A g-C3N4-based heterogeneous photocatalyst for visible light mediated aerobic benzylic C-H oxygenations

Article abstract of DOI:10.1039/c9gc02870f

A metal-free heterogeneous photocatalytic system has been developed for highly efficient benzylic C-H oxygenations using oxygen as an oxidant. This visible light mediated oxidation reaction utilizes graphitic carbon nitride (g-C₃N₄) as a recyclable, nontoxic and low cost photocatalyst. Mild reaction conditions allow for the generation of synthetically and biologically valued isochromannones, phthalides, isoquinolinones, isoindolinones and xanthones from readily accessible alkyl aromatic precursors in good yields. The heterogeneous nature of the g-C₃N₄ catalytic system enables easy recovery and recycling as well as the use in multiple runs without loss of activity. The synthetic utility of this "green" methodology was further demonstrated by applying in bioactive and drug valued target syntheses.

Full text of DOI:10.1039/c9gc02870f

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PAPER
A g-C N -based heterogeneous photocatalyst
3
4
for visible light mediated aerobic benzylic C–H
oxygenations†
Cite this: Green Chem., 2019, 21,
116
6
a,b
b
b
c
Pengxin Geng,
Yurong Tang,
Guanglong Pan,
Wentao Wang,
a
b
Jinchuan Hu * and Yunfei Cai
*
A metal-free heterogeneous photocatalytic system has been developed for highly efficient benzylic C–H
oxygenations using oxygen as an oxidant. This visible light mediated oxidation reaction utilizes graphitic
3 4
carbon nitride (g-C N ) as a recyclable, nontoxic and low cost photocatalyst. Mild reaction conditions
allow for the generation of synthetically and biologically valued isochromannones, phthalides, isoquinoli-
nones, isoindolinones and xanthones from readily accessible alkyl aromatic precursors in good yields. The
Received 15th August 2019,
Accepted 7th October 2019
3 4
heterogeneous nature of the g-C N catalytic system enables easy recovery and recycling as well as the
DOI: 10.1039/c9gc02870f
use in multiple runs without loss of activity. The synthetic utility of this “green” methodology was further
rsc.li/greenchem
demonstrated by applying in bioactive and drug valued target syntheses.
from low conversion, limited scope, and using precious, toxic
transition metal catalysts and expensive ligands (Scheme 1A).
Introduction
Benzylic oxidations, which offer straightforward access to Therefore, new convenient and sustainable synthetic strategies
highly valued building blocks and scaffolds from readily avail- in a more cost-effective and green fashion are highly desirable.
able alkyl aromatic precursors, are of considerable importance
Recently, homogeneous visible-light mediated photocata-
in synthetic organic chemistry and the pharmaceutical indus- lysis employing ruthenium- or iridium-based transition metal
1
try. These transformations are traditionally carried out with a complexes or organic dyes as sensitizers has become a power-
large excess of environmentally undesirable metal oxidants,
2
such as chromium and manganese reagents. Even though
transition metal complex based catalytic methods have been
developed, sacrificial expensive terminal oxidants such as tert-
butyl hydroperoxide or phenylmethylsulfoxide, additives (e.g.,
bases, acids) and/or harsh reaction conditions (e.g., high temp-
3
erature) have limited their practical applications. Thus, sig-
nificant efforts have been directed toward the development of
sustainable and environmentally friendly benzylic oxidation
catalyst platforms with molecular oxygen as an oxidant, which
1
b,4
features virtually no waste and high atom economy.
A range
5
6
7
8
of metal (e.g., copper, manganese, cobalt, rhodium, or
iron ) or metal-free catalyst systems have been developed for
9
10
aerobic benzylic oxidation reactions, but many of these suffer
a
College of Chemistry, Chongqing Normal University, Chongqing 401331,
P. R. China. E-mail: hujc20@139.com
b
School of Chemistry and Chemical Engineering & Chongqing Key Laboratory
of Theoretical and Computational Chemistry, Chongqing University,
1
74 Shazheng Street, Chongqing 400030, P. R. China. E-mail: yf.cai@cqu.edu.cn
c
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
P. R. China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Aerobic benzylic C–H oxygenations of alkyl aromatic pre-
c9gc02870f
cursors to esters, amides and ketones.
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Paper
1
1
a
ful tool for organic synthesis and visible light-driven reac- Table 1 Optimization of reaction conditions
tions are considered as a green chemical process due to the
utilization of inexpensive, nontoxic and widely abundant light
1
2
as a reagent (Scheme 1A). Nevertheless, from the perspec-
tives of basic science and practical applications, there is an
urgent demand for the development of heterogeneous photo-
redox catalysis using a recyclable, metal-free and low cost
b
1
3
Entry
PC
Solvent
Yield (%)
visible light-driven photocatalyst. Solid polymeric graphitic
1
4
carbon nitride (g-C
3
N
4
), an organic semiconductor with easy
1
2
3
4
5
6
7
8
g-C
g-C
g-C
g-C
g-C
g-C
g-C
g-C
g-C
TiO2
CdS
BiVO
Ru(bpy)
Ir(ppy)3
Eosin Y
Mes-AcrBF
None
3
3
3
3
3
3
3
3
3
N
N
N
N
N
N
N
N
N
4
4
4
4
4
4
4
4
4
2
H O
77
0
0
Toluene
THF
MeOH
AcOEt
DCE
preparation from inexpensive precursors, shows an appropri-
ate electronic band structure, which makes it a promising can-
didate to achieve this goal. Unlike the most widely studied
Trace
5
25
30
34
24
Trace
29
21
17
38
31
64
0
metal semiconductor TiO
2
with a band gap of 3.2 eV (onset of
3
CH CN
absorption: 380–390 nm), g-C N with adjustable conduction
3
4
DMSO
H O
2
H O
2
c
and valence band positions absorbs light in the visible region
9
1
4j
10
(
band gap <2.7 eV; onset of absorption: <450–460 nm). In
1
1
1
2
2
H O
2
H O
2
H O
addition, g-C N is capable of activating molecular oxygen,
3
4
4
which has already been applied in photoredox oxidations of 13
3
Cl
2
1
5,16
14
2
H O
benzyl alcohols and thioethers.
In particular, Wang
1
1
5
6
2
H O
2
H O
2
H O
2
H O
2
H O
reported boron doped carbon nitride photocatalyzed aerobic
oxidation of alkylarenes to ketones, which was a pioneering 17
4
d
e
1
1
8
9
g-C
g-C
3
N
N
4
0
0
study but with a very limited scope (4 examples) requiring high
1
6a
3
4
reaction temperatures (115–160 °C).
Based on all of the
a
above information, we wish to disclose an environmentally
benign, metal-free, heterogeneous photoredox based protocol
Reaction conditions: 1a (0.2 mmol), PC (10 mg), solvent (2 mL), O
2
b 1
balloon, blue LEDs, 24 h, rt. Yields determined by H NMR analysis
c
using 1,3,5-trimethoxybenzene as an internal standard. Air balloon.
employing g-C N as a nontoxic and low cost visible light-
d
e
3
4
In the dark. N balloon instead of the O balloon.
2
2
3
driven photocatalyst for the oxidation of benzylic sp C–H
bonds using oxygen as the oxidant. Via this new synthetic para-
digm, a broad range of structurally diverse alkyl aromatic pre- 24 h in 77% yield using g-C
N
(prepared by pyrolysis of mela-
3
4
cursors can undergo selective aerobic oxidation smoothly, mine at 600 °C) as the catalyst and water as the solvent at
affording various synthetically and biologically valued isochro- room temperature under an atmosphere under the
O
2
mannones, phthalides, isoquinolinones, isoindolinones and irradiation of 6 W blue LEDs (Table 1, entry 1). Among the
xanthones in good yields under mild reaction conditions screened solvents, toluene, THF or MeOH did not promote the
(
room temperature and water as the solvent in most cases). reaction at all (Table 1, entries 2–4). EtOAc, DCE, CH
3
CN or
Meanwhile, the g-C N catalyst shows very good recyclability DMSO only afforded the desired product 2a in low yields
3
4
and can be reused in multiple runs due to its heterogeneous (5–34% yields, Table 1, entries 5–8). The observation that water
nature. The application of the newly developed protocol has was a significantly better medium for the reaction was possibly
also been demonstrated in the synthesis of bioactive and drug due to higher oxygen concentration and enhanced dispersion
valued targets including indoprofen, indobufen and corydal- of heterogeneous g-C
dine (Scheme 1B). As a complementary option to known carbon nitrides including mpg-C
3
N
4
in water. Evaluation of different
and g-C from different
3
N
4
3 4
N
methods, this work has been identified with several distinct precursors (e.g., dicyandiamide, urea, guanidine, hydro-
advantages such as (a) using recyclable and metal-free semi- chloride) and different preparation procedures (450, 500, 600,
heterogeneous photocatalysts, (b) using water as the most 650 °C) did not result in further improvement (see ESI,
“
green” solvent in most cases and (c) good application scope Table S1†). When switching the reaction atmosphere from an
to a variety of different substrates.
2
O balloon to an air balloon, an inferior yield of 2a was
obtained (24% yield, Table 1, entry 9). We also tested other
classical heterogeneous or homogeneous photoredox catalysts
(
Table 1, entries 10–16). The reaction did not proceed in the
Results and discussion
presence of TiO
as
2
and a low yield of 2a was observed with CdS
In the initial assessment of aerobic benzylic C–H oxygenation or BiVO
a
metal semi-heterogeneous photocatalyst
4
reactions, isochroman 1a was selected as the mode substrate (Table 1, entries 10–12). Metal-based homogeneous photo-cat-
using g-C
as the heterogeneous photocatalyst. To obtain alysts Ru(bpy) Cl and Ir(ppy) gave 2a only in 17% and 38%
optimized conditions for this reaction, we tried a range of sol- yield, respectively (Table 1, entries 13 and 14). Metal-free
vents (Table 1, entries 1–8) and carbon nitrides prepared using homogeneous photocatalysts Eosin Y and Mes-AcrBF afforded
different procedures or precursors (see the ESI† for details). 2a in moderate to good yields (Table 1, entries 15 and 16).
The anticipated isochromanone 2a could be obtained after These results indicated that g-C is one of the most promis-
3
N
4
3
2
3
4
N
3 4
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ing visible light-driven photocatalysts for this reaction when Table 2 g-C
using water as the most “green” solvent. Furthermore, chan-
3
N
4
photocatalyzed aerobic oxidation of isochromans and
a
phthalans
ging the light source with different wavelengths diminished
the reaction yield, giving product 2a in 18–59% yields when
red, yellow, green, purple or white LED light was employed
(see ESI, Fig. S2†). Notably, the reactivity could be improved
with purple LED light but with the formation of increasing
amount of by-products. In the control experiments, no desired
product 2a was detected in the absence of g-C N or light,
3 4
which unambiguously proved the photocatalytic activity of
g-C N (Table 1, entries 17 and 18). In addition, no reaction
3
4
was observed under a N
the source of oxygen for this reaction (Table 1, entry 19).
With the optimized reaction conditions (Table 1, entry 1) in
hand, the substrate scope of the g-C photocatalyzed
benzylic CH oxidation of different isochromans was investi-
2 2
atmosphere, which confirmed O as
3 4
N
2
gated (Table 2). The position of the substituent on the aryl
ring with an electron-withdrawing group or electron-donating
group had little influence on the efficiency of this transform-
ation (2a–2k, 58–80% yields). It needs to be mentioned that
DMSO was used as an alternative solvent in the case of 1d–1k
due to its poor solubility in water. Furthermore, the mild
oxidation method enables selective oxidization of the very
electron-rich thienopyrans to give the dehydrogenative
α-oxygenation products 2l and 2m in acceptable yields (30%
and 47%) without affecting the sensitive heterocyclic ring.
Next, we examined the reactions of isochromans with a methyl
group on the saturated ‘O’ ring. Again, the processes pro-
ceeded smoothly with high yields (2n and 2o, 70% and 75%,
respectively). Moreover, the reaction can be scaled up and a
gram-scale reaction of 1a (10 mmol) was conducted, affording
the corresponding product 2a (1.04 g) in a yield of 70% (see
the ESI† for details).
Notably, the optimal reaction conditions were also appli-
cable for the aerobic oxidation of phthalan derivatives and the
desired phthalide products 2p–2s could be obtained in excel-
lent yields (71–94%, Table 2). Substituted phthalans 1q and 1r
have two benzylic positions for oxidation, and the para oxi-
dation product was slightly favoured over the isomer presum-
ably because of the electronic effect. In the case of 4-chloro-
1
,3-dihydroisobenzofuran 1s, less hindered product 4-chloroi-
sobenzofuran-1(3H)-one was observed as the major product.
The structure of the phthalide products was unambiguously
confirmed by comparison with literature data, and no over-oxi-
dation to the phthalic anhydride was observed, possibly due to
the fact that the introduction of the carbonyl group deactivates
the ring towards further oxidation.
a
3 4 2 2
Reaction conditions: 1 (0.2 mmol), g-C N (10 mg), H O (2 mL), O
b
balloon, blue LED strips, rt, 24 h. Yields after isolation. The reaction
was conducted on a 10 mmol scale. DMSO was used as the solvent.
c
Having showed the viability of the catalyst in oxidizing iso- ing substituents on the phenyl ring had little influence on the
chromans and isoindolines, we turned our attention to the reaction, affording 4f and 4g in 60% yield and 68% yield,
aerobic oxidation of N-substituted tetrahydroisoquinolines to respectively. Furthermore, the present catalytic system was also
dihydroisoquinolones (Table 3). We examined the effect of applicable for the α-oxygenation of isoindolines to isoindol-
amine protecting groups including phenyl, acetyl, benzoyl, inones (4h–4l, Table 3). For example, N-Boc isoindoline 3i
Boc, and Cbz groups on the reaction. The results showed that could be smoothly converted to N-Boc isoindolinone 4i in 54%
these protecting groups were all suitable under the minor yield. Various N-arylisoindolines with electron-donating
modified standard conditions, giving the desired products (3j, 3k) or electron-withdrawing (3l) substituents provided the
4
a–4e in 41–73% yields. Electron-donating or electron-withdraw- desired N-arylisoindolinones 4j–4l in good yields (63–72%).
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Table 3 g-C
Paper
3
N
4
photocatalyzed aerobic oxidation of tetrahydroisoqui- For N-aryl isoquinolines or isoindolines, the presence of a
a
nolines and isoindolines
base was required for the efficiency of this reaction. It is note-
worthy that minor modification of the standard conditions
made this protocol applicable to the aerobic oxidation of alkyl-
arenes to the corresponding ketones 6a–6e (Table 4, 28–80%
yields). The reactivity of diphenylmethane 5d and ethylben-
zene 5e was sluggish; however, when the excitation light was
changed to violet LED light with higher energy, the desired
products 6d and 6e were obtained in 54% yield and 28% yield,
respectively. Meanwhile, the methodology could also be suc-
cessfully applied to the α-oxidation of open chain benzylic
ethers. Electron-rich 4-methoxybenzylmethyl ether 7 was oxi-
dized to the corresponding methyl 4-methoxybenzoate 8 and
4-methoxybenzoic acid 9 in 68% and 23% yield, respectively
(
Scheme 2).
To further illustrate the practicability of the heterogeneous
photocatalysis protocols, a recycling procedure was estab-
lished. g-C was recovered after reaction and subsequently
reused. As shown in Fig. 1, the catalyst maintains its high
3 4
N
a
3 4 2
Reaction conditions: 3 (0.2 mmol), g-C N (20 mg), DMSO (4 mL), O
b
balloon, blue LED strips, 8 h, rt. Yields after isolation. 3 (0.2 mmol),
balloon K CO (0.1 mmol), g-C (20 mg), DMSO (4 mL), blue
LEDs, rt, 24 h.
O
2
2
3
3 4
N
3 4
Scheme 2 g-C N photocatalyzed aerobic oxidation of open chain
benzylic ether to ester and acid.
Table 4 g-C
ketones
3
N
4
photocatalyzed aerobic oxidation of alkylarenes to
a
a
3 4 2 2
Reaction conditions: 5 (0.2 mmol), g-C N (20 mg), H O (2 mL), O
b
balloon, blue LED strips, 24 h, rt. Yields after isolation. DCE was
c
used as the solvent. 5d (0.2 mmol), g-C
N
3 4
(20 mg), DMSO (2 mL),
0 °C, violet LEDs, 24 h. 5e (0.2 mmol), g-C (20 mg), d6-DMSO
2 mL), 50 °C, violet LEDs, 48 h; 28% yield of 6e determined by
d
5
(
3 4
N
1
H
NMR analysis using 1,3,5-trimethoxybenzene as an internal standard
was obtained and 72% of 5e remained unreacted.
Fig. 1 Evaluation of the catalyst recycling.
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Scheme 3 Application in the synthesis of bioactive and medicinal compounds corydaline, indoprofen and indobufen.
Scheme 4 Plausible mechanism.
photocatalytic activity when applied in the aerobic oxidation of cation B, followed by base-promoted deprotonation to generate
isochroman 1a to isochromanone 2a.
benzylic radical C. A and C react with the peroxide radical
OOH from superoxide anions to generate intermediates E and
•
In order to prove the synthetic utility of the newly developed
protocol, direct application of this metal-free α-oxygenation F. Finally, they transformed to the products with the simul-
reaction as the key step for the synthesis of biologically rele- taneous formation of water as the side product.
vant and drug valued targets was performed (Scheme 3).
Under the standard reaction conditions, lactams 4m–4o were
obtained in 58–65% yields. The removal of the Boc group of
Conclusions
4
m by trifluoroacetic acid afforded corydaldine 11, which is a
In summary, we have developed an entirely metal-free hetero-
geneous photocatalyst system for aerobic oxidation of structu-
rally diverse benzylic sp C–H bonds of ethers, amines and
alkylarenes. This environmentally friendly protocol employs
3 4
readily accessible g-C N as a visible light-driven photocatalyst,
featuring low cost, recyclability, and high atom economy. The
mild reaction conditions allow for the generation of a broad
range of synthetically and biologically valued isochroman-
nones, phthalides, isoquinolinones, isoindolinones and
xanthones from readily accessible alkyl aromatic precursors in
good yields. The application of this “green” approach in the
synthesis of biologically relevant and drug valued targets has
also been demonstrated.
1
7
naturally occurring isoquinolone alkaloid.
Hydrolysis of 4n and 4o leads to indoprofen 12a and indo-
bufen 12b, which are known as anti-inflammatory drugs and
3
1
8
platelet aggregation inhibitors.
Finally, the reaction mechanism is proposed as shown in
Scheme 4. Owing to the structural and electronic properties of
g-C
3 4
N with a band gap of 2.7 eV, visible-light excitation leads
to the efficient separation of photogenerated electron–hole
1
4
3 4
pairs. The photoelectrons (VBM of g-C N located at −0.88 V
•
1
4a
−
vs. RHE)
the reduction potential of residing at −0.56 V vs. SCE). The
photogenerated hole of g-C further undergoes single elec-
2 2
can activate O to superoxide radical anion O
(
3 4
N
tron transfer (SET) with the substrate including isochromans,
benzylarenes and N-acyl derived tetrahydroisoquinolines or
isoindolines to afford benzylic radical A. Regarding substrate
N-aryl derived tetrahydroisoquinolines or isoindolines, SET
Conflicts of interest
leads to the formation of the corresponding nitrogen radical There are no conflicts to declare.
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2
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