Photocatalysis with Quantum Dots and Visible Light: Selective and Efficient Oxidation of Alcohols to Carbonyl Compounds through a Radical Relay Process in Water

Article abstract of DOI:10.1002/anie.201700243

Selective oxidation of alcohols to aldehydes/ketones has been achieved with the help of 3-mercaptopropionic acid (MPA)-capped CdSe quantum dot (MPA-CdSe QD) and visible light. Visible-light-prompted electron-transfer reaction initiates the oxidation. The thiyl radical generated from the thiolate anion adsorbed on a CdSe QD plays a key role by abstracting the hydrogen atom from the C?H bond of the alcohol (R¹CH(OH)R²). The reaction shows high efficiency, good functional group tolerance, and high site-selectivity in polyhydroxy compounds. The generality and selectivity reported here offer a new opportunity for further applications of QDs in organic transformations.

Full text of DOI:10.1002/anie.201700243

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201700243
German Edition: DOI: 10.1002/ange.201700243
Photocatalysis
Hot Paper
Photocatalysis with Quantum Dots and Visible Light: Selective and
Efficient Oxidation of Alcohols to Carbonyl Compounds through
a Radical Relay Process in Water
Lei-Min Zhao, Qing-Yuan Meng, Xiang-Bing Fan, Chen Ye, Xu-Bing Li, Bin Chen,
Vaidhyanathan Ramamurthy, Chen-Ho Tung, and Li-Zhu Wu*
Abstract: Selective oxidation of alcohols to aldehydes/ketones
has been achieved with the help of 3-mercaptopropionic acid
(MPA)-capped CdSe quantum dot (MPA-CdSe QD) and
visible light. Visible-light-prompted electron-transfer reaction
initiates the oxidation. The thiyl radical generated from the
thiolate anion adsorbed on a CdSe QD plays a key role by
Figure 1. Examples of some polyhydroxy natural products.
À
abstracting the hydrogen atom from the C H bond of the
alcohol (R1CH(OH)R2). The reaction shows high efficiency,
good functional group tolerance, and high site-selectivity in
polyhydroxy compounds. The generality and selectivity
reported here offer a new opportunity for further applications
of QDs in organic transformations.
spectra in the visible region, QDs offer new and versatile
possibilities to harvest light energy for chemical transforma-
tion.[6–8] We have established earlier that thiyl radicals could
be generated on the surfaces of 3-mercaptopropionic (MPA)-
capped CdSe QD in visible light.[7] However, the generated
thiyl-centered radicals coupled before being able to diffuse
into the solution and prompt further reaction. The high
homocoupling efficiency implied that the desorption of thiyl
radicals from the surface of QDs is very slow. Recent studies
have established the value of thiyl radicals in inducing
a variety of synthetically useful reactions.[9] Prompted by
these reports, we have explored the possibility of thiyl radicals
generated on the surface of QDs in performing cascade
catalysis. The value of visible-light catalysis is extended into
the realm of radical chemistry.
B
ecause of its wide applications in fragrance, confectionary,
beverage, and pharmaceutical industries, selective conversion
of alcohols to carbonyl compounds without over-oxidation
remains
a
sought-after organic key transformation.[1]
Although a few methods for the acceptor-less oxidation
have been reported,[2] most of the existing methods employed
to achieve this transformation involve toxic oxidants and
generate undesired over-oxidized products and lead to
tremendous waste.[3] Site-selective oxidation of natural prod-
ucts containing multiple hydroxy groups (examples shown in
Figure 1) continues to be a challenging task.[4] Considering the
current emphasis on green and sustainable chemistry,[5]
identification of an oxidant-free method that employs mild
and reusable reagents as catalysts to achieve the above
transformation is timely.
To achieve selective conversion of alcohols to carbonyl
compounds, we intercepted the thiyl radical generated on the
À
QDs surface with benzyl alcohols containing abstractable C
H hydrogen atoms. Using this approach, we demonstrate
herein that alcohols can be converted to aldehydes/ketones by
an efficient, selective, and mild method. Remarkably, this
method does not use any external oxidant and produces
hydrogen as the only by-product (Scheme 1). By examining
over two dozen examples, we have established that alcohols
are converted to aldehydes/ketones in water with MPA-
capped CdSe QD as catalyst and with visible light as the
energy source. Detailed mechanistic studies establish that the
initial key step is the electron transfer that promotes the
generation of reactive thiyl radical. In turn the thiyl radical
initiates cascade catalysis by a radical relay process. The
Herein, we report that the combination of quantum dots
(QDs) and visible light is a powerful method to achieve the
above goal. Owing to the quantum confinement effects, rich
surface-binding properties, broad and intense absorption
[*] L.-M. Zhao, Dr. Q.-Y. Meng, X.-B. Fan, C. Ye, Dr. X.-B. Li, Dr. B. Chen,
Prof. Dr. C.-H. Tung, Prof. Dr. L.-Z. Wu
Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and
Chemistry & Graduate University the Chinese Academy of Sciences,
The Chinese Academy of Sciences, Beijing, 100190 (P.R. China)
E-mail: lzwu@mail.ipc.ac.cn
Homepage: http://lsp.ipc.ac.cn/
Prof. Dr. V. Ramamurthy
Department of Chemistry, University of Miami
Miami, FL 33124-0431 (USA)
Supporting information, including experiment details and com-
pounds characterization, and the ORCID identification number(s)
for the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700243.
Scheme 1. Applications of surface-bound thiyl radicals in organic
chemistry.
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results reported here highlight the value of QDs as a reagent
in synthetic chemistry.
substituted benzyl alcohols, heterocyclic substituted alcohols,
and a-alkylated benzyl alcohols (Table 1 and Table 2).
Perusal of Table 1 and Table 2 leads to the following
conclusions: 1) Oxidation occurred with benzyl alcohols
Details of the preparation of MPA-capped CdSe QD and
the procedure for the oxidation of the alcohols are provided
in the Supporting Information. Briefly, a typical procedure
consisted of irradiating with purple LEDs (410 nm) a stirred
solution of an aromatic or an aliphatic alcohol (0.2 mm),
MPA-capped CdSe QDs (5.0 ꢀ 10À6 m), and NiCl2 (0.01m) in
aqueous solution at pH 9 in argon atmosphere. Following
irradiation for 6 h organic products were diluted with
acetonitrile or extracted with ethyl acetate and filtered
through a membrane filter (22 mm in diameter). Products in
the filtrate were analyzed by either GC or HPLC. One of the
products of oxidation, hydrogen gas, was analyzed by GC
using a thermal conductivity detector (TCD) with methane as
the external standard. The optimum condition for the
oxidation was established by irradiating the samples for
different lengths of time, at different pH values, different
concentrations of Ni2+, and in the presence of different co-
catalysts. Results of these studies are presented in Figure S1 in
the Supporting Information.
Table 2: Oxidation of secondary alcohols.[a]
[c]
Entry Substrate
Convers. Selec.[b] H2
[%]
[%]
1
2
3
4
5
6
7
1ba, R1 =H, R2 =H, R3 =Me
99
86
96
80
95
94
62
98
66
82
87
45
89
42
98
71
98
57
89
94
53
50
31
1bb, R1 =OMe, R2 =H, R3 =Me
1bc, R1 =H, R2 =OMe, R3 =Me
1bd, R1 =H, R2 =H, R3 =Et
1be, R1 =H, R2 =H, R3 =Ph
1bf, R1 =OMe, R2 =H, R3 =PMP 90
1bg, R1 =Cl, R2 =H, R3 =PCP
1bh, cyclopentanol
81
55
52
8[d]
9[d]
When benzyl alcohol was subjected to oxidation by the
above procedure, it was quantitatively transformed to ben-
zaldehyde with evolution of a stoichiometric amount of
hydrogen gas. The produced benzaldehyde was stable under
the irradiating conditions (6 h). The generality of the above
oxidation was established by investigating over two-dozen
1bi, cyclohexanol
[a] Conditions: 0.1 mmol 1, MPA-CdSe QDs (5.0ꢀ10À6 m, 5 mL), NiCl2
(1ꢀ10À4 m, 200 mL) at a pH value near 9, bubbled under Ar, irradiation of
l=410 nm at room temperature, unless noted, reactions were irradiated
for 6 h. [b] Determined by HPLC with naphthalene as an internal
standard. [c] Determined by GC-TCD with methane as an internal
standard. [d] Irradiated for 24 h, determined by GC-FID with octane as an
internal standard.
Table 1: Oxidation of primary alcohols.[a]
substituted with both electron-donating and withdrawing
groups at the para-position (R1) with the latter being a bit
slow. 2) The bulkiness of the group at the ortho-position of the
aryl ring (R2) had no effect on the conversion and yield of the
reaction, which is in sharp contrast to a previous study.[7] 3) a-
Alkylated benzyl alcohols (secondary benzyl alcohol) reacted
as efficiently as the primary benzyl alcohols (Table 2).
4) Aliphatic primary alcohols such as propanol, 3-phenyl-
propanol, cyclopentanol and cyclohexanol reacted slowly
compared to the benzyl alcohols.
[c]
Entry
Substrate
Convers.
[%]
Select.[b]
[%]
H2
1
2
3
4
5
6
7
8
1aa, R1 =H, R2 =H
1ab, R1 =F, R2 =H
1ac, R1 =Cl, R2 =H
1ad, R1 =Br, R2 =H
1ae, R1 =Me, R2 =H
1af, R1 =H, R2 =Me
1ag, R1 =OMe, R2 =H
1ah, R1 =H, R2 =OMe
1ai, R1 =i-C3H7, R2 =H
1aj, R1 =OH, R2 =H
1ak, R1 =H, R2 =OH
1al, R1 =CF3, R2 =H
1am, R1 =CN, R2 =H
1an, 2-furyl-methanol
1ao, 2-pyridylmethanol
1ap, 3-phenylpropanol
1aq, propanol
92
99
85
87
87
88
85
95
92
62
93
72
46
95
79
43
22
98
99
98
98
98
85
93
90
98
47
88
68
47
14
74
46
12
87
91
80
78
92
62
72
96
55
52
92
44
49
19
83
70
15
We believe the faster reaction of benzyl alcohols with
respect to the aliphatic alcohols (Table 1 and Table 2) is due
to the difference in bond dissociation energies of the benzylic
and aliphatic C H bonds.[10] This prompted us to test whether
À
9
selective oxidation of the benzylic alcohol could be achieved
in the presence of an aliphatic alcohol. In this context results
of the reactions carried out with 1-phenyl-1,4-butylene glycol,
1-methyl-3-phenyl-propylene glycol and 6b-hydroxy-17b-
estradiol (detailed synthetic processes see the Supporting
Information), the molecules that contain both aliphatic and
benzylic alcohols are revealing. As illustrated in Scheme 2 in
all three examples the benzylic alcohol was selectively
oxidized within six hours. To establish the value of our
method in large-scale oxidation of alcohols, we have success-
fully performed gram-scale oxidation reactions of estradiol
[Scheme 2, Equation (III)] and para-bromobenzyl alcohol
2ad under standard conditions with isolated yields of 70%
and 80%, respectively (details are found in the Supporting
10
11
12
13
14
15
16
17[d]
[a] Conditions: 0.1 mmol 1, MPA-CdSe QDs (5.0ꢀ10À6 m, 5 mL), NiCl2
(1ꢀ10À4 m, 200 mL) at a pH value near 9, bubbled under Ar, irradiation at
l=410 nm at room temperature, unless noted, reactions were irradiated
for 6 h. [b] Determined by HPLC with naphthalene as an internal
standard. [c] Determined by GC-TCD with methane as an internal
standard. [d] Irradiated for 24 h, determined by GC-FID with octane as an
internal standard.
2
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adduct of thiyl radical with DMPO (labeled as b, aN = 14.6 G,
aH = 16.1 G) appeared (Figure S4) similar to our previous
observation.[7] This signal was merged with another one
(labeled as a, aN = 16.5 G, aH = 22.1 G) corresponding to the
adduct of the hydrogen radical with DMPO. The coupling
constants are consistent with reported values for the spin
adducts of thiyl and hydrogen radicals with DMPO.[7,13]
Except for these two trapped DMPO radicals no other signals
were detected. Unlike in our earlier study where the thiyl
radicals had no choice except to dimerize to yield disulfides,[7]
in the current study they were provided with a choice to
generate a new organic radical by abstracting a hydrogen
atom from a substrate present in aqueous solution.
Scheme 2. Selective oxidation of polyhydroxy compounds.
To establish there is indeed hydrogen abstraction, oxida-
tion of p-tolyl alcohol with partially and fully deuterated
benzylic hydrogen atoms was conducted. The extent of the
kinetic isotope effect (KIE) was estimated using deuterated
p-tolyl alcohols HH-1ae, HD-1ae, and DD-1ae (Scheme 3),
Information). Remarkable selectivity and generality noted
above illustrate the potential of this green methodolgy in
converting benzylic alcohols to the corresponding aldehydes/
ketones in various skeletons including the ones that occur in
natural products.
To establish the mechanism of oxidation we carried out
a series of experiments with benzyl alcohol as the model
compound. Control experiments established that the oxida-
tion does not proceed in the absence of visible light, co-
catalyst NiCl2, and capping agent MPA. The best yield of the
oxidized product was obtained when the mixture was
irradiated between 6–10 h (Figure S1 and Table S4). Oxida-
tion performed in the presence of NiCl2, CoCl2, FeCl2, and
CuCl2 revealed that the best co-catalyst is NiCl2 (Figure S1
and Table S1). The fact that the reaction worked best in the
basic pH range 8–11 (Figure S1 and Table S3), close to the
pKa value of aliphatic thiol,[10b] suggested that MPA is
adsorbed on CdSe QD as thiolate anion rather than as
a neutral thiol. Upon excitation at 410 nm, MPA-CdSe QDs
showed two emissions one centered around 477 nm (band
edge emission) and the other at 607 nm (trap emission;
Figure S2a).[11] Both were quenched upon addition of Ni2+.
The fact that the Stern–Volmer plots based on emission
intensity and lifetime quenching (Figure S2b) did not overlap
and the apparent quenching rate constant estimated (1.09 ꢀ
1012 molÀ1·sÀ1·LÀ1) from the emission quenching studies was
more than a diffusion constant in water[12] indicated that the
Ni2+ ion is physically adsorbed on the QD. The above
quenching data and our previous results[7] are consistent with
the conclusion that the initial step of oxidation is electron
transfer from the conduction band of the CdSe QD to the
physisorbed Ni2+. Quenching of the oxidation by the good
electron acceptor AgNO3 (Figure S3) is in agreement with the
above conclusion. The fact that the reaction was slowed by the
good “hole quencher” ammonium oxalate suggested that the
second step in the sequence is the “hole transfer” from the
valence band of the catalyst to the thiolate anion (Figure S3).
The above hole transfer would be expected to result in
a thiyl radical (RS·) as the product. Although attempts to
detect thiyl radical by an electron paramagnetic resonance
(EPR) spectrum were not successful, the spectrum of the thiyl
Scheme 3. Probing the role of deuterium on the photocatalyzed
oxidation of alcohols: effect of deuterium labeling on the reactive
alcohol and solvent.
and the kH/kD ratio was calculated according to 1H NMR
detection. As illustrated in Scheme 3, the KIE for intra-
molecular and intermolecular competition were 2.32 Æ 0.09
À
and 3.35 Æ 0.12, respectively. This suggested that C H
abstraction by the thiyl radical indeed occurs and it is the
slow step in the oxidation process.[14] Unfortunately, we could
not detect the expected benzyl radical by EPR spectroscopy.
When the oxidation of p-toluyl alcohol was conducted in D2O
instead of H2O, D2 was detected in place of H2. This suggested
that the hydrogen abstracted from benzyl alcohol quickly
equilibrated with water and the product hydrogen resulted
from the reduction of H+ from bulk water by Ni2+ adsorbed
on the surface of MPA-CdSe QDs.
To ascertain the formation of disulfide, as reported in our
earlier work,[7] during the course of the alcohol oxidation, we
monitored the signals of disulfide in water carefully by
1H NMR spectroscopy. After 6 h irradiation of the QD
samples with visible light a very small amount of disulfide
was formed (Figure S9-a). It is quite likely that the disulfides
formed may have reversed to active thiols. To examine such
a possibility, we conducted the oxidation of 3,3’-dithiodipro-
panoic acid-functionalized (the disulfide of MPA) QDs
sample. Upon irradiation of the reaction mixture of the
disulfide-stabilized QD sample signals due to anions of the
radical
trapped
by
5,5-dimethyl-1-pyrroline-N-oxide
(DMPO) was recorded after irradiation (l = 400 nm) of an
aqueous solution of benzyl alcohol, MPA-capped CdSe QD-
Ni2+, and DMPO for 15 s. A signal corresponding to the spin
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disulfide and MPA were observed while the yield of the
carbonyl product from alcohol was reduced by 30% as
compared with that of MPA-stabilized QDs (Figure S9-b).
When increasing the concentration of disulfide from 1 equiv-
alent to 10 equivalents, the reactivity was almost the same as
that of MPA-CdSe QDs under identical conditions (Fig-
ure S9-c). These results show that disulfide could also be an
active relay species though less effective than MPA.
Because MPA interacts with the surface of CdSe QDs
much more strongly than the product disulfide, the latter
would be expected to diffuse into solution leading to the
amount of sulfur on the surface of QDs to decrease. This is
indeed the case. A set of characteristic X-ray photoelectron
spectroscopy (XPS) study of the MPA-CdSe-QDs-Ni2+ cata-
lyst before and after the reaction showed that the main
elements of MPA-CdSe QDs remained unchanged (Table S7)
except for the reduced amount of sulfur upon irradiation
(Table 3). The longer the time of irradiation, the greater is the
Scheme 4. Proposed mechanism for the MPA-CdSe QD-Ni2+-photo-
catalyzed oxidation of alcohols in water.
the Ni2+ catalyst adsorbed on the MPA-CdSe QD. Overall,
the conversion requires two photons, the catalyst MPA-
capped CdSe QD, the co-catalyst Ni2+, and bulk water. Water
being the source of hydrogen atoms has been established by
the H2O/D2O studies detailed above. Adsorbed MPA and
Ni2+ act as primary electron donor and acceptor, respectively,
to the excited CdSe QDs. The oxidation is clean and the
products aldehyde/ketone and hydrogen gas are formed
nearly quantitatively.
Table 3: XPS composition of MPA-CdSe QDs and the precipitates, and
the atomic percentage of elements in each sample obtained from raw
XPS data.[a]
In conclusion, we have designed a novel dehydrogenative
oxidation method by which a number of water-insoluble
alcohols can be selectively transformed to the corresponding
aldehydes/ketones under oxidant-free condition. This method
employs water-soluble CdSe QDs as catalyst, Ni2+ ions as co-
catalyst, and MPA and water as relay reagents. The procedure
shows high efficiency, high selectivity, and good functional
group tolerance. Generation of a reactive radical by a relay
process has opened up a new opportunity to explore QDs
chemistry for organic transformations. We believe that the
thiyl radicals generated in situ from QDs in visible light have
considerable potential in facilitating radical relays to build
complex molecules, which is being currently actively pursued
in our laboratory.
Cd [%] S [%] Se [%] Cd/Se S/Se Cd/S
MPA-CdSe-QDs
7.13
10.46 1.87
13.68 2.50
3.81
3.27
4.71
5.59 0.68
5.47 0.60
4.55 1.03
MPA-CdSe-QDs-Ni2+ 8.17
MPA-CdSe-QDs-Ni2+ 7.25
7.01
1.54
3 h
MPA-CdSe-QDs-Ni2+ 7.46
6 h
6.75
1.46
5.11
4.62 1.10
[a] Cd, S, and Se values were obtained from XPS. Atoms ratio of Cd, S and
Se (Cd/Se, Cd/S, and S/Se) was calculated from integrated peak area
fitted from Cd, S and Se signals in XPS data.
decrease of the amount of sulfur in the QDs. This observation
is consistent with the redshifts found in the UV/Vis absorp-
tion and emission spectra (Figures S5 and S6) and the slight
aggregation of QDs seen by transmission electron microscopy
(TEM; Figure S7). The effective conversion of alcohols to
carbonyl compounds imply a higher efficiency of cascade
catalysis initiated by a thiyl radical relay and a slower
desorption of thiyl radicals from the surface of QDs to form
disulfides in the course of irradiation.
Based on the above experimental observations, we
propose a mechanism for the visible-light-induced photo-
catalytic oxidation of benzyl alcohols to benzaldehydes, which
involves the steps illustrated in Scheme 4. The first step in this
sequence is the absorption of light by MPA-CdSe QDs to
generate a hole (in the valence band) and an excess electron
(in the conduction band) pair.[15] The key steps in the
oxidation process are: 1) hole transfer from the valence
band to the adsorbed thiolate anion or disulfide to generate
thiyl radical. 2) Abstraction of a hydrogen atom from the
reactant benzyl alcohol by the thiyl radical to generate
a benzylic radical (Table S5), and 3) disproportionation of
two benzylic radicals generated by the above process to
benzyl alcohol and benzaldehyde. Finally, the abstracted
hydrogen atoms are released as protons by equilibration of
RSH with bulk water and these are reduced to hydrogen by
Acknowledgements
We are grateful for financial support from the Ministry of
Science and Technology of China (grant numbers
2013CB834804, 2013CB834505, and 2014CB239402), the
National Science Foundation of China (grant numbers
21390404 and 91427303), the Strategic Priority Research
Program of the Chinese Academy of Science (grant number
XDB17030200), and the Chinese Academy of Sciences.
Ramamurthy V. acknowledges the Chinese Academy of
Sciences for a fellowship and the US National Science
Foundation (grant number CHE-1411458) for its support.
Conflict of interest
The authors declare no conflict of interest.
4
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Manuscript received: January 9, 2017
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Photocatalysis
Selective oxidation of alcohols to alde-
hydes/ketones has been achieved with 3-
mercaptopropionic acid-capped CdSe
quantum dots (QDs) and visible light. A
visible-light-prompted electron-transfer
reaction initiated the oxidation. The thiyl
radical generated from the thiolate anion
adsorbed on a CdSe QD plays a key role
L.-M. Zhao, Q.-Y. Meng, X.-B. Fan, C. Ye,
X.-B. Li, B. Chen, V. Ramamurthy,
C.-H. Tung, L.-Z. Wu*
&&&& — &&&&
Photocatalysis with Quantum Dots and
Visible Light: Selective and Efficient
Oxidation of Alcohols to Carbonyl
Compounds through a Radical Relay
Process in Water
À
by abstracting the hydrogen atom of C H
bond from the alcohol.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!