Visible-light induced enhancement in the multi-catalytic activity of sulfated carbon dots for aerobic carbon-carbon bond formation

Article abstract of DOI:10.1039/c9gc02658d

The development of carbonaceous materials as metal-free catalysts integrating different types of catalysis in a single system represents a significant advance in cascade/tandem organic synthesis. Zero-dimensional carbon dots with tuneable optical properties and easily modifiable surface functionalities can be harnessed as a carbocatalyst for merging photooxidation and acid-catalyzed reactions in one pot. Herein, we explore carbon dots decorated with hydrogen sulfate groups as a photocatalyst for the dehydrogenative cross-coupling of xanthenes with ketones, arenes and 1,3-dicarbonyl compounds that showed high efficiency and selectivity under visible-light irradiation. The sulphated carbon dots demonstrate dual catalytic properties, wherein they induced the rapid photooxidation of xanthenes in the presence of molecular oxygen to form a hydroperoxy intermediate followed by coupling of nucleophiles catalysed by the acidic surface functional groups. The methodology represents an operationally simple pathway for the generation of C-C coupling products in a short reaction time with wide substrate scopes under mild conditions. The catalyst is easily separable and can be reused over multiple cycles with good efficiency.

Full text of DOI:10.1039/c9gc02658d

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Visible-light induced enhancement in the multi-
catalytic activity of sulfated carbon dots for
aerobic carbon–carbon bond formation†
Cite this: Green Chem., 2019, 21,
6717
Daisy Sarma, Biju Majumdar and Tridib K. Sarma
*
The development of carbonaceous materials as metal-free catalysts integrating different types of catalysis
in a single system represents a significant advance in cascade/tandem organic synthesis. Zero-dimen-
sional carbon dots with tuneable optical properties and easily modifiable surface functionalities can be
harnessed as a carbocatalyst for merging photooxidation and acid-catalyzed reactions in one pot. Herein,
we explore carbon dots decorated with hydrogen sulfate groups as a photocatalyst for the dehydrogena-
tive cross-coupling of xanthenes with ketones, arenes and 1,3-dicarbonyl compounds that showed high
efficiency and selectivity under visible-light irradiation. The sulphated carbon dots demonstrate dual cata-
lytic properties, wherein they induced the rapid photooxidation of xanthenes in the presence of molecular
oxygen to form a hydroperoxy intermediate followed by coupling of nucleophiles catalysed by the acidic
surface functional groups. The methodology represents an operationally simple pathway for the gene-
ration of C–C coupling products in a short reaction time with wide substrate scopes under mild con-
ditions. The catalyst is easily separable and can be reused over multiple cycles with good efficiency.
Received 29th July 2019,
Accepted 28th October 2019
DOI: 10.1039/c9gc02658d
rsc.li/greenchem
pathway under mild reaction conditions.12–14 Usually another
catalyst (inorganic Lewis acid or organocatalyst) is combined
Introduction
The use of carbonaceous materials as catalysts for various with the photoredox system to act on different substrates in
organic transformations has become increasingly attractive as the same reaction pot. For example, photochemical organic
metal-free alternatives to inorganic catalysts. The interest in transformations using tandem photoredox-Lewis acid catalysts
using 2-dimensional graphene oxide and its derivatives as car- have been reported recently.15,16 Although ruthenium and
bocatalysts is growing as they demonstrate both oxidative and iridium photoredox catalysts demonstrate high efficiency, the
acidic catalytic activity making it possible to integrate multiple rarity and the cost of the resources are matters of concern.
“tandem” or “relay” catalysis in one pot.1–6 On the other hand, Furthermore, the possibility of poisoning of one catalyst by
zero-dimensional carbon dots, with interesting emission pro- another is often associated with such multi-catalytic combi-
perties tunable through the modification of surface functional- nations.17 In this regard, carbon dots, which can be easily syn-
ities or doping with heteroatoms, can offer an added advan- thesized from commonly available biomolecules and bio-
tage, as they demonstrate efficient visible-light photocatalytic masses, can be developed as a cheap, biocompatible and
activity.7–9 The tunable photoluminescence, arising from both metal free catalyst having both photooxidative and acidic pro-
quantum-confinement effects and surface functional groups perties in
along with upconversion emission properties, offers exciting engineering.
a
single entity through judicious surface
opportunities for photocatalytic applications.10,11 Visible-light
Cross-dehydrogenative coupling (CDC) has been developed
driven photocatalytic processes have emerged as an important as an important methodology in synthetic chemistry to activate
tool for various organic transformations, where the transfer of C–H bonds resulting in the formation of carbon–carbon and
energy and electrons from a light absorbing photocatalyst to carbon–heteroatom bonds in complex natural products and
non-absorbing molecules provides a green and sustainable pharmaceuticals.18–20 Under oxidative conditions, two C–H
bonds from two different molecules can be coupled to form a
new C–C bond with hydrogen as the only leaving group and
water as the sole byproduct in most cases. Usually transition
Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology
Indore, Khandwa Road, Indore 453552, India. E-mail: tridib@iiti.ac.in
metal catalysts are used as catalysts for the CDC reactions in
†Electronic supplementary information (ESI) available: Detailed description of
the presence of external oxidants.21–24 A metal-free CDC
the materials and methods, supplementary figures and tables and characteriz-
pathway was reported by Klussmann et al. where the aerobic
ation data of the synthesized compounds. See DOI: 10.1039/c9gc02658d
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oxidative coupling of activated benzylic electrophiles with of glucose and further treatment of the c-CDs with fuming sul-
several nucleophiles such as aldehydes, ketones and 1,3-dicar- furic acid respectively (details in the ESI†).32 Transmission
bonyl compounds, could be realized using organic acids as electron microscopy studies of s-CDs showed the formation of
catalysts.25–27 Although a wide range of substrates could be quasi spherical and fairly mono disperse particles. The par-
coupled, the yield of the coupling products was poor and ticles had a size distribution in the range of 2–6 nm (average
required a longer reaction time. The use of carbocatalysts for diameter of 3.2 1.5 nm) as calculated from the measurement
C–C cross-coupling was very recently reported by Loh et al. of over hundred particles (Fig. 1a and c). The high resolution
where graphene oxide was coupled with triflic acid for the TEM image of s-CDs showed the appearance of lattice fringes
aerobic cross-coupling of xanthenes and 1,2-dimethoxy- signifying the (102) lattice of graphitic carbon (Fig. 1b). The
benzene.28 However, in the absence of triflic acid, only the oxi- selected area electron diffraction (SAED) pattern confirmed the
dized product of xanthene was obtained. The use of strong crystallinity of the synthesized CDs (Fig. 1c inset). The powder
mineral and organic acids such as sulphuric acid, methane- X-ray diffraction spectra of s-CDs show a broad peak centred at
sulphonic acid, triflic acid and p-toluensulphonic acid as catalysts 2θ = 23.1°, corresponding to a d-spacing of 3.9 Å, which sup-
is often associated with serious disadvantages like corrosion, ports the introduction of functional groups on the graphitic
toxicity and separation from the reaction mixture as well as plane as the interfacing distance is greater than that of graph-
waste stream neutralization.29,30 Therefore, the development of ite (3.4 Å) (Fig. 1d). The s-CD aqueous solution showed exci-
a mild and non-toxic carbocatalytic system for C–C cross-coup- tation-dependent emission properties with the maximum
ling reactions is much desired for maintaining overall emission at 424 nm upon excitation at 300 nm and the photo-
sustainability.
luminescence shifted to a longer wavelength with increasing
We envisioned that a combination of photooxidation and excitation wavelength (Fig. 1e). The Raman spectrum shows
surface acidity might be influential in obtaining a high yield both the G band at 1586 cm−1 owing to in-plane vibrations of
of the coupling products in a shorter reaction time. We earlier the sp2 carbons and the D band at 1346 cm−1 signifying the
exploited the inherent photocatalytic and surface acidic pro- presence of sp3 defects (Fig. 1f). UV-visible and FTIR spec-
perties of carbon dots for benzylic C–H oxidation and carbon– troscopy were also carried out to further evaluate the physico-
heteroatom bond formation.31 Herein, we report that sulfo- chemical properties of the synthesized CDs (Fig. S1†). A quan-
nated carbon dots can be used as cost-effective and metal-free titative assessment of the surface functional groups was
catalysts for the CDC coupling of benzylic hydrocarbons with a carried out by standard titration methods, which suggests the
variety of nucleophilic coupling partners such as ketones, dike- presence of –OH (1.3 × 10−3 mol L−1), –COOH (0.5 × 10−3 mol
tones, and arenes (Scheme 1) under mild reaction conditions L−1) and –SO3H (0.4 × 10−3 mol L−1) functionalities on the
by taking advantage of their visible-light induced photo- s-CD surface.
catalytic and surface acidic properties.
The initial assessment of the catalytic activity of carbon
dots bearing acidic functionalities was conducted for the
aerobic cross-coupling of xanthene and cyclohexanone. On
performing the reaction using c-CDs as a catalyst under a
solvent-free and O2 rich environment (1 bar) at 70 °C for 10 h
Results and discussion
Acidic carbon dots surface functionalized with –COOH (c-CDs) (entry 2, Table 1), the coupling product (3ab) was obtained in
and –SO3H groups (s-CDs) were synthesized by carbonization 22% yield with xanthenone (3a) as the major byproduct. On
Scheme 1 Synthesis methods for the aerobic C–C coupling reaction.
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Fig. 1 (a) TEM image of s-CDs; (b) high resolution TEM image showing the lattice fringes; (c) histogram for the size distribution of s-CDs, inset
SAED pattern; (d) powder XRD spectra of s-CDs showing a broad peak at 2θ = 23.1°; (e) emission spectra of s-CDs, while excited at variable wave-
lengths; and (f) Raman spectra of s-CDs, showing the D and G bands of graphitic carbon.
Table 1 Optimization of reaction conditionsa
byproduct in both cases (Fig. 2, entries 5 and 6, Table S2†). For
comparison, we also studied the carbocatalytic activity of gra-
phene oxide and sulfonated graphene oxide (s-GO), which
afforded the coupling product with a much lower yield (39%
and 48% respectively), at 70 °C for 10 h (Fig. 2, entries 9–11
Table S2†). Using commonly available acids such as H2SO4,
benzoic acid and p-toluene sulfonic acid also could afford the
coupling product with significant yield, although they required
a much longer reaction time (Fig. 2, entries 2–4, Table S2†).
The oxidized product of xanthene, 3a, was obtained as the
major product when the reaction carried out in the absence of
any catalyst. Performing the photocatalytic reaction using s-CD
as the carbocatalyst under an argon atmosphere resulted in
insignificant yield of the product (4%), which proves that O2 is
the terminal oxidant and the s-CDs can serve as an oxidant
only to a small extent (entry 6, Table 1). The use of external oxi-
dants such as H2O2 or tert-butyl hydroperoxide (TBHP) resulted
in lower yields (entries 7 and 8, Table 1). Solvent screening
studies using a variety of solvents suggested that the best results
are obtained under solventless conditions (Table 1).
Entry
Catalyst
Solvent
Time (h)
Yieldb (%)
1
—
Ethyl acetate
—
—
—
—
—
—
—
Ethyl acetate
DCM
ACN
MeOH
—
12
10
5
5/10
3
5
3
Trace
22
31
23/15
91
4
17
26
46
10
2
c-CD (Δ)
s-CD (Δ)
s-CD (hν)/(Δ)
s-CD (hν)
s-CD (hν)
s-CD (hν)
s-CD (hν)
s-CD (hν)
s-CD (hν)
s-CD (hν)
s-CD (hν)
s-CD (hν)
3c
4d
5
6e
7f
8g
9
3
12
12
12
12
3
10
11
12
13h
18
15
86
a Unless otherwise specified, all the reactions were carried out with
xanthene (0.5 mmol) and cyclohexanone (3 mmol) as the model sub-
strates; s-CD as the catalyst (5 mg) illuminated under a 34 W blue LED
lamp (hν = 425 nm) at 25 °C under an O2 environment. b Isolated yield.
c Reactions under dark conditions at 70 °C. d Under air. e Under an Ar
environment. f TBHP. g H2O2. h Catalase.
Using the optimized conditions, the effectiveness of s-CDs
the other hand, under similar reaction conditions, s-CDs as a photocatalyst was evaluated for coupling a range of
afforded 31% of the coupling product (entry 3, Table 1). Under benzylic hydrocarbons and nucleophiles. Cyclic α-ketones
open air conditions, the auto-oxidative coupling was rather such as cyclopentanone, cyclohexanone, and cycloheptanone
sluggish and only 15% of the coupling product was obtained could be coupled with xanthene to give their corresponding
at 70 °C after 10 h (entry 4, Table 1). A dramatic enhancement 1-(9′-xanthyl)-substituted ketones with good isolated yield
in the formation of the coupling product (91% isolated yield) (80%–93%, entries 3aa–3ae Table 2) within 3–5 h under visible
in a shorter reaction time (3 h) was observed when the reaction light irradiation. Cyclic α-ketones could also be amicably
was performed under visible-light illumination at room temp- coupled with thioxanthene leading to the formation of C–C
erature (entry 5, Table 1). In comparison, c-CDs or pyrene-1- coupling products with high yield (entries 3ba–3bb, Table 2).
carboxylic acid (PCA) could afford only 20% and 15% respect- Acyclic ketones such as 2-pentanone could also be coupled
ively of the coupling product under visible-light irradiation efficiently with xanthene and thioxanthene to afford the
and a significant amount of xanthenone was formed as a corresponding coupling products in high yield within a short
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Table 2 Photocatalytic C–C coupling reaction of benzylic hydro-
carbons and various nucleophilesa
Time Yieldb
Product
(h)
(%)
Entry
n = 1, R = H R′ = H
n = 2 R = H R′ = H
n = 2 R = H R′ = –OCH3
n = 2 R = OH R′ = H
n = 3 R = H R′ = H
3
3
5
3
5
93
91
86
77
89
3aa
3ab
3ac
3ad
3ae
n = 2 R = H R′ = H
n = 3 R = H R′ = H
5
8
87
83
3ba
3bb
Fig. 2 Cross-coupling of xanthene (0.5 mmol) and cyclohexanone
(3 mmol) using various acid catalysts and carbocatalysts, using a 34 W
blue LED lamp (hν = 425 nm) at 25 °C under an O2 environment. The
reactions (1 to 7) were performed for 12 h, whereas the reactions (8 to
10) were carried out for 3 h, 7 mol% of acid catalysts were used in the
reactions (3, 4, 5, 6 and 9).
R = –CH3 R′ = –C2H5
R = –C6H5 R′ = H
R = –C6H5 R′ = –CH3
12
5
10
76
82
88
3ca
3cb
3cc
R = 4 Me-C6H5 R′ = –CH3
R = –C6H5 R′ = H
10
9
86
85
3cd
3da
reaction time (entry 3ca, Table 2). Next, we evaluated the
coupling of acetophenone derivatives with both xanthene and
thioxanthene under photocatalytic reaction conditions, all of
which resulted in high yield of the coupling products
(yield, 82%–88%, entries 3cb–3da Table 2).
R, R′ = –OCH3
R, R′ = –OC2H5
R = –CH3 R′ = –OCH3
12
12
12
93
96
74
3ea
3eb
3ec
Other nucleophiles such as β-keto esters and 1,3-diesters
also could be coupled with xanthene and thioxanthene under
the optimal reaction conditions. When ethyl acetoacetate, di-
methylmalonate or diethylmalonate was coupled with
xanthene and thioxanthene, their corresponding C–C coupling
products were obtained with high isolated yield (74%–96%,
entries 3ea–3fc Table 2). In addition, we expanded the
substrate scope of the C–C coupling reaction to other
heteroaromatic benzylic hydrocarbons with various nucleo-
philes under visible light irradiation. Isochromane and
N-phenyltetrahydroisoquinoline could be coupled effectively
with cyclic α-ketones and acetophenone derivatives to obtain
the coupling products with significant yield (entries 3ga–3jc,
Table 2). In several of the coupling products, the formation of
R, R′ = –OCH3
R, R′ = –OC2H5
R = –CH3 R′ = –OCH3
12
12
15
89
91
68
3fa
3fb
3fc
n = 1
n = 2
n = 3
5
5
5
83
81
78
3ga
3gb
3gc
1
diastereomers was clearly observed from the H NMR studies,
in most cases with a high diastereomeric ratio (Table S3†).
Furthermore, we investigated the activity of other benzylic
compounds, such as diphenylmethane, indane and tetraline,
to form coupling products with cyclohexanone; however all
these substrates were found to be inactive under the present
reaction conditions.
R = –C6H5 R′ = H
R = –C6H5 R′ = –CH3
R = 4 Me-C6H5 R′ = –CH3
5
5
5
73
86
78
3ha
3hb
3hc
Next, the compatibility of the methodology for the CH–CH
coupling reaction was evaluated through the reaction of
xanthene derivatives with various arenes under visible light
irradiation (Scheme 2). s-CDs show excellent photocatalytic
activity for the CH–CH coupling of xanthene and its derivatives
and an electron-rich arene, meta-dimethoxybenzene to afford the
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Table 2 (Contd.)
The recovered s-CDs after the fourth cycle were analyzed to
determine the relationship between the catalytic reactivity and
surface functionalities. The TEM study revealed that there is
no pronounced morphological change (Fig. S4a†). From the
emission spectra of the recovered s-CDs, a minor blue shift in
the emission peak as well as an enhancement in the fluo-
rescence intensity was observed as compared to the pristine
ones (Fig. S4b†). This suggests a possible partial reduction of
the surface in the –SO3-H functionality in the recovered
s-CDs.33 This was further evidenced from FTIR studies, where
the band at 1038 cm−1 associated with OvSvO and the band
at 1166 cm−1 corresponding to –SO3H groups are reduced sig-
nificantly (Fig. S4c†). The elemental analysis also supports the
result (Table S4†). The S 2p core level XPS spectra show that
the amount of the SO3-H functionality (169.7 V) is reduced in
the recovered s-CDs as compared to the pristine ones
(Fig. S5†). A comparative C 1s core level XPS spectra of the pris-
tine and recovered s-CDs show no significant change in the
graphitic content as well as the CvO functionalities (Fig. S6a
and b†). Raman spectroscopy studies further revealed that the
intensity ratio of D and G bands (ID/IG = 0.93) in the case of
Time Yieldb
Product
(h)
(%)
Entry
n = 1
n = 2
n = 3
5
5
10
76
80
74
3ia
3ib
3ic
R = –C6H5 R′ = H
R = –C6H5 R′ = –CH3
R = 4 Me-C6H5 R′ = –CH3
8
8
8
87
81
82
3ja
3jb
3jc
a Unless specified, all the reactions were carried out with the substrate
(0.5 mmol) and ketone (3.0 mmol) using s-CD as the catalyst (5.0 mg)
under solventless conditions using a 34 W blue LED lamp (hν =
425 nm) at 25 °C under an O2 environment. b Isolated yield.
corresponding coupling products in 80–92% isolated yields, pristine s-CDs did not change appreciably in the recovered
although the reaction required a relatively longer duration (12 h).
s-CDs (Fig. S6c†). This revealed that the graphitic pool of s-CD
To further ensure whether the catalysis process is surface is not affected during the reaction and the sulfonated groups
catalyzed or due to any leached active species, we performed play a significant role in the reaction kinetics.
the model coupling reaction under optimal reaction con-
For further investigation into the effect of surface acid func-
ditions. The reaction was stopped after 50% conversion and tional groups on the catalytic activity for the C–C coupling
the s-CD catalyst was removed from the reaction mixture by reaction, a small amount of a base (1 mL of a 500 mM
centrifugation. The reaction was further continued with the aqueous NaOH solution) was added in the reaction mixture of
supernatant for 10 h under visible light irradiation; however the model coupling reaction. The conversion of the coupling
no further coupling product was formed (Fig. S2†). The results product was reduced to 25% under visible-light illumination.
clearly suggest that the catalytic process is truly heterogeneous This suggests the effect of deprotonating the acid sites present
in nature. The s-CDs could be readily recovered and recycled on the catalyst surface on the catalytic activity. Furthermore,
for at least four runs without any significant impact on the the s-CDs were thermally treated at 200 °C, which results in
yield of the product (76%, isolated yield) (Fig. S3†).
the reconstitution of the graphitic core and removal of some of
Scheme 2 Photocatalytic CH–CH coupling reaction of benzylic hydrocarbons and various arenes. All the reactions were carried out with the sub-
strate (0.5 mmol) and arene (3 mmol) using s-CD as the catalyst (5 mg); using a 34 W blue LED lamp (hν = 425 nm) at 25 °C for 12 h under an O2
environment.
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the surface functional groups, as confirmed by elemental ana- much longer reaction time, as only the coupling step is cata-
lysis (Table S5†). The catalytic activity of the resultant s-CD for lysed by the acids. Thus, it is apparent that the photocatalytic
the coupling reaction is greatly reduced (17%) suggesting that activity of s-CDs was greatly influential towards the enhanced
the acidic functional groups along with the photocatalytic activity and selectivity for the cross-dehydrogenative coupling
activity of the carbon dots play a major role in the formation of reaction even in comparison with their two-dimensional
the coupling product.
counterpart s-GO. In the case of s-GO as the catalyst, we
Following earlier reports,25–27 a two-step mechanism is observed that the selectivity for the coupling reaction was
suggested for the C–C coupling reaction, where the first step rather poor and the oxidized product xanthenone was formed
involves the oxidation of xanthene to form a peroxo intermedi- as a byproduct with a significant amount, although acid
ate, which undergoes acid-catalyzed substitution of the per- surface-functionalized GO is known to demonstrate both oxi-
oxide group by nucleophiles to form the product. In order to dizing as well as acidic properties.6 Even s-CDs show much
substantiate the mechanistic pathway in the visible-light lower selectivity for the coupling product when the reactions
enhanced coupling reaction, a control reaction of 2-methoxy- were carried out under thermal conditions (70 °C, Table 1,
xanthene and cyclohexanone was performed under an O2 atmo- entry 3 and Table S2,† entry 8). We and others5,6 have demon-
sphere (Scheme 3a). We can observe the in situ formed hydro- strated earlier that the oxidizing and acidic properties of
peroxide intermediate by stopping the reaction after 2 hours, various carbonaceous catalysts originated from the acid func-
1
which can be isolated and structurally confirmed by H-NMR tionalities present on the surface. As the same surface func-
and HRMS (Fig. S7†). The hydroperoxide intermediate sub- tional groups participate in both steps in a consecutive reac-
sequently converts to the desired coupling product. This tion (oxidation and acidic steps), there might be a possibility
suggests the dual role of s-CD as both an oxidative as well as for reduction in acid functionalities during the oxidation
acidic catalyst. The oxidizing activity of –SO3H groups on s-CD process. The inability of the catalytic system to create a
is responsible for the formation of the xanthene hydroperoxide sufficiently stronger acidic environment might hamper the for-
intermediate, whereas the acidic functionalities on the s-CD mation of the desired C–C coupling product and the oxidized
surface catalyzed the formation of the final coupling product product is also obtained in a significant amount. FTIR studies
from the xanthene hydroperoxide intermediate. Furthermore, of s-GO nanocatalysts recovered after one cycle of the model
based on this mechanism, the formation of H2O2 is imminent coupling reaction showed a slight decrease in intensity for the
(Scheme 3b). When the 2-methoxyhydroperoxide intermediate acid functional groups (Fig. S9†). On the other hand, earlier
was reacted with cyclohexanone under thermal conditions (at reports suggested an enhancement of the surface acidity of the
50 °C) for 30 min, the formation of H2O2 was detected by the sulphonated carbon dots (s-CD) under light irradiation.35 For
iodometric method and monitored by UV-visible spectroscopy the coupling step, the enhanced surface acidity probably plays
(Fig. S8†). The results suggest the possibility of involvement of an important role in the selectivity of the coupling product
the in situ generated H2O2 as an oxidant for the coupling reac- formation.
tion. Surprisingly, even the presence of catalase as a H2O2
scavenger in the reaction medium did not have any impact on ling reaction on the s-CD surface can be substantially
the yield of the coupling product (86%) (Table 1). enhanced under visible light irradiation; we next investigated
It appears from the data in Table 1 that the rate of the coup-
It is worth mentioning that the oxidation step (conversion the involvement of free radicals in the reaction pathway. A few
of benzylic hydrocarbon to give the hydroperoxide intermedi- controlled reactions were carried out using a mixture of
ates) can also be achieved in the absence any catalyst through xanthene (1), 2-methoxyxanthenehydroperoxide (X) and cyclo-
an auto-oxidation process under an oxygen environment, hexanone and the product formation was monitored in the
however the reaction takes a longer time.34 Thus the use of a presence of free radical scavengers under visible-light
strong acid such as PTSA or H2SO4 as catalysts required a irradiation (Fig. 3a). Upon the addition of butylated hydroxy-
Scheme 3 Formation of the hydroperoxide intermediate followed by the C–C coupling product.
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Fig. 3 (a) Free radical scavenging experiment, using a mixture of xanthene, 2-methoxyxanthenehydroperoxide and cyclohexanone; (b) effect of
various radical scavengers on the photocatalytic C–C coupling reaction; (c) DMPO trapped EPR spectra resulting from DMPO binding with super-
oxide radicals (•O2−).
toluene (BHT), 3ac was obtained in 85% yield, while 90% of 1 ent photocurrent experiment further revealed the formation of
was recovered. This suggests that radical species are involved electron–hole pairs during visible light exposure (Fig. S11†).
in the oxidation of xanthene to its hydroperoxide and the The Kubelka–Munk-transformed reflectance spectrum of s-CD
Brønsted acid catalyzed substitution of the peroxide group by was used to evaluate the optical band gap, which was found to
nucleophiles was not affected by the radical scavenger. In be 2.55 eV. Cyclic-voltammetry experiments revealed the lowest
order to understand the specific radical involved in the oxi- unoccupied molecular orbital (LUMO) of s-CD at −0.68 V vs.
dation pathway, we performed a series of control experiments Hg/HgCl2 (Fig. S12†). The corresponding highest occupied
•
(Fig. 3b). When an OH radical scavenger, tert-butyl alcohol molecular orbital (HOMO) level is calculated by subtracting
(TBA), was added, no effect on the conversion was observed. the LUMO level from the optical band gap, and a value of
However, upon the addition of p-benzoquinone (BQ) as an +1.87
V indicates the high oxidizing nature of s-CD.
•O2 scavenger to the photocatalytic reaction, a reduced con- Furthermore, the release of additional protons from the ioniza-
−
version of 17% was noted. The results suggest the possible role tion of –SO3H groups on the s-CD surface under visible light
•
−
of O2 radicals in the in situ generation of the hydroperoxide irradiation creates a stronger acidic environment,36 which
intermediate. Furthermore, we employed a spin trapping elec- might enhance the conversion and selectivity of hydroperoxide
tron paramagnetic resonance technique using 5,5-dimethyl-1- intermediates to the desired coupling products.
pyrroline N-oxide (DMPO) as the probe to detect the super-
Based on the experimental results, a photocatalytic mecha-
oxide radical formed over the s-CD surface during the reaction. nism for the C–C coupling reaction is proposed (Scheme 4).
As expected, we observed the characteristic fingerprint of spin Upon visible light irradiation, the excitation of electrons from
adducts resulting from the superoxide radical in the EPR the HOMO in the s-CD framework generates reductive elec-
signal (Fig. 3c). While the addition of EDTA-2Na as a hole (h+) trons and oxidative holes. The effective electron transfer from
scavenger has no effect on the reaction, the addition of CuCl2 the LUMO of s-CD to O2 results in the formation of the super-
as an electron (e−) scavenger in the photocatalytic reaction oxide radical (•O2−) which reacts with the benzylic position of
resulted in reduced conversion (26%), suggesting electron xanthene to give the hydroperoxide intermediate. This is fol-
transfer during oxidation. Next, we explored the photo-induced lowed by the formation of a highly stabilized carbocation with
electron transfer properties of s-CDs, which were confirmed by the release of H2O2, driven by a stronger acidic environment
observing the emission quenching in the presence of either resulting from the ionization of surface –SO3H groups. Finally,
electron acceptor or electron donor molecules in solution the enol form of cyclohexanone couples with the carbocation
under visible light irradiation.
to give the desired product.
When a known electron acceptor 2,4-dinitrotoluene (DNT,
In summary, we demonstrate that s-CDs can act as a high
0.9 V vs. NHE) or electron donor N,N-diethylaniline (DEA, 0.88 performance visible-light induced photocatalyst for the aerobic
V vs. NHE)36 was added to the s-CD aqueous solution, its emis- oxidative coupling of xanthenes (or thioxanthenes) with a
sion intensity at 424 nm was efficiently quenched. The Stern– variety of nucleophiles such as ketones, 1,3-dicarbonyls and
Volmer quenching constants (KSV = τF°kq) were calculated from arenes under mild reaction conditions. The s-CDs demonstrate
the linear regression and found to be 0.4619 mM−1 and dual catalytic properties that include photoactivation of
2.53 M−1 for DNT and DEA, respectively (Fig. S10†). The transi- benzylic –CH2 groups in xanthenes in the presence of O2
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The –OSO3H functionalized carbon dots (s-CD) were syn-
thesized by treating c-CDs (1 g) with fuming sulphuric acid
(40 ml) at room temperature for 72 h under a nitrogen environ-
ment to give a dark coloured suspension. The suspension was
added into anhydrous diethyl ether with vigorous stirring in
an ice bath. The precipitates were separated from the solution
through centrifugation and washed with water and ethanol.
Synthesis of graphene oxide
Graphene oxide (GO) was synthesized from graphite powder by
a modified Hummer’s method as reported earlier38 and was
purified by dialyzing against ultra-pure water for 24 h to
remove any metal impurity. The –SO3H groups were intro-
duced by treating graphene oxide (500 mg) with fuming sul-
phuric acid (20 ml) at room temperature for 72 h under a nitro-
gen environment to prepare s-GO. The synthesized GO and
s-GOs were characterized by various spectroscopy and
microscopy techniques (details in the ESI†).
Scheme 4 Proposed mechanism for the photo-induced CDC reaction
of benzylic hydrocarbon and ketone using s-CD as the photocatalyst in
the presence of O2.
Quantitative measurement of functional groups on the s-CD
surface
resulting in the formation of the hydroperoxy intermediate,
followed by coupling of the nucleophiles catalyzed by acid
surface functionalities. In the case of carbon dots, the photo-
catalytic activity can be tailored through proper surface engin-
eering with various surface functionalities, for example, in
–CvO and –COOH terminated carbon dots, hole transpor-
tation towards the surface is more favourable than electron/
hole pair recombination thus resulting in high photocatalytic
activity. Furthermore, the carbon dot surfaces can be easily
modified or conjugated to various other functionalities to
divulge multi-catalytic sites on the same heterogeneous entity.
The high efficiency and selectivity of the coupling reaction in a
short period under visible-light irradiation clearly demonstrate
that these tiny carbonanceous materials can be used for multi-
step organic transformations in a one-pot synthesis. Several
advantages, such as easy synthesis from cheap precursors, bio-
compatibility, recyclability, possibility of integrating catalyti-
cally active entities on the surface and inherent enzyme-
mimetic activities can make carbon dots a green alternative to
metal catalysts for various multi-step organic synthesis.
A quantitative estimation of hydroxyl, carboxyl, and sulfonic
functionalities on the s-CD surface was evaluated following an
earlier report.39 First the concentration of the functional
groups was evaluated by treating a 5.0 mL aqueous solution
containing s-CDs (1.0 mg mL−1 concentration) with NaOH
(0.05 mol L−1) for 60 min at 25 °C in an ultrasonic bath. After
the removal of s-CDs by centrifugation, the supernatant was
titrated against aliquots of aqueous HCl (0.05 mol L−1) using
phenolphthalein as an indicator. The concentration of func-
tional groups was calculated to be 2.2 × 10−3 mol L−1
.
For the measurement of carboxyl and sulfonic functional
groups, a similar method was adopted except for treating a
5 mL aqueous dispersion of s-CDs (1 mg mL−1 concentration)
with sodium bicarbonate (0.05 mol L−1). The supernatant was
titrated against aqueous HCl (0.05 mol L−1) using phenolphtha-
lein as an indicator. The concentration of –COOH and –SO3H
functional groups was found to be 0.9 × 10−3 mol L−1. Similarly,
for the measurement of –SO3H functional groups, the s-CD cata-
lyst was stirred with NaCl (0.05 mol L−1) and the supernatant
was titrated against sodium hydroxide (0.05 mol L−1) using
phenolphthalein as an indicator. The concentration of –SO3H
functional groups was found to be 0.4 × 10−3 mol L−1. All the
measurements were repeated thrice to ascertain precision. From
the measurements, the concentration of –OH, –COOH and
Experimental procedures
Synthesis of carbon dots
First the carboxyl functionalized carbon dots (c-CDs) were syn- –OSO3H on the s-CD surface was calculated to be 1.3 × 10−3
thesized on a large scale by following a previously reported mol L−1, 0.5 × 10−3 mol L−1 and 0.4 × 10−3 mol L−1 respectively.
procedure37 with slight modifications. 5.0 g of dextrose in The surface functional group measurements were also carried
10 ml of oleic acid was subjected to microwave irradiation out for the s-CDs recovered after the catalytic cycles.
using a focused microwave CEM discover reactor at 150 W and
Similarly, the concentration of surface functional groups on
180 °C for 7 min. The resultant solid brownish product was graphene oxide (GO) and sulphonated graphene oxide (s-GOs)
dispersed in water and hexane and was used several times to was quantified by acid-base titration. The carboxyl and
extract the excess oleic acid. Finally the product was purified hydroxyl group concentration was found to be 1.86 × 10−4 mol
by dialyzing against ultra-pure water for 24 h to remove any L−1 and 1.94 × 10−3 mol L−1 respectively in an aqueous solu-
excess precursor. 3.2 g of c-CDs were obtained as a solid tion of 1 mg mL−1 concentration of GO. Similarly, the carboxyl,
product after the removal of water using a lyophilizer.
hydroxyl and sulfonic group concentration was found to be
6724 | Green Chem., 2019, 21, 6717–6726
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1.8 × 10−4 mol L−1 and 1.73 × 10−3 mol L−1 and 1.65 × 10−4 Jain for help and technical discussions. D. S. thanks the
mol L−1 respectively in an aqueous solution of 1 mg mL−1 con- MHRD, New Delhi for her fellowship. Help from Mr Rajan
centration of s-GO.
Kumar and Prof. Raja Shunmugam is duly acknowledged.
Photocatalytic C–C coupling of benzylic hydrocarbon with
ketones
References
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column chromatography (100–200 mesh) where a mixture of
hexane and ethyl acetate was used as the eluent.
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