the corresponding cation
radicals under visible light illumination. Oxygen serves as
both the terminal oxidant and the electron mediator, which is
distinct from the previous reports using homogeneous
photoredox systems. N-arylated compounds are obtained in
good to excellent yields with various nitrogen nucleophiles.
Although the semiconductor photoredox catalytic system for
C-H functionalization is currently limited to electron-rich
arenes, further improvement of BCN materials is currently on
the way in our laboratory. The semiconductor system for
organic synthesis expands our knowledge of chemical
reactivity and provides a complementary environmentally
friendly synthetic protocols to C-H functionalization.
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thank BayChina for financial support.
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: Heterogeneous Catalysis • Visible Light•
Semiconductor Photoredox Catalysis • C-H Fuctionalizaition •
Radical
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[
2
12] Upon the addition of KI and starch to the solution of the N flushed
reactor, the solution did not change colour; but under the standard
reaction conditions with oxygen, the solution were observed in blue
colour. The observations indicate the presence of
photocatalytic process.
2 2
H O in the
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ChemCatChem
10.1002/cctc.201801948
CCOMMUNICATION
COMMUNICATION
1
2
Semiconductor photoredox catalysis
Meifang Zheng, Indrajit Ghosh,
2
*
1
(
SPC)
offers
a
simple
and
Burkhard König, and Xinchen Wang *
environment-friendly access to the
construction of C–N bonds. This
Page No. – Page No.
heterogeneous system provides
a
Metal-free Semiconductor
Photocatalysis for sp C-H
Functionalization with Molecular
Oxygen
green route to the selective C–H
amination of arenes with molecular
oxygen as the only oxidant.
2
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