Bromination of 1,1-diarylethylenes with bromoethane

Article abstract of DOI:10.1080/00397911.2021.1943751

Aliphatic bromide was used as a halogenation reagent in the presence of DMSO, resulting in 2,2-diarylvinyl bromides from the corresponding 1,1-diarylethylenes. This protocol not only provides a convenient and straightforward strategy for the rapid construction of various 2,2-diarylvinyl bromides without bromine and extra oxidants, but also can improve the atom economy of Kornblum oxidative reaction.

Full text of DOI:10.1080/00397911.2021.1943751

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Bromination of 1,1-diarylethylenes with
bromoethane
Ze Yu, Jialiang Jiang, Hongtai Chen & Xiangyang Tang
To cite this article: Ze Yu, Jialiang Jiang, Hongtai Chen & Xiangyang Tang (2021) Bromination
of 1,1-diarylethylenes with bromoethane, Synthetic Communications, 51:16, 2544-2552, DOI:
10.1080/00397911.2021.1943751
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2021, VOL. 51, NO. 16, 2544–2552
https://doi.org/10.1080/00397911.2021.1943751
Bromination of 1,1-diarylethylenes with bromoethane
Ze Yu, Jialiang Jiang, Hongtai Chen , and Xiangyang Tang
Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of
Science, Tianjin University, Tianjin, China
ABSTRACT
ARTICLE HISTORY
Received 29 April 2021
Aliphatic bromide was used as a halogenation reagent in the pres-
ence of DMSO, resulting in 2,2-diarylvinyl bromides from the corre-
sponding 1,1-diarylethylenes. This protocol not only provides a
convenient and straightforward strategy for the rapid construction of
various 2,2-diarylvinyl bromides without bromine and extra oxidants,
but also can improve the atom economy of Kornblum oxidative reac-
tion.
KEYWORDS
Vinyl bromides;
Bromination; Bromoethane
GRAPHICAL ABSTRACT
Introduction
2,2-Diarylvinyl bromides are privileged building blocks and can be easily converted to
organic molecules bearing 2,2-diarylvinyl moieties, which have wide applications in
organic synthesis and functional materials.[1–10] Therefore, it is important to develop an
efficient protocol for the synthesis of 2,2-diarylvinyl bromides. Currently, the bromina-
tion of 1,1-diarylethylenes with bromine is still the most widely used method for the
synthesis of 2,2-diarylvinyl bromides.[11–13] However, bromine is highly toxic and the
reaction will generate hazardous HBr as a byproduct. Therefore, the green chemistry
calls for the development of safe and environmentally benign bromination strat-
egies.[14–16] For example, Lin’s group recently reported the use of N-bromosuccinimide
(NBS) as a safer bromination reagent heated in HOAc.[14]
Recently, synthetic chemists have developed oxidative halogenations by simulating the
biological halogenation in nature, and the utilization rate of halogen atoms can be as
high as 100%. Bromide salts can serve as brominating reagent in the presence of oxi-
dants such as oxone, NaIO4, selectfluor, PhI(OAc)2, O2, and H2O2 (Scheme 1, Eq.
(1)).[17–27] For example, Jiao’s group reported the bromination of olefins using DMSO/
CONTACT Hongtai Chen
chenht@tju.edu.cn; Xiangyang Tang
txy@tju.edu.cn
Tianjin Key Laboratory of
Molecular Optoelectronic Science, Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2021 Taylor & Francis Group, LLC

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Scheme 1. Oxidative bromination of arenes and olefins.
HBr system affording dibrominated alkanes and bromohydrins.[28–31] Zhang’s group
reported the use of sodium bromide as bromination reagent of quinolines in presence
of copper(II) and PhI(OAc)2.[32] Bromination with alkyl bromide as a bromine source
has also been reported in combination with dimethyl sulfoxide (DMSO). In 1956, Pan
reported the first example of the bromination of aniline with ethyl bromide in DMSO
(Scheme 1, Eq. (2)).[33] In 2009, Guo also realized the bromination of anilines using 2-
bromopentane and DMSO in the presence of NaH (Scheme 1, Eq. (3)).[34] Despite the
significance of these novel strategies, there are still several unsolved problems in this
transformation. Direct C-H halogenation of aromatic compounds has been accom-
plished, however, the application of this strategy into preparation of alkenyl halides
from olefins remains elusive. For olefin substrates, bromination addition products such
as dibrominated alkanes and bromohydrins are resulted. Therefore, it is useful to
develop a mild and environmentally friendly strategy for the direct bromination of ole-
fins. Herein, we report a novel oxidative bromination of 1,1-diarylethylene using DMSO
and bromoethane (Scheme 1, Eq. (4)).
Results and discussion
The optimized conditions were obtained by screening the two reaction parameters
(bromo source and solvent) by using 1,1-diphenylethylene as a substrate. When the
bromide salt such as NaBr and LiBr were used as a bromo source, no target product 2a
was yielded (entries 1 and 2, Table 1). When NBS was used as a bromination reagent,
the target product 2a was obtained in 8% yield (entry 3). When BrCH2CO2Et was used,
the yield of 2a increased to 59% (entry 4). To our delight, when BrCH2CH3 was used,
2,2-diphenylvinyl bromide (2a) was produced with 94% GC yield and 90% isolated
yield. The solvent also plays a key impact on the results of this reaction. When toluene,
CH3CN, and N,N-dimethylformamide was used instead of DMSO, no bromination
product 2a was afforded (entries 6–8). These results indicated that DMSO might play a

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Table 1. Optimization of reaction conditions.a,b
Entry
Bromo source/solvent
Yield (%)b
1
2
3
4
5
6
7
8
9
NaBr/DMSO
LiBr/DMSO
NBS/DMSO
BrCH2CO2Et/DMSO
BrCH2CH3/DMSO
BrCH2CH3/Tol
BrCH2CH3/ACN
BrCH2CH3/DMF
80 oC instead of 110 oC
n.r.
n.r.
8
69
94 (90)
n.r.
n.r.
n.r.
70
aReaction conditions: 1a (0.2 mmol, 1.0 equiv.), BrCH2CH3 (0.24 mmol, 1.2 equiv.), DMSO (0.5 mL), 110 ꢀC, under N2, 5 h.
bThe yield was determined by GC analysis with mesitylene as an internal standard. The value in parentheses is the iso-
lated yield. n.r. ¼ no reaction.
key role in the formation of product 2a. When we attempted to lower the temperature
from 110 ꢀC to 80 ꢀC, the yield of 2a decreased to 70% (entry 9). Therefore, the optimal
conditions for the oxidative bromination involved treatment of 1,1-diphenylvinyl (1a) at
o
110 C in the presence of 1.2 equiv. of BrCH2CH3 in DMSO (entry 5).
With the optimized conditions in hand, the scope of the bromination reactions was
explored by testing a series of 1,1-diarylethylene containing different substituents on
phenyl ring and the representative examples are shown in Table 2. This method exhib-
ited good functional group compatibility and excellent chemoselectivity. The bromine
atom can be selectively incorporated onto vinyl fragment instead of diaryl motifs.
Satisfactory yields were obtained for the substrates in which the phenyl groups are
mono- or disubstituted with either electrondonating groups (such as methyl) or elec-
tron-withdrawing groups (such as fluoro, chloro, and bromo) (2a–2l). When one or
both of phenyl groups were replaced by naphthyl group, 2-naphthyl-2-phenyl vinyl
bromide 2m or 2,2-dinaphthyl vinyl bromide 2n could be obtained with high yields.
Interestingly, when 9-methylene-9H-fluorene was subjected to the above reaction condi-
tions, the target product 2o was afforded in good yield.
Subsequently, various tri-substituted 1,1-diarylethylene were also evaluated (Table 3),
which could proceed smoothly to give the corresponding vinyl bromides in good yields
(2p–2u). A scale-up reaction has also been performed on a gram level to demonstrate
the practicability of this strategy. When 1a (1.08 g) was subjected to the standard condi-
tions, the desired product 2a could be gained in 90% yield (1.40 g) (Scheme 2).
To gain some insights into the reaction mechanism, several control experiments have
been carried out. When benzyl bromide was applied instead of bromoethane, the
desired product 2a were yielded with 88% yield together with 90% of benzaldehyde,
which proved that Kornblum oxidation of bromoalkane by DMSO provide a convenient
bromination strategy toward 1,1-diarylethylene (Scheme 3(a)). When 1a was treated
with [(DMS)Brþ]Brꢁ in DMF at 110 ꢀC, the desired product 2a in 88% yield, proving
that [(DMS)Brþ]Brꢁ serve as the reactive bromo source (Scheme 3(b)).

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Table 2. Scope of diarylethylene.a,b
aReaction conditions: 1a (0.5 mmol, 1.0 equiv.), BrCH2CH3 (0.6 mmol, 1.2 equiv.), DMSO (0.5 mL), 110 ꢀC, under
N2, 5 h.
bIsolated yield.
Table 3. Scope of trisubstituted diarylethylene.a,b
aReaction conditions: 1a (0.5 mmol, 1.0 equiv.), BrCH2CH3 (06 mmol, 1.2 equiv.), DMSO (0.5 mL), 110 ꢀC, under
N2, 5 h.
bIsolated yield.

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Scheme 2. Gram-scale experiment.
Scheme 3. Control experiments for mechanism insight.
Scheme 4. Proposed mechanism.
According to the above experimental results and previous reports,[35–40] a plausible
DMSO-mediated oxidative bromination mechanism was proposed (Scheme 4). Firstly,
the reaction starts from an SN2 reaction of bromoethane with the DMSO to form sulfo-
nium salt A. Then deprotonation of A affords B and HBr, which was oxidized by
DMSO to afford bromodimethylsulfonium bromide C according to Jiao’s report.[28–31]
Meanwhile, intermediate B undergoes [2, 3]-rearrangement to give acetaldehyde while

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releasing dimethylsulfide (DMS). The electrophilic reaction between C and 1,1-diphenyl-
ethylene 1a leads to three-membered bromonium ion species D. Then rearrangement of
D affords E, which undergoes b-H elimination to provide 2a as the final product.
Conclusion
In summary, a novel DMSO-mediated oxidative bromination has been developed with
bromoethane as the bromo source. This reaction was mild, convenient, and environ-
mentally friendly due to the commercial availability, low cost, and air stability of bro-
moethane avoiding the use of bromine and extra oxidants for the synthesis of 2,2-
diarylvinyl bromides, which are very important intermediates in many complex organic
molecules. Further application of this strategy into organic synthesis is underway in
our laboratory.
General procedure for the preparation of 2,2-diarylvinyl bromides
A solution of 1,1-diphenylethylene (1a) (88 lL, 0.5 mmol) and bromoethane (45 lL,
0.6 mmol) in DMSO (1.0 mL) in a sealed reaction tube was stirred at 110 ꢀC (oil bath)
for 5 hrs under N2 atmosphere. The reaction was quenched with ethyl acetate and water,
and then extracted with ethyl acetate (3 ꢂ 10 mL). The combined organic layers were
dried over anhydrous Na2SO4. After filtration, the filtrate was concentrated in vacuum
and the residue was purified by silica gel column chromatography (petroleum ether) to
give the pure product.
1H NMR and 13C NMR spectra for the products are available in the Supplementary
information.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
This work was supported by the Natural Science Foundation of Tianjin [19JCYBJC20200] and
Tianjin University.
ORCID
Hongtai Chen
http://orcid.org/0000-0002-7765-9781
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