Indium-Mediated Synthesis of Benzylic Hydroperoxides

Article abstract of DOI:10.1021/acs.orglett.9b01070

An indium(0)-metal-mediated efficient synthesis of benzylic hydroperoxides is described. The reaction proceeds efficiently with a broad range of benzyl bromides under aerobic conditions at room temperature to afford benzyl hydroperoxides in good to excellent yields. In addition, the tandem hydroperoxidation-Michael addition of (E)-1-(bromomethyl)-2-(2-nitrovinyl)benzene was also demonstrated.

Full text of DOI:10.1021/acs.orglett.9b01070

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Indium-Mediated Synthesis of Benzylic Hydroperoxides
Yuxuan Hou, Jinjin Hu, Ruigang Xu, Shulei Pan, Xiaofei Zeng,* and Guofu Zhong*
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
S
* Supporting Information
ABSTRACT: An indium(0)-metal-mediated efficient synthesis of benzylic hydro-
peroxides is described. The reaction proceeds efficiently with a broad range of benzyl
bromides under aerobic conditions at room temperature to afford benzyl hydro-
peroxides in good to excellent yields. In addition, the tandem hydroperoxidation-
Michael addition of (E)-1-(bromomethyl)-2-(2-nitrovinyl)benzene was also demon-
strated.
eroxides and hydroperoxides exist widely in natural
products, and many of them have therapeutic values.1
Scheme 1. Chemistry of Benzylic Hydroperoxides
P
For instance, Artemisinin (Qinghaosu) (1),2 a traditional
Chinese medicine, is widely used as an antimalarial drug;
Yingzhaosu C (2)3 and many 1,2,4-troxiane analogues4 were
shown to possess potential antimalarial activity and Peroxyfer-
olide (3),5,1c from the leaves of L. tulipifera showed cytotoxic
and anticancer activities. 15-Hydroperoxydehydroabietic acid
(4)6,1d and a peroxy-generated derivative 5 of the indole
alkaloid ibogaine exhibit contact allergenic properties and have
been related to delayed hypersensitivity toward Portuguese
gum rosin (colophony) and Swedish tall oil rosin.
Furthermore, the mimic synthesis of peroxidic compounds as
antimalaria or anticancer drugs became very interesting. For
instance, lophine peroxide (6)1d and its derivatives have been
synthesized, tested for anticancer activities, and obtained
satisfactory results (see Figure 1). In addition, benzylic
such types of compounds, it came as a surprise to us that the
facile synthesis of hydroperoxides from readily available
starting materials is still rare. Consequently, we reported
herein a highly efficient indium(0)-mediated synthesis of
benzylic hydroperoxides from benzylic halides.
Traditionally, there are several synthetic approaches to
benzylic hydroperoxides: (1) oxidation of alcohols by H2O2,8
(2) oxidation and hydrolysis of Grignard reagents,9 (3)
addition of singlet oxygen with alkene and autoxidation of
alkane,10 etc.11 However, none of these methods is truly
general, and many of them suffer from stringent conditions or
poor yields, require a high concentration of H2O2, need inert
conditions for the preparation of Grignard reagents, or require
a high temperature for the direct oxidation of alkenes.
Recently, Dussault and co-workers reported a convenient
approach for synthesis of alkyl hydroperoxides via alkylation of
1,1-dihydroperoxides followed by acidic deprotection of the
derived bisperoxyacetals.12 However, the starting materials are
complicated and the atom efficiency is low. In consideration of
the advantages of low cost, safety, environmental protection,
and fewer side reactions, molecular oxygen has become a
preferred oxidant to replace H2O2 in hydroperoxidation
reaction.13 Woerpel reported a CuI-catalyzed synthesis of
propargyl hydroperoxides via the oxidation of enynes, which
employed molecular oxygen as the source of oxygen atoms.14
Figure 1. Important peroxides and hydroperoxides.
hydroperoxides are widely used as starting materials in the
synthesis of hydroxyl-substituted arenes, and act as active
oxygen-atom compounds in oxidation catalysis, or as
precursors for the generation of oxygen-center radicals in
polymer science, physical organic chemistry, organometallics,
and bioorganic investigations (see Scheme 1).7 In biological
systems, the radical or ion generated by the cleavage of
peroxide bond plays an important role in biological activities.1b
Considering the importance and potential synthetic value of
Received: March 26, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01070
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Makino disclosed a manganese-catalyzed aerobic hydro-
peroxidation method for conjugated alkenes.15 Su and co-
workers reported a hydroperoxidation/lactonization of α-
ethereal C−H bonds by singlet O2 under mild conditions,
with modest to high yields and excellent site selectivity.16
Dussault and Berrien independently introduced a hemi-
ketalized version of hydrogen peroxides as hydroperoxidation
reagents in a two-step hydroperoxidation of organohalides and
alkenes.17 Despite the progresses made in this field, the
development of more atom economic and practical methods
such as using molecular oxygen as environmentally friendly
oxidant for the hydroperoxidation reaction is still highly
desirable.
Indium, because of its unique properties of being tolerant to
oxygen and moisture, has been demonstrated to be an efficient
and promising metal to mediate organic reactions in recent
years.18,19 In principle, neutral radical species are not affected
by the presence of water,20 and indium can be used as radical
initiator through the single electron transfer (SET) process.
The utilization of indium species as a radical initiator in several
organic transformations has been reported by Naito and co-
workers21 and Loh and co-workers.22 We envisioned that the
generation of neutral free radicals would be the key factor for
the success of the reaction. Herein, we report an efficient
method for the hydroperoxidation reaction of benzyl halides to
form benzylic hydroperoxides by using indium(0) metal under
an aerobic atmosphere.
1, entries 1−3). Attempts to improve the reaction efficiency by
using more polar solvents were performed. Gratifyingly,
moderate yields (65% and 55%) could be achieved in
CH3CN and dimethyl sulfoxide (DMSO) (Table 1, entries 4
and 5). It was found that protonic solvents metahnol (MeOH)
and ethanol (EtOH) accelerated the reaction rate significantly
and improved the yields to 72% (Table 1, entries 6 and 7).
However, when water was used, only 18% yield was afforded
(Table 1, entry 8). When dimethyl formamide (DMF) was
employed, the yield could be increased to 79% (Table 1, entry
9). We then moved to study the stoichiometry of indium
metal. A higher yield (86%) was achieved when 1.3 equiv of
indium was used and the reaction time was shortened to 12 h
(Table 1, entries 10 and 11). It was found that the reaction was
significantly influenced by the surface and particle size of the
indium powder. The finer indium particles (200 mesh), which
have a larger specific surface area, gave the hydroperoxidation
product in higher yield with shorter reaction time (Table 1,
entries 12 and 13). In addition, other metals, such as zinc, were
also investigated (Table 1, entry 14) and a lower yield was
achieved. These results demonstrated that indium is highly
unique and efficient for the reaction. After the establishment of
the optimized reaction conditions, we subsequently set up a
study toward exploring the scope by using different benzylic
bromides as substrates.
As shown in Table 2, a wide range of primary benzylic
halides effectively underwent the hydroperoxidation reaction
under air in DMF to produce the corresponding products in
good to excellent yields. Substrates with electron-donating
groups (Table 2, entries 2−4), electron-withdrawing groups
(Table 2, entries 6−10), and electron-neutral groups (Table 2,
entries 11 and 12) on the phenyl ring could all be well-
tolerated; a series of important functional groups (e.g., cyano,
fluoro, and bromo groups) survived the present reaction
conditions and were kept intact during the course of the
reaction. In addition to benzylic bromide (2a), benzylic
chloride (2b) and iodide (2c) (Table 2, entries 1−3) were
proven to be suitable partners for the reactions; however, the
yield of benzylic chloride was lower. It is noteworthy that the
secondary benzylic bromides were also demonstrated to be
appropriate substrates for the reaction, leading to the
anticipated products 2o and 2p in good yields (62% and
65%; see Table 2, entries 13 and 14). Moreover, heterobenzyl
bromide containing thienyl substituents worked equally well
under the optimal reaction conditions to give the desired
product in moderate yield (Table 2, entry 15). In addition,
allylic bromide 1r was also a good substance for the reaction,
the corresponding product could be obtained in 60% yield.
Unfortunately, when alkyl halide 1s was used, no reaction
happened. The structure of 4-(hydroperoxymethyl)-
benzonitrile (2l) was confirmed by X-ray crystallography
(Figure 2) and analysis of the NMR data of the product.
Having established the hydroperoxidation of benzyl halides,
we investigated the feasibility of a tandem hydroperoxidation-
Michael addition reaction of (E)-1-(bromomethyl)-2-(2-
nitrovinyl)benzene (I) under the optimized reaction con-
ditions, it was found that the corresponding product II
containing cyclic peroxide moiety was afforded in moderate
yield (Scheme 2a). Furthermore, when the indium mediated
hydroperoxidation reaction was performed by using AcOH as a
solvent, benzyl alcohol (III) instead of benzyl hydroperoxide
was formed (Scheme 2b). Furthermore, the reaction could be
To begin with, benzyl bromide (1a) was employed as a
model substrate in the presence of indium(0) metal (100 mesh
powder) as the radical initiator under a variety of conditions to
study the feasibility of radical initiation, as well as the
subsequent hydroperoxidation under aerobic conditions. A
series of solvents were first screened with 1.1 equiv of indium
powder at room temperature, as shown in Table 1. When the
reaction performed in a less-polar solvent, such as toluene,
ethyl acetate (EtOAc), and tetrahydrofuran (THF), proceeded
sluggishly to give trace amount of the desired product (Table
a
Table 1. Optimization of the Reaction Conditions
b
entry
solvent
metal (equiv)
time (h)
yield (%)
c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
toluene
EtOAc
THF
CH3CN
DMSO
MeOH
EtOH
H2O
DMF
DMF
DMF
DMF
In (1.1)
24
24
24
16
37
5
14
24
30
23
12
24
4
trace
trace
trace
65
55
72
72
18
79
81
In (1.1)
In (1.1)
In (1.1)
In (1.1)
In (1.1)
In (1.1)
In (1.1)
In (1.1)
In (1.2)
In (1.3)
86
54
89
8
d
In (1.3)
In (1.3)
e
DMF
DMF
Zn (1.3)
20
a
Conditions: indium(0) powder, benzyl bromides (0.3 mmol) in
b
solvent (1.0 mL) under air at room temperature (rt). Yield of
isolated product. 100 mesh indium powder was used. 60 mesh
indium powder was used. 200 mesh indium powder was used.
c
d
e
B
DOI: 10.1021/acs.orglett.9b01070
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 2. Reaction Scope of the Hydroperoxidation
a
Reaction
Figure 2. X-ray crystal structure of 2l.
Scheme 2. Synthetic Utility of the Hydroperoxidation
Reaction
In order to investigate the reaction mechanism, some
control experiments were performed (see Scheme 3). TEMPO
Scheme 3. Control Experiments
was added into the model reaction system. The fact that the
desired product was not formed in the reaction indicated that
the reaction proceeded through a radical mechanism based on
the SET process. Butsugan and co-workers reported an
example of aerobic oxygenation of allylic indium reagents to
generate hydroperoxide species, which was further reduced to
alcohols.23 We then prepared the benzylic indium compound
according to the literature22e and applied in the aerobic
hydroperoxidation reaction. It was found that the organo-
indium compound IV is quite stable in air and the reaction
could not proceed, which means that the indium compound IV
is relatively unreactive19a,b,24 and the reaction is unlikely to
proceed through an oxidative addition of indium to the
carbon−halogen bond and subsequent C−O bond formation
process. Then, benzyl zinc bromide prepared from benzyl
bromide and zinc powder under anhydrous conditions was
applied in the aerobic hydroperoxidation reaction. It was found
that the benzyl hydroperoxide could be obtained in moderate
yield, which indicates that benzyl zinc bromide is more reactive
than benzylic indium compound and the reaction proceeds
through an oxidative addition of zinc to the carbon−halogen
bond and subsequent C−O bond formation process. However,
a
Conditions: indium(0) (0.39 mmol, 1.3 equiv), benzyl bromides
(0.3 mmol, 1.0 equiv) in DMF (1.0 mL) under aerobic conditions at
rt. Yield of isolated product. No reaction.
b
c
scaled-up: 1.00 g of 1a afforded the desired 2a in 83% yield
under standard conditions.
C
DOI: 10.1021/acs.orglett.9b01070
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
when we use tribenzylboron as the starting material, no
reaction happened. (See the Supporting Information.)
To gain a deep insight into the radical formation, electron
paramagnetic spectroscopy (EPR) experiments were per-
formed in the presence of 5,5-dimethyl-1-pyrrolidine-N-oxide
(DMPO) (see Scheme 4). When we mixed 1a with indium
Experimental procedures, characterization data H and
13C NMR spectra for compounds, and X-ray data for 2l
(PDF)
Accession Codes
CCDC 740902 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_ request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
Scheme 4. ESR Spectra Obtained from the Indium
Mediated Hydroperoxidation Reaction of Benzyl Bromide
in the Presence of DMPO
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chemzxf@hznu.edu.cn (X.-F. Zeng).
*E-mail: zgf@hznu.edu.cn (G.-F. Zhong).
ORCID
Xiaofei Zeng: 0000-0003-4222-1365
Guofu Zhong: 0000-0001-9497-9069
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
powder in DMF under degassed conditions, a strong benzyl
radical signal (gfactor = 2.003) was detected (Scheme 4, black
line), and a strong BnOO• radical signal (gfactor = 2.003) was
observed when the radical trapping test was performed in air
(see Scheme 3, red line). Inspired by the above results, a
plausible reaction mechanism was proposed and illustrated in
Scheme 5; the reaction is initiated by indium to generate
■
We gratefully acknowledge the Natural Science Foundation of
China (No. 21672048), the Natural Science Foundation of
Zhejiang Province (No. LY18B020015), and Hangzhou
Normal University for financial support. X.Z. acknowledges a
Xihu Scholar award from Hangzhou City, and G.Z. acknowl-
edges a Qianjiang Scholar from Zhejiang Province in China.
REFERENCES
Scheme 5. Proposed Reaction Mechanism
■
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