Facile Synthesis of α-Haloketones by Aerobic Oxidation of Olefins Using KX as Nonhazardous Halogen Source

Article abstract of DOI:10.1002/cjoc.201900372

An operationally simple and safe synthesis of α-haloketones using KBr and KCl as nonhazardous halogen sources is reported. It involves an iron-catalysed reaction of alkenes with KBr/KCl using O₂ as terminal oxidant under the irradiation of visible-light. This strategy avoids the risks associated with handling halo-contained electrophiles (Cl₂, Br₂, NCS, NBS). The process is tolerant to several functional groups, and extended to a range of substituted styrenes in up to 89% yield. A radical reaction pathway is proposed based on control experiments and spectroscopy studies.

Full text of DOI:10.1002/cjoc.201900372

Chinese Journal
of Chemistry
CJC
中 国 化 学 - An International Journal
Accepted Article
Title: Facile Synthesis of α-Haloketones by Aerobic Oxidation of Olefins Using KX as
Nonhazardous Halogen Source
Authors: Zhibin Luo, Yunge Meng, Xinchi Gong, Jie Wu, Yulan Zhang, Long-Wu Ye,
and Chunyin Zhu*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2020, 38, 10.1002/cjoc.201900372.
The final Version of Record (VoR) of it with formal page numbers will soon be
published online in Early View: http://dx.doi.org/10.1002/cjoc.201900372.
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Chinese Journal
of Chemistry
α-haloketone,α-bromoketone,α-chloroketone, alkene, aerobic
Report
CJC
中
国 化 学 - An International Journal
Facile Synthesis of α-Haloketones by Aerobic Oxidation of Olefins Using
KX as Nonhazardous Halogen Source
Zhibin Luo,a Yunge Meng,a Xinchi Gong,a Jie Wu,a Yulan Zhang,a Long-Wu Ye,b and Chunyin Zhu*,a, b
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
b State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P.R. China
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Summary of main observation and conclusion An operationally simple and safe synthesis of α-haloketones using KBr and KCl as nonhazardous
halogen sources is reported. It involves an iron-catalysed reaction of alkenes with KBr/ KCl using O2 as terminal oxidant under the irradiation of
visible-light. This strategy avoids the risks associated with handling halo-contained electrophiles (Cl2，Br2，NCS, NBS). The process is tolerant to several
functional groups, and extended to a range of substituted styrenes in up to 89% yield. A radical reaction pathway is proposed based on control
experiments and spectroscopy studies.
(Scheme 1).
Scheme 1 Synthetic pathways to α-haloketones
Background and Originality Content
α-Haloketones play an important role as key intermediates in
organic syntheses. [1] Therefore, numerous methods have been
developed for accessing this class of compounds and one classical
strategy is α-halogenation of carbonyl compounds. This method
usually involves halo-contained electrophiles (Cl2，Br2，NCS, NBS)
that suffer from either associated risks in handling and transport,
low atom efficiency or use of molecular halogen for their
preparation.[2] Despite some research using K2S2O8 to generate
Br2 insitu for the synthesis of α -bromoketones[3]
, the
development of facile synthetic methods for α-haloketones from
simple starting materials using nonhazardous halogen source is
highly desirable.
Alkenes are extremely abundant chemical feedstocks that are
produced extensively in petrochemical industry. As a result, they
have been exploited as starting materials for the synthesis of a
wide range of fine chemicals, including various ketone derivatives.
One famous example is Wacker-type reactions, which convert
olefins to ketones/aldehydes.[4] However, this type of
transformations usually relies on expensive palladium(II) salts as
the catalyst to activate π-orbitals of alkenes, and sometimes high
catalyst loading is required, leading to cost and environmental
concerns. An alternative option is to subject alkenes to certain
radical sources, which would generate some very reactive species
Results and Discussion
At the outset, we examined the synthesis of
α
-bromoketones using alkenes as the starting materials, and
styrene 1a was chosen as the model substrate to react with
different bromine sources at room temperature under the
irradiation of 12W blue LEDs in the presence of catalyst and O2
(Table 1). Subjecting 1a to catalytic FeBr3 using nBu4NBr as the
bromine source in ethanol provided the desired product 2a with a
30% yield. A variety of solvents were screened including protic
solvents (ethanol, isopropanol, water, and acetic acid), polar
aprotic solvents (MeCN, DMSO and DMF), ether (THF and MTBE)
and DCM (entries 1-10). Notably, ether appeared to be more
suitable to this reaction and MTBE gave a yield up to 45% (entry
9). As for the catalyst, eliminating FeBr3 from the reaction led to
no desired product (entry 11), while replacing FeBr3 with other
iron salts did not improve the reaction (entries 12-14). Then the
evaluation of different bromine sources revealed that KBr was
superior to other bromides including nBu4NBr, NaBr and LiBr
(entries 15-17). Finally, different acids were tried as the additive
under mild conditions, leading to the functionalization of the
[5]
double bond without using expensive metal catalysts.
These
radical sources can be generated through visible light photoredox
[6], [7]
chemistry
that is receiving increasing attention from across
the chemical community. For example, Itoh and coworkers have
developed an efficient synthesis of phenacyl halides using
molecular iodine or 48% aqueous HBr under visible-light
irradiation with fluorescent lamps[8]
.
With this perspectives, we have previously reported a
synthesis of ketones involving the oxidative addition of alkenes
with aryl radicals that are generated by visible-light induced
aerobic oxidation of arylhydrazines.[9] We speculate that the
replacement of arylhydrazines with halides would lead to the
formation of α-haloketones. On the other hand, iron catalysts
have been intensively explored in the functionalization of simple
alkenes,[10] which inspired us to engage iron catalysts in this study.
Herein we would like to report the details of this investigation
*E-mail: zhucycn@gmail.com
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through the copyediting, typesetting, pagination and proofreading process which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cjoc.201900372
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First author et al.
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to promote the reaction, and TsOH was found to be the best
among various inorganic and organic acids, resulting in the
formation of 2a with 80% yield (entry 22). Thus, the optimal
conditions for the transformation of alkenes to α-bromoketones
was defined as follows: FeBr3 (10 mol %) as the catalyst, KBr (1.5
equiv) as the bromine source, TsOH (2.0 equiv) as the additive,
MTBE as the solvent.
standard condition (2o), which can be ascribed to the radical
mechanism of the reaction, as phenol group is known to inhibit
radical reactions.
Table 2 Substrate scope for the synthesis of α-bromoketones.a
Table 1 Optimization for the synthesis of α-bromoketones.a
Entry
1
Catalyst
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
--
[Br]
Additive Solvent
Yield b
30%
41%
10%
36%
37%
30%
trace
43%
45%
27%
N.D.
33%
27%
36%
50%
37%
29%
68%
32%
55%
58%
80%
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
nBu4NBr
KBr
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
EtOH
iPrOH
H2O
2
3
4
AcOH
MeCN
DMSO
DMF
5
6
7
8
THF
a Reaction conditions: 1 (1 mmol), KBr ( 1.5 mmol), FeBr3( 0.1 mmol)，TsOH
(2 mmol), MTBE (2 mL)，12W blue LEDs，room temperature，O2 ballon.
9
MTBE
DCM
10
11
12
13
14
15
16
17
18
19
20
21
22
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
Next, we turned our attention to the synthesis of α
-chloroketones from alkenes. With styrene as the model substrate,
the screen of chlorination conditions was observed to follow a
similar trend to bromination (Table 3). MTBE was the best
among various solvents including protic solvents (ethanol,
isopropanol, water, and acetic acid), polar aprotic solvents (MeCN,
DMSO and DMF), ether (THF and MTBE) and DCM (entries 1-10).
Fe(OTf)3
Fe(NO3)3
Fe2(SO4)3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
n
NaBr
KCl was proved to be superior as the chlorine source to Bu4NBr,
Et3N•HCl and NaCl (entries 11-13). Iron catalyst was found to be
necessary for the reaction and FeCl3 was better than other tested
iron salts (entries 14-17). Also, the reaction with TsOH as the
additive gave the highest yield compared to other acids (entries
18-22). So the optimal conditions for the chlorination is: FeBr3 (10
mol %) as the catalyst, KBr (1.5 equiv) as the bromine source,
TsOH (2.0 equiv) as the additive, MTBE as the solvent.
LiBr
KBr
H2SO4 MTBE
H3PO4 MTBE
KBr
KBr
AcOH
TFA
MTBE
MTBE
MTBE
KBr
Table 3 Optimization for the synthesis of α-chloroketones.a
KBr
TsOH
a Reaction conditions: 1a (1 mmol), [Br] ( 1.5 mmol), catalyst( 0.1 mmol)，
additive (2 mmol), solvent (2 mL)，12W blue LEDs，room temperature，
O2 ballon. b isolated yield.
Under the optimized reaction conditions, a wide range of
styrenes with different substituents could be transformed to α
-bromoketones as shown in Table 2. Electronic effect was
Entry
Catalyst
FeCl3
[Cl]
KCl
KCl
Additive
AcOH
Solvent
EtOH
iPrOH
Yield b
24%
1
2
FeCl3
AcOH
32%
observed
for
this
reaction,
as
substrates
with
electron-withdrawing groups (-CN, -Br, -F and -NO2) tend to give
higher yields than the ones with electron-donating groups (-CH3,
-OCH3, -isopropyl) (2e, 2f, 2j, 2k, 2n vs 2b, 2c, 2h, 2i, 2m). As for
steric effect, all the ortho-/meta-substituted substrates work as
well as their para-substituted counterpartner, leading to
corresponding α-bromoketones in good yields (2h, 2i vs 2c; 2g
vs 2d). Importantly, this reaction is compatible to functional
groups such as –OMe, -CN and –NO2, demonstrating its good
functional group tolerance. Notably, substrate 1o bearing a –OH
group did not give any desired product when being subjected to
3
4
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
KCl
KCl
KCl
KCl
KCl
KCl
KCl
KCl
AcOH
--
H2O
trace
33%
26%
23%
trace
33%
36%
22%
AcOH
MeCN
DMSO
DMF
5
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
6
7
8
THF
9
MTBE
DCM
10
2
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Running title
Chin. J. Chem.
and BHT were carried out. The reactions were found to be
inhibited, as no desired 2a was isolated (eq. 1 and 2, Scheme 2).
This observation supports the idea that the reaction probably
proceeds through a radical pathway. Also UV-Vis absorption of
FeBr3[11] in MTBE was studied and there is a maximum absorption
at 430 nm (Scheme 3), indicating FeBr3 can be irradiated by blue
light.
11
12
13
14
15
16
17
18
19
20
21
22
FeCl3
FeCl3
NaCl
nBu4NCl
Et3N•HCl
KCl
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
H3PO4
HNO3
H2SO4
TFA
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
31%
27%
29%
31%
27%
33%
trace
32%
21%
44%
38%
55%
FeCl3
Fe(OTf)3
Fe(NO3)3
Fe2(SO4)3
--
KCl
KCl
KCl
Scheme 2 Control reactions
FeCl3
KCl
FeCl3
KCl
FeCl3
KCl
FeCl3
KCl
FeCl3
KCl
TsOH
a Reaction conditions: 1a (1 mmol), [Cl] ( 1.5 mmol), catalyst( 0.1 mmol)，
additive (2 mmol), solvent (2 mL)，12W blue LEDs，room temperature，
O2 ballon. b isolated yield.
With the optimal conditions, the substrate scope of the
chlorination was also studied. Generally, lower yields were
observed for this reaction compared to the bromination (Table 4).
Similar electronic effect was observed, as substrates with
electron-withdrawing groups (-CN, -F and -NO2) gave better yields
(3e, 3k and 3n), among which 1-fluoro-4-vinylbenzene could
undergo this transformation to produce corresponding product
3k with a yield up to 67%. The reaction works well for
ortho-/meta-/para-substituted substrates with tolerance for
functional groups including –OMe, -CN and –NO2. Notably,
substrate 1o bearing a –OH group failed to work under this
condition either, indicating a strong quenching effect of phenol
group. For the poor yields in some cases, reactions were found to
be sluggish and most remaining starting alkenes were recovered.
For example, 44% of 2l were recovered along with the desired
product after 48-hour reaction.
Scheme 3 UV-Vis absorption of FeBr3.
2.0
1.5
1.0
0.5
0.0
Table 4 Substrate scope for the synthesis of α-chloroketones.[a]
350
400
450
500
550
600
Wavelength (nm)
On the basis of these experimental observations, a radical
mechanism is depicted in Scheme 4. The reaction is initiated by
the split of FeBr3 into FeBr2 and bromine radical (Br•) upon
irradiation of blue light. The resulted Br• is trapped by styrene 1a,
leading to the formation of intermediate I, which could be
captured by O2 and FeBr2 to form intermediate II. In the presence
of bromide source and acid, intermediate II is then converted to
intermediate III along with the regeneration of FeBr3. Finally,
elimination of H2O from intermediate III generates the desired
product 2a.
Scheme 4 Proposed mechanism
a Reaction conditions: 1 (1 mmol), KCl ( 1.5 mmol), FeCl3( 0.1 mmol)，TsOH
(2 mmol), MTBE (2 mL)，12W blue LEDs，room temperature，O2 ballon.
b Recovery yield of 2l.
To gain some insights into the mechanism of this reaction,
control experiments in the presence of radical trapper TEMPO
Chin. J. Chem. 2019, 37, XXX－XXX
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3

First author et al.
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silica-supported sodium hydrogen sulfate as
a heterogeneous
Conclusions
In conclusion, we revealed that KBr and KCl can be used as
nonhazardous halogen sources in the synthesis of
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Suzuki, T.; Nishida, Y.; Satsumabayashi, K.; Horaguchi, T. A mild and
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N-bromosuccinimide catalysed by ammonium acetate. Chem.
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α-halogenation reactions of 1,3-dicarbonyl compounds catalyzed by
Lewis acids. J. Org. Chem. 2002, 67, 7429–7431. (j) Bertelsen, S.;
Halland, N.; Bachmann, S.; Marigo, M.; Braunton, A.; Jørgensen, K.A.
Organocatalytic asymmetric α-bromination of aldehydes and ketones.
Chem. Commun. 2005, 4821–4823.
α
-haloketones. It involves the iron-catalysed reaction of alkenes
with KBr/ KCl in the presence of oxygen and visible-light. This
strategy avoids the need for the use of halo-contained
electrophiles (Cl2，Br2，NCS, NBS) that are of potential risks in
handling and transport. The process is tolerant to –OMe, -CN and
–NO2, and extended to a range of substituted styrenes in up to
89% yield. Preliminary mechanistic studies confirmed the radical
pathway induced by visible-light. The operational simplicity and
readily available reagents of this green reaction will lead to some
practical application in organic synthesis.
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K2S2O8-mediated tandem hydroxybromination and oxidation of
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Experimental
General procedure for the synthesis of products 2 and 3: An
oven-dried flask tube was charged with KBr/KCl (1.5 mmol),
catalyst FeBr3/ FeCl3 (0.1 mmol), acid TsOH·H2O (2 mmol), MTBE
(2 mL), finally, alkene 1 (1 mmol) was added in the solution. The
reaction mixture was stirred at room temperature under O2
atmosphere and irradiated by blue LED (12 W). The reaction was
monitored by thin layer chromatography (TLC). When the
reaction was completed, it was diluted with water and extracted
with ethyl acetate 3 times. The organic phase was concentrated
using the rotary evaporator. Removal of solvent followed by
column chromatography afforded desired products.
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Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement
We thank the financial support from the National Natural
Science Foundation of China (21622204 and 21772161).
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Chin. J. Chem.
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2019
Manuscript revised: XXXX, 2019
Manuscript accepted: XXXX, 2019
Accepted manuscript online: XXXX, 2019
Version of record online: XXXX, 2019
Chin. J. Chem. 2019, 37, XXX－XXX
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Facile Synthesis of α-Haloketones by Aerobic
Oxidation of Olefins Using KX as Nonhazardous
Halogen Source
An operationally simple and safe synthesis of α-haloketones using KBr and KCl as nonhazardous
halogen sources is reported. It involves an iron-catalysed reaction of alkenes with KBr/ KCl using O2
as terminal oxidant under the irradiation of visible-light. This strategy avoids the risks associated
with handling halo-contained electrophiles.
Zhibin Luo, Yunge Meng, Xinchi Gong, Jie Wu,
Yulan Zhang, Long-Wu Ye, and Chunyin Zhu*
6
www.cjc.wiley-vch.de
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, XXX－XXX
This article is protected by copyright. All rights reserved.