Silver/manganese dioxide nanorod catalyzed hydrogen-borrowing reactions and tert-butyl ester synthesis

Article abstract of DOI:10.1177/1747519821989963

Silver/manganese dioxide (Ag@MnO₂) nanorods are synthesized and characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray powder diffraction, and X-ray photoelectron spectroscopy. It was discovered that Ag@MnO₂ nanorods can realize hydrogen-borrowing reactions in high yields and are also effective for the synthesis of tert-butyl esters from aryl cyanides and tert-butyl hydroperoxide in a short period of time. Mechanistic experiments revealed that this catalytic system acts as a Lewis acid in hydrogen-borrowing reactions, while the synthesis of tert-butyl esters occurs through a radical pathway. This is the first report on the excellent catalytic activity of Ag@MnO₂ nanorods as a catalyst.

Full text of DOI:10.1177/1747519821989963

989963CHL0010.1177/1747519821989963Journal of Chemical ResearchLuo et al.
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Journal of Chemical Research
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Silver/manganese dioxide nanorod catalyzed
hydrogen-borrowing reactions and tert-butyl
ester synthesis
https://doi.org/10.1177/1747519821989963
DOI: 10.1177/1747519821989963
journals.sagepub.com/home/chl
Huanhuan Luo1, Yike Yang1,2, Bobin Yang1,
Zhaojun Xu1 and Dawei Wang1
Abstract
Silver/manganese dioxide (Ag@MnO2) nanorods are synthesized and characterized by scanning electron microscopy,
transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray powder diffraction, and X-ray
photoelectron spectroscopy. It was discovered that Ag@MnO2 nanorods can realize hydrogen-borrowing reactions in
high yields and are also effective for the synthesis of tert-butyl esters from aryl cyanides and tert-butyl hydroperoxide
in a short period of time. Mechanistic experiments revealed that this catalytic system acts as a Lewis acid in hydrogen-
borrowing reactions, while the synthesis of tert-butyl esters occurs through a radical pathway. This is the first report on
the excellent catalytic activity of Ag@MnO2 nanorods as a catalyst.
Keywords
alkylation, dehydrogenation, hydrogen-borrowing, nanoparticle, nanorod
Date received: 23 August 2020; accepted: 6 January 2021
NH2
OH
+
Ag@MnO2
Ag@MnO2
N
H
to
95%
Up
O
R
CN
+
R
O
OOH
to
94%
Up
High catalytic activity, Easy recyclability
enzymes and Aspergillus oryzae, are also used in hydrogen-
borrowing reactions along with the development of
enzymes.16 Although some homogeneous noble metals are
Introduction
Hydrogen-borrowing methodology, also known as hydro-
gen auto-transfer, is a powerful strategy for reduction reac-
tions that avoids using molecular hydrogen directly and the
synthesis of complexes using other reactions.1–3 This meth-
odology has been developed for many years as a useful tool
in synthetic organic chemistry.4 It also provides a sustaina-
ble and cost-effective approach for the synthesis of substi-
tuted amines and ketones, which are important groups in
many bioactive molecules (Figure 1), without the need for
toxic alkyl halides or borohydrides.5 It has been demon-
strated that transition-metal complexes are effective cata-
lysts for hydrogen-borrowing reactions, as are noble and
1Key Laboratory of Synthetic and Biological Colloids, Ministry of
Education, School of Chemical and Material Engineering, Jiangnan
University, Wuxi, P.R. China
2Department of Criminal Science and Technology, Henan Police
College, Zhengzhou, P.R. China
Corresponding author:
Dawei Wang, Key Laboratory of Synthetic and Biological Colloids,
Ministry of Education, School of Chemical and Material Engineering,
Jiangnan University, Wuxi 214122, P.R. China
non-noble metals.6–15 Besides, biocatalysts, such as known Email: wangdw@jiangnan.edu.cn
Creative Commons Non Commercial CC BY-NC: This article is distributed under the terms of the Creative Commons
Attribution-NonCommercial 4.0 License (https://creativecommons.org/licenses/by-nc/4.0/) which permits non-commercial use,
reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and
Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage).

2
Journal of Chemical Research 00(0)
effective catalysts, there are catalyst-product separation and gold, while others have reduced catalytic ability after
and recycling issues involved in the use of these catalytic several cycles. Inspired by this, we decided to study
systems. Therefore, heterogeneous catalysts with recycla- nanorods as heterogeneous catalysts in hydrogen-borrow-
bility have been developed such as supported catalysts. Our ing reactions and in the synthesis of tert-butyl esters, a
group has engaged in developing heterogeneous catalysts functional group present in many natural products, poly-
for dehydrogenation and hydrogen-borrowing reactions mers, and pharmaceuticals.23
that are recyclable, such as polymer Zr-CIA,17 POP-Ir,18
Nanoparticles, in the form of nanorods, are novel sus-
Cu(binap)I]2,19 TTA-Au-NG,20 NQ-MR,21 and Au@HT.22 tainable nano catalysts that solve many issues in modern
However, some catalytic systems are effective as catalysts, synthesis and catalysis.24,25 Such nano catalytic systems
but demonstrate poor recyclability and stability, for have effective active sites and usually demonstrate much
example, catalysts with noble metal cores, such as iridium better catalytic performance and good recovery characteris-
tics.26 Manganese oxides (MnO2) nanorods are known as
inexpensive and nontoxic materials, which can be used as
environmental friendly catalysts.27–30 A recent survey of
manganese oxide (MnO2) nanorods indicates that when
incorporating noble metals, for example, Pt, Au, and Ag,
the nano particles have improved activity. In addition, Ag
nano particles have been shown to have outstanding ability
as catalyst in many organic reactions.30–33 Based on our
recent discoveries on hydrogen-borrowing reactions34–38
and the properties of manganese oxides (MnO2) nanorods
and Ag nanoparticles, we herein report the synthesis and
application of Silver/manganese dioxide (Ag@MnO2)
nanorods as a catalyst, which has higher catalytic activity
after several cycles and which is much cheaper than other
catalysts. The Ag@MnO2 nanorods are characterized by
several physical techniques. This synthetic catalytic system
Figure 1. Examples of biologically active amines.
Figure 2. ((a) and (b)) SEM images of Ag@MnO2. ((c) and (d)) TEM images of Ag@MnO2.

Luo et al.
3
Figure 3. EDS images of Ag@MnO2.
was successfully utilized in hydrogen-borrowing reactions
and for the synthesis of tert-butyl esters.
Results and discussion
Characterization of the Ag@MnO2 nanorod
The supported Ag@MnO2 nanorods were synthesized by a
hydrothermal method39 and were characterized through
scanning electron microscopy (SEM), energy dispersive
X-ray spectroscopy (EDX), X-ray powder diffraction
(XRD), transmission electron microscopy (TEM), and
X-ray photoelectron spectroscopy (XPS). The SEM and
TEM images in Figure 2 reveal that silver nanoparticles are
supported on α-MnO2 nanorods. The morphologies and
structural characteristics of the Ag@MnO2 nanorods could
be observed diametrically by SEM and TEM. As shown in
Figure 2, it is clearly observed that the surface of the MnO2
nanorods is coated with many small silver particles. The
Figure 4. XRD pattern of Ag@ MnO2.
EDS images revealed a peak at 2.93keV demonstrating that peaks located at 368.1eV for Ag 3d3/2 and 374.2eV for Ag
metallic silver nanoparticles are present on the MnO2 3d5/2, with a spin energy separation of 6.1eV, which
nanorods (Figure 3).
explains that silver is present in the metallic state.
Figure 4 shows the X-ray diffraction patterns of the syn-
thetic α-MnO2 and Ag@MnO2. By comparing to previous Catalytic activity. In order to test the catalytic activity of
wok,40–42 the XRD pattern of Ag@MnO2 revealed the pres- the Ag@MnO2 nanorods, the classical hydrogen-borrow-
ence of α-MnO2 (JCPDFS 44-0141) and metallic silver ing reaction between benzyl alcohol (1a) and aniline (2a)
(JCPDFS 87-0597). There is no Ag, AgOx, or other silver was selected (Table 1). After a series of experiments, it
oxide species’ diffraction signal, which may reveal good was found that Ag@MnO2 revealed catalytic activity in
dispersion of silver particles on the surface of α-MnO2. toluene (entry 8), with only small amount of the target
From the two patterns delineated in Figure 4, the diffraction product, N-benzylaniline (3a) produced in solvents such
peaks at 38.12°, 44.28°, 64.43°, and 81.54° were assigned as EtOAc, DCM, and MeOH (Table 1, entries 1–7), which
to the (111), (200), (220), and (222) crystal faces of silver indicates that there may be a similar solvent effect to our
indicating that silver nanoparticles had been successfully previous work.37,38 Besides, compared to room tempera-
supported on the α-MnO2 nanorods.
ture and 60°C, it was observed that this reaction produced
The chemical composition and surface chemical states a higher yield at 110°C (Table 1, entries 8 and 9). This
of the Ag@MnO2 composite were also examined by XPS. result may disclose that it has a slightly higher energy bar-
Four elements (C, Ag, Mn, and O) were identified from the rier. Further screenings showed that N-benzylaniline (3a)
full-survey-scan spectrum of the Ag@MnO2 composite is obtained in a higher yield by replacing Na2CO3 with
(Figure 5(a)). The Ag 3d spectrum shows two obvious NaOH as the base (Table 1, entries 9–13). It seems that

4
Journal of Chemical Research 00(0)
the stronger the alkalinity is, the higher the isolated yield
of N-benzylaniline (3a) is. Therefore, the optimal condi-
tions are as follows: Ag@MnO2 as the catalyst, NaOH as
the base, and at reflux in toluene.
(a)
Having established the optimum reaction conditions, the
substrate scope of this hydrogen-borrowing reaction was
explored. As shown in Table 2, all of the results are similar
to the previous work,43 different amines and alcohols could
be employed, and it was found that this transformation was
not influenced to any great extent by donating or withdraw-
ing groups on the aromatic ring of the amine substrates
such as methoxy (–OCH3), hydroxyl (–OH), chloro (–Cl),
bromo (–Br), and fluoro (–F) (Table 2, 3a–c and 3e–h).
However, for the alcohol substrates, a benzyl alcohol con-
taining the electron-withdrawing group, chloro (–Cl) (Table
2, 3l) gave a lower yield than examples with an electron-
donating group such as methyl (–CH3) and methoxy
(–OCH3) (Table 2, 3j and 3k). Comparing the yields of 3b,
3c, and 3g, there was no apparent steric hindrance effect in
this reaction. A substrate with a heteroatom, 2-aminopyri-
dine (Table 2, 3d), could also be tolerated in this transfor-
mation. In addition, furfuryl alcohol (Table 2, 3o) and alky
1-substituted products 3i, 3m, and 3n were isolated in
good-to-high yields.
(b)
Considering the importance and application of tert-butyl
esters in organic synthesis,23 the reaction of aryl cyanides
with tert-butyl hydroperoxide (TBHP) using the Ag@
MnO2 catalytic system was explored. A range of pheny-
lacetonitriles and TBHPs containing electron-rich or elec-
tron-deficient groups were employed. The desired products
were obtained through C–CN bond cleavage and esterifica-
tion in good to excellent yields and the results are summa-
rized in Table 3. Phenylacetonitrile derivatives bearing
electron-rich groups, like methoxy, methyl, and hydroxyl,
reacted smoothly and were transferred into the correspond-
ing esters 6a, 6b, 6g, and 6h in high yields, while the sub-
strates containing electron-withdrawing groups, such as
nitro, bromo, and chloro, were also converted into desired
products in moderate-to-high yields (6d–f and 6i–k).
Comparing the yields of 6f and 6i, it was found that the
ortho-substituted arylacetonitrile substrate produced a
slightly lower yield of the desired product, which indicated
that a weak steric hindrance effect might exist in this reac-
tion. Similar results were also observed by comparing 6e,
6j, and 6k, with the yields being slightly lower for meta-
and ortho-substituted substrates. To our surprise, tert-amyl
hydroperoxide and (2-hydroperoxypropan-2-yl)benzene
could also be converted into the corresponding esters 6n
and 6o, respectively, in high yields. A substrate with a
hydroxy group, 4s, was also compatible providing an excel-
lent yield of product 6g.
Figure 5. (a) XPS wide-scan spectrum of Ag@MnO2 and (b)
high-resolution spectrum of Ag.
Table 1. Optimization of reaction conditions.a,b
Entry
Solvent
Base
T (°C)
Yield (%)b
1
2
3
4
5
6
7
8
Toluene
EtOAc
DCM
MeOH
Et2O
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaOH
rt
rt
rt
rt
rt
rt
rt
60
110
110
110
110
110
13
<5
<5
<5
<5
<5
<5
21
56
88
81
67
DMF
DCE
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
9
10
11
12
13
Cs2CO3
K2CO3
NaOAc
Control experiments and activity
explorations
73
To gain further insight into the mechanism, control experi-
ments were performed (Scheme 1). Therefore, 2,2,6,6-tetra-
methyl-1-piperidinyloxy (TEMPO) was used in the Ag@
MnO2 nanorod catalyzed reactions as a radical scavenger,
and the experiments revealed that nearly the same yields of
DCM: dichloromethane; DMF: dimethylformamide; DCE:
dichloroethane.
aConditions: 1a (0.5mmol), 2a (0.75mmol), catalyst (10mg), base
(1.0mmol), solvent (3.0mL), and 12h.
bIsolated yields.

Luo et al.
5
Table 2. Substrate scope experiments.a,b
aConditions: 1 (0.50mmol), 2 (0.75mmol), Ag@MnO2 (10mg), NaOH (0.75mmol), toluene (3.0mL), 12h, and 110°C.
bIsolated yields.
the desired products were obtained. However, totally differ- radical pathway. Although the exact mechanism is still
ent results were observed for the reaction of arylcyanide 4a, unclear, based on the literature33,34 and the control experi-
with TBHP. Therefore, these control experiments imply ments, we have proposed a tentative mechanism (Scheme 3).
that radicals are not involved in the hydrogen-borrowing According to the previous work,35 it is possible to generate
reaction, but that they are involved in the synthesis of tert- tert-butoxyl and tert-butylperoxy radicals in this system.
butyl esters.
Intermediate radical I might then be generated, which is able
Based on these experiments and the work of other to form the intermediate II by trapping TBHP. Intermediate
groups,33–35 we propose a tentative mechanism for the II could be oxidized by TBHP to form intermediate III. The
hydrogen-borrowing reaction (Scheme 2). First, the target ester is finally obtained through the attack of tert-butyl
alcohol is oxidized to an aldehyde via dehydrogenation alcohol on III.
by the Ag@MnO2. At the same time, a silver-hydride
complex is formed by hydride coordination. Next, the
aldehyde condenses with the amine to form imine.
Recyclability of the catalyst
Finally, the imine intermediate is reduced to give the tar- Recycling the Ag@MnO2 nanorods was explored by
get product.
repeating the hydrogen-borrowing reaction of benzyl alco-
As for the reaction of the arylcyanide with TBHP, we hol (1a) and aniline (2a). The Ag@MnO2 nanorods were
inferred from the control experiment that it proceeded via a separated conveniently by centrifugation, followed by

6
Journal of Chemical Research 00(0)
Table 3. Expansion of the substrate scope of arylcyanides with hydroperoxides.a,b.
aConditions: 4 (1.0mmol), 5 (3.0mmol), Ag@MnO2 (10mg), KOH (3.0mmol), CH3CN (3.0mL), 60°C, and 1.5h.
bIsolated yields.
Scheme 1. Control experiments.
Scheme 2. The proposed reaction pathway.
washing with EtOH and H2O, and then vacuum freeze-dry-
To help explain these observations involving the Ag@
ing for 12h. Subsequent experiments with the recovered MnO2 nanorod catalyst, SEM studies on recycled nanorod
catalyst revealed that only slightly lower yields of product catalyst showed that the recovered catalyst had excellent
3a were achieved over five cycles (Figure 6).
stability (Figure 7).

Luo et al.
7
moderate-to-high yields. Further investigation on the appli-
cation of this catalyst in other reactions systems is in
progress.
Conclusion
In conclusion, we have reported the synthesis and applica-
tion ofAg@MnO2 nanorods, which have been characterized
through SEM, TEM, EDX, XRD, and XPS. This synthetic
catalytic system was successfully applied in hydrogen-bor-
rowing reactions and for the synthesis of tert-butyl esters in
Experimental
Synthesis of Ag@MnO2
The α-MnO2 nanorods were synthesized by hydrothermal
method according to the reported procedure.39 Next, 400mg
of α-MnO2 nanowires and 130mg of PVP (polyvinyl pyr-
rolidone) were added to 100mL of EG (ethylene glycol).
The obtained suspension was transferred to a 250 mL
round-bottom flask and kept under vigorous stirring at
90°C for 20min. Then, NaBH4 (1.0mmol) and 2mL of
0.1M AgNO3 (aq., 5%) solutions were sequential added to
the reaction flask. This mixture was kept under vigorous
stirring for another 12h to produce Ag@MnO2 (Ag:
5%w/w), which were washed three times with ethanol
(15mL) and water (15mL) by successive rounds of cen-
trifugation at 7000r/min for 8min and removal of the
supernatant. Finally, the product was dried under vacuum
condition.
Scheme 3. A possible reaction mechanism for the synthesis of
the tert-butyl esters.
General procedure for synthesis of 3a. To 20-mL colorimet-
ric tube was added 1a (46.5mg, 0.5mmol), 2a (81mg,
0.75mmol), Ag@MnO2 nanorod catalyst (10mg), NaOH
(30.0mg, 0.75mmol), and toluene (3.0mL). The mixture
was refluxed in air for 12h and then cooled to room tem-
perature. The resulting solution was directly purified by
column chromatography with petroleum ether/ethyl ace-
tate as eluent to give the desired product N-benzylaniline
(3a). Colorless oil (80.5mg, 88%), 1H NMR (400MHz,
CDCl3) δ 7.34–7.24 (m, 4H), 7.23–7.17 (m, 1H), 7.11 (t,
J=7.8Hz, 2H), 6.65 (t, J=7.2Hz, 1H), 6.57 (d, J=7.8Hz,
2H), 4.26 (s, 2H), 4.07–3.81 (m, 1H); 13C NMR (101MHz,
CDCl3) δ 148.2, 139.5, 129.3, 128.7, 127.5, 127.3, 117.6,
112.9, 48.4.
General procedure for synthesis of 6a. To 20 mL colorimetric
tube was added 4a (147.1mg, 1.0mmol), 5a (270.3mg,
3.0mmol), Ag@MnO2 (10mg), KOH (168.3mg, 3.0mmol),
and CH3CN (3.0mL). The mixture was stirred at 60°C for
Figure 6. Recyclability of the Ag@MnO2 nanorods in
hydrogen-borrowing reactions.
Figure 7. SEM images: (a) fresh Ag@MnO2 nanorods catalyst and (b) Ag@MnO2 nanorod catalyst after five runs.

8
Journal of Chemical Research 00(0)
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Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
paper.
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Funding
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this paper: The
authors gratefully acknowledge the financial support of this work
by the National Natural Science Foundation of China (Nos.
21776111 and 21861039).
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