Synthesis of α,β-dibromo ketones by photolysis of α-bromo ketones with N-bromosuccinimide: Photoinduced β-bromination of α-bromo ketones

Article abstract of DOI:10.1016/j.tet.2019.130684

Irradiation of α-bromopropiophenones in the presence of NBS results in the formation of α,β-dibromopropiophenones, which can be viewed as β-bromination of α-bromopropiophenones. The reaction is believed to go through a series of reactions; photoinduced C–Br bond cleavage, elimination of HBr to give α,β-unsaturated ketone intermediates, and addition of Br₂, which are formed by the reaction between HBr and NBS. From mechanistic studies of the reaction, we have also found a very convenient method for α-debromination of the α,β-dibromopropiophenones which is by simple irradiation of the dibromo ketones in acetone or 2-propanol without the use of any additives. Our results demonstrate that bromine can be added into or eliminated from the alpha, beta, or both positions to the carbonyl group by photochemical methods, which make synthetic options of bromine containing carbonyl compounds versatile.

Full text of DOI:10.1016/j.tet.2019.130684

Journal Pre-proof
Synthesis of α,β-dibromo ketones by photolysis of α-bromo ketones with N-
bromosuccinimide: Photoinduced β-bromination of α-bromo ketones
Da Yoon Moon, Sejin An, Bong Ser Park
PII:
S0040-4020(19)31059-2
https://doi.org/10.1016/j.tet.2019.130684
TET 130684
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 31 August 2019
Revised Date: 5 October 2019
Accepted Date: 8 October 2019
Please cite this article as: Moon DY, An S, Park BS, Synthesis of α,β-dibromo ketones by photolysis
of α-bromo ketones with N-bromosuccinimide: Photoinduced β-bromination of α-bromo ketones,
Tetrahedron (2019), doi: https://doi.org/10.1016/j.tet.2019.130684.
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Synthesis of α,β-Dibromo Ketones by Photolysis of
α-Bromo Ketones with N-bromosuccinimide:
Photoinduced β-Bromination of α-Bromo Ketones
Da Yoon Moon, Sejin An and Bong Ser Park*
Department of Chemistry, Dongguk University(Seoul Campus), Seoul 04620, Korea

Synthesis of α,β-Dibromo Ketones by Photolysis of α-Bromo Ketones
with N-bromosuccinimide: Photoinduced β-Bromination of α-Bromo Ketones
Da Yoon Moon, Sejin An and Bong Ser Park*
Department of Chemistry, Dongguk University (Seoul Campus), Seoul 100-715, Korea
Abstract: Irradiation of α-bromopropiophenones in the presence of NBS results in theformation of
α,β-dibromopropiophenones, which can be viewed as β-bromination of α-bromopropiophenones. The
reaction is believed to go through a series of reactions; photoinduced C-Br bond cleavage, elimination
of HBr to give α,β-unsaturated ketone intermediates, and addition of Br2, which are formed by the
reaction between HBr and NBS. From mechanistic studies of the reaction, we have also found a
very convenient method for α-debromination of the α,β-dibromopropiophenones which is by simple
irradiation of the dibromo ketones in acetone or 2-propanol without the use of any additives. Our
results demonstrate that bromine can be added into or eliminated from the alpha, beta, or both
positions to the carbonyl group by photochemical methods, which make synthetic options of bromine
containing carbonyl compounds versatile.
1. Introduction
Bromination is undoubtedly one of the most important methods in organic functional group
interconversion. Thus, various techniques for bromination have been developed from many
different starting compounds.1 Among them, there has recently been much interest over the
preparation of α.β-dibromo carbonyl compounds because they serve as useful intermediates
in organic synthesis. For example, dibromo compounds have been used as precursors for α-
bromoacrylates,2 vinyl azides,3 β-nitro ketones,4 indanones,5 pyrazoles,6 etc. Furthermore,
a facile preparation method for the dibromo compounds is a key feature in using them as a
protecting group in vinyl ketones.7 The vicinal dibromides can also be used as UV photoacid
generators for photoresist because photodecomposition of such compounds can provide a
remarkably clean source of bromine atoms.8 Even though the dibrominated compounds
could be obtained easily by Br2 addition to vinyl ketones,9 the use of hazardous molecular
bromine was unsuitable hence, there has been the need for more convenient and safer
methods.10
Recently, we reported that α-bromopropiophenones could be converted into β-
bromopropiophenones via photochemical C-Br bond cleavage reaction, which we coined as
photoinduced 1,2-Br shift reaction.(Scheme 1)11 It was suggested that the 1,2-Br shift
occurred by conjugate addition of HBr into α,β-unsaturated ketone intermediates formed by
the photoinitiated C-Br bond cleavage and disproportionation of the resulting radical pairs.
We envisioned that the α,β-unsaturated ketone intermediates would be trapped by Br2 to give
α.β-dibromo ketones if the photolysis of α-bromopropiophenones is done under Br2 enriched
condition. It is known that Br2 can be formed by the reaction of N-bromosuccinimde (NBS)
with HBr.12 Since HBr is one of initial photoproducts of the 1,2-Br shift reaction, we
speculated that α.β-dibromo ketones would be formed when α-bromopropiophenones are
irradiated in presence of NBS. The formation of the α,β-dibromo ketones from α-bromo
ketones would be considered as a formal β-bromination, which is very rare in organic
synthesis.13 To our delight, β-bromination was observed to occur in a very effective way,
which will be described below.
1

O
O
O
O
h
CH3
CH3
CH2
Br
Br
+ HBr
Br
Scheme 1. Photoinduced 1,2-Br shift reaction.
2. Result and discussion
α-Bromopropiophenones, which were prepared by α-bromination of the corresponding
propiophenones using CuBr2, were irradiated as 0.02-0.03 M solution in the presence of NBS
using a photolysis setup whose light source was a Pyrex filtered light of a 450W Hanovia
medium pressure mercury arc lamp. The photolysis products were separated by column
chromatography, whereby the structure of each product was characterized by routine
spectroscopic analysis. To obtain the optimum condition for the photolysis reaction, reactant
1a was chosen as the base compound and several solvents were tested shown in Table 1.
Table 1. Product distribution in photolysis of 1a using various solvents
Product Distribution (%)a
Solvent
Acetonitrile
Acetone
2a
100
-
81
59
38
54
3a
-
4a
-
22
-
-
10
-
78
19
41
52
46
Benzene
Dichloromethane
Chloroform
Ethyl acetate
a The numbers were obtained by comparing integration of 1H NMR signal of each product after irradiation until all the
starting ketones disappeared. The photolysis was done in 0.025 M solution containing 1a and NBS(1.1 equivalents).
It was observed that product distribution was strongly dependent on the type of solvents used.
In acetonitrile, the β-bromination reaction product 2a was formed exclusively, while the 1,2
bromine shift product 3a became a major product in acetone. The product selectivity of 2a
over 3a decreased in benzene by ca. 10% relative to the result in acetonitrile, and comparable
amounts of 2a and 3a were obtained in other solvents such as dichloromethane, chloroform
and ethyl acetate. In alcoholic solvents such as methanol and 2-propanol, a large amount of
polymeric products were formed, whose structure could not be determined. The amount of
NBS did not have a significant effect on product distribution, however, the product ratios
seemed to be dependent on the irradiation time, which was due to photochemical reactions of
2a under the given photolysis condition, vide infra. Since the product selectivity of 2a was
the best and the irradiation time dependence of the yield of 2a was the smallest out of all the
solvents tested, acetonitrile was chosen for the rest of the experiments.
With the optimum condition at hand, we investigated photochemical reactions of several
compounds, as shown in Table 2.
2

Table 2. Product distribution in photolysis of various α-bromoalkyl phenyl ketones
Product distribution (%)b
Yield of 2 c
Substrate
(%)
Other Products
2
1a
1b
100
-
83(72)
100
100
100
100
-
-
-
-
82(71)
95(86)
80(70)
99(89)
1c
1d
1e
1f
100
100
-
-
99(82)
79(68)
1g
Benzylic
bromination (38)
1h
1i
62
-
52
-
Ring
brominationd(100)
Benzylic
bromination(100)
1j
-
-
Norrish/Yang
photoreaction(10)
e
1k
90
-
3

Norrish/Yang
photoreaction(25)
e
1l
1m
1n
1o
1p
1q
75
-
-
Polymeric
products
(15)f
75(55)g
78(65)
-
100
80
-
-
Tri-bromo
product(20)
-
(100)
100h
-
90i
a
b
Reaction condition: 0.025 M solution of 1 in acetonitrile, NBS(1.1 eq.)
Values obtained
1
C
1
by integration of H NMR. The yields were determined by integration of H NMR spectra
using methyl benzoate as an internal standard. (In parenthesis are isolated yields. During the
isolation, HBr elimination occurred in small extent in most cases.)
d It turned out to be a thermal reaction. e The yield was not determined because it was strongly
f
dependent on irradiation time due to secondary photoreactions. The relative yield using
g
NMR integration could not be obtained due to broadness of the spectrum. Only the trans
product was detected and isolated. h cis : trans= 1 : 4. iThe cis and trans isomers could not be
separated.
The dibromo ketones 2 were obtained as a major product in good yields from photolysis of
most ketones that we analyzed. In cases where one or more methyl substituents were present
in a benzene ring such as 1h and 1j, benzylic bromination became a major competing
pathway with the desired β-bromination. For 1i containing a hydroxyl group in the benzene
ring, both ortho positions to the hydroxy group were brominated, which turned out to be a
thermal reaction later from further investigation. However, 1b and 1e, which have a methoxy
group at either para or ortho position to the carbonyl, went through the desired β-
bromination with no interference of other side reactions.
Other alkyl ketones apart from propiophenones, 1k to 1q, were also tested under the same
reaction condition. For 1k and 1l, Norrish/Yang photoreaction14 products were also formed,
which made the reaction mixtures more difficult to analyze compared to other ketones.
Apparently, the β-bromination reaction is slower than photoinduced 1,2-Br shift reaction
because the Norrish/Yang products are not formed from photolysis of 1k and 1l in a given
4

reaction condition with no NBS present. As a reference, rates of the Norrish/Yang reaction of
α-substituted phenyl alkyl ketones are known to be in the order of 108 s-1.15
The yield of the desired β-bromination product from 1m was further reduced mainly due to
the formation of polymeric precipitates during the reaction. It can be speculated that the β-
bromination of 1m is hampered by steric hindrance caused by an extra methyl group at β
position to the carbonyl, resulting to polymerization of the α,β-unsaturated ketone
intermediate. Further characterization of the polymeric precipitates was nor carried out due to
their insoluble nature.
In photolysis of 1o, the products from a secondary photoreaction were always contained in
the reaction mixture. The initially formed β-bromination product seemed to go through the
same reaction sequence as the first one, which was α C-Br bond cleavage, disproportionation,
and Br2 addition. The secondary photoreaction of the initial product occurred at the other
methyl group of 1o to form a tribromo ketone as shown in Scheme 2.
O
O
O
O
CH3
CH3
CH3
CH2Br
CH2Br
CH2Br
h
h
CH2Br
+Br2
-HBr
Br
Br
Br
1o
2o
Scheme 2. Photochemical reaction of α-bromoisobutyrophenone (1o).
In the case of 1p, chromone was the only product formed. Apparently, β-bromination reaction
could not occur due to the stability of the chromone ring. In contrast, β-bromination reaction
easily occurred during photolysis of an indanone derivative, 1q.
Our proposed mechanism for the formation of β-bromination products as shown in Scheme 3
is similar to the one proposed for photoinduced 1,2-Br shift reaction of α-
bromopropiophenones, except that Br2 addition to α.β-unsaturated carbonyl intermediates
becomes a predominant reaction pathway under the Br2 enriched reaction condition whereby
bromine molecules are formed by the reaction between NBS and HBr. One could argue that
the dibromo ketones would be formed by α-bromination of initially formed β-bromo ketones
under the influence of NBS. In fact, there have been a few reports showing photochemical α-
bromination of ketones using NBS.16 In order to get a better understanding of the reaction
mechanism, we performed the photolysis of β-bromopropiophenone. Under the same reaction
condition as described above, the ketone was essentially inert, which led us to exclude the α-
bromination of β-bromo ketones from possible reaction mechanisms for the formation of β-
bromination reaction products.
5

O
O
O
O
CH3
CH3
h
Br
Br
Br
-HBr
3
+HBr
O
CH2
+Br2
Br
Br
2
O
O
HBr
+
Br2
+
N Br
N H
O
O
Scheme 3. Proposed mechanism for photochemical β-bromination of α-
bromopropiophenones.
The reaction mechanism implies that the product ratios of 2 and 3 are determined by the
relative concentration of Br2 and HBr in solution, since they are derived from the same α,β-
unsaturated ketone intermediates. In the presence of NBS, Br2 addition to give 2 becomes the
dominant reaction pathway, whereas the formation of 3 becomes the predominant reaction
pathway in the absence of NBS. Even though the above reaction mechanism provides a
reaction route to the observed dibromo ketones, their fate under photolysis condition remain
doubtful, since they possess a photolabile alpha C-Br bond and therefore, could be involved
in the reaction process. Can photolysis of 2 form the α,β-unsaturated ketone intermediate and
convert it into 3, if the reaction condition changes whereby the concentration of HBr exceeds
that of Br2? We may have already encountered such a case. Table 1 shows that the β-
bromination reaction product 2a is absent in the reaction mixture when photolysis is done in
acetone. The result was obtained after complete conversion of the starting ketone, but further
examination revealed that the dibromo ketone was in fact formed at an earlier stage of the
reaction and significantly decreased with concomitant increase of 3a as the photolysis
continued. Figure 1 shows the irradiation time dependence of product ratios in photolysis of
1a in acetone.
6

Figure1. Irradiation time dependence of product ratios in photolysis of 1a in acetone.
The dibromo ketone 2a was initially formed as a result of photolysis of 1a in the presence of
NBS, consequently, photolysis of 2a under the same condition led to the formation of α.β-
unsaturated ketone and Br2 via the C-Br bond cleavage followed by disproportionation. The
latter process is known to occur very fast, with rate constants exceeding 5 x 107 s-1.17 The
bromine molecules produced at this stage reacted with acetone to form brominated acetone
and HBr, which consequently added into the α.β-unsaturated ketone to form 3a as observed
in our experiment. To ascertain the accuracy of the mechanism, we irradiated 2a in acetone-d6
under the same reaction conditions and observed that 3a-d2 was formed as the major product,
as shown in Scheme 3. This suggests that the major product 3a from photolysis of 1a in
acetone was probably formed from the secondary photoreaction of 2a. Besides its
mechanistic implication, the reaction can find useful application in α-debromination of α,β-
dibromo carbonyl compounds without using any added reducing agents.
O
O
hν
Br
Br
acetone-d6
D D
3a-d2
Br
2a
H-D exchange
with acetone-d6
-Br2
O
O
+DBr
Br
H D
Br2 + acetone-d 6
DBr + acetone-d5
Scheme 3. Photoinduced α-debromination reaction of 2a.
α-Debromination of 2 to form 3 can also be achieved if the acetone is replaced with 2-
propanol as the solvent. The reaction in 2-propanol is photoreduction that is known to occur
via radical chain mechanism, as discussed in our previous report.18
7

We have also tested whether the α,β-unsaturated ketone intermediate can be observed upon
completion of photolysis in the presence of aniline where it can act as a quencher for Br2.19
When 2 was irradiated in acetonitrile in the presence of 1.2 equivalents of aniline, α,β-
unsaturated ketone was obtained as the major product with a minimal amount of 3 formed. As
mentioned above, the α,β-dibromo ketones have been utilized as a protecting group of vinyl
ketones. The photochemical reaction of the dibromo ketones in the presence of aniline would
be a valuable addition to other known methods.20
From our previous results showing that photolysis of α-bromopropiophenones leads to
efficient conversion into β-bromopropiophenones, we envisioned that the α,β-dibromo
compound 2 can also be prepared photochemically from α,α-dibromopropiophenones. To
fulfil our expectation, photolysis of α,α-dibromopropiophenone gave the desired product 2 in
good yields both in solution and neat condition. This reaction completes a valuable set of
photochemical Br-shift reactions starting from α-bromopropiophenones, as shown in Scheme
4.
Scheme 4.Photochemical interconversion of various Br containing ketones.
3. Conclusions
In summary, photolysis of α-bromopropiophenones in acetonitrile in the presence of NBS
resulted in the formation of α,β-dibromopropiophenones in good yields. The facile and eco-
friendly reaction procedure, in which no hazardous chemical such as bromine was used,
would provide a valuable addition to the known synthetic methods of the dibromo ketones.
From studies on the reaction mechanism, we have also found a very efficient method for α-
debromination of the α,β-dibromopropiophenones which is by simple irradiation of the
dibromo ketones in acetone or 2-propanol without using any additives. Our results
demonstrate that bromine can be added into or eliminated from alpha, beta, or both positions
to the carbonyl group by photochemical methods, which make synthetic options of bromine
containing carbonyl compounds versatile.
4. Experimental section
General Information
All reagents and solvents were purchased from commercial suppliers and were used without
further purification. The starting materials, α-bromo ketones, were synthesized using the
purchased reagents. The reaction was monitored by thin layer chromatography on silica gel
8

60 F254 plate (Merck, Darmstadt, Germany). Column chromatography was done on silica gel
(BioSphere silica gel 60um 60A; BioMec, Seoul, South Korea) with dichloromethane and n-
1
hexane. H and 13C NMR spectra were recorded on Bruker Auto-Sampler-HR-MAS 500
MHz-NMR Spectrometry in CDCl3. IR spectra were recorded on Functional Anal-ATR-FTIR
Spectrometer (IdentifyIR® Portable FT-IR Spectrometer; Smiths Detection, London, United
Kingdom). High-resolution massspectra (HR-MS) were acquired under fast atom
bombardments (FAB) condition on a JMS-700 MStation (JEOL, Tokyo,Japan).
General experimental procedure
The α-bromo ketones tested for photolysis were prepared by CuBr2 bromination of the
corresponding ketones. The synthetic process has been described in our previous paper.21 α-
Bromo ketone (0.2 mmol) and N-bromosuccinimide (0.22 mmol) were dissolved in the
solvent (8 ml). The reaction mixture was placed in a test tube, and it was degassed by
charging Ar gas for a few minutes. Photolysis of the mixture was done by using a typical
photolysis setup with a light source as a 450W Hanovia medium pressure mercury arc lamp
filtered by Pyrex. The reaction was monitored by thin layer chromatography (TLC), and the
irradiation continued until the α-bromo ketone could not be detected. Then the mixture was
concentrated by a rotary evaporator at room temperature. The products were then purified
through silica gel column chromatography (dichloromethane/hexane).
2,3-dibromo-1-phenylpropan-1-one (2a) 11
1H NMR (500 MHz, CDCl3) δ 8.02 (dd, J = 7.8, 7.4 Hz, 2H), 7.64 (t, J = 7.4 Hz, 1H), 7.52
(t, J = 7.8 Hz, 2H), 5.40 (dd, J = 10.7, 4.1 Hz, 1H), 4.22 (dd, J = 10.7, 9.8 Hz, 1H), 3.80 (dd,
13
J = 9.8, 4.1 Hz, 1H) ppm; C NMR (126 MHz, CDCl3) δ 190.9, 134.5, 134.3, 129.2, 129.1,
41.5, 28.9 ppm.
2,3-dibromo-1-(4-methoxyphenyl)propan-1-one (2b)
1H NMR (500 MHz, CDCl3) δ 8.01 (d, J = 8.9 Hz, 4H), 6.98 (d, J = 8.9 Hz, 4H), 5.37 (dd, J
= 10.7, 4.1 Hz, 1H), 4.22 (d, J = 10.7, 9.7 Hz, 1H), 3.89 (s, 6H), 3.79 (dd, J = 9.7, 4.1 Hz, 1H)
ppm; 13C NMR (126 MHz, CDCl3) δ 189.4, 164.6, 131.6, 127.1, 114.4, 55.8, 41.5, 29.2 ppm;
IR(CHCl3) 1677 cm-1 (C=O), HRMS (FAB+): m/z calcd for C10H1179Br2O2 [M+H]+:
320.9126, found: 320.9128.
2,3-dibromo-1-(4-bromophenyl)propan-1-one (2c)
1H NMR (500 MHz, CDCl3) δ 7.89 (d, J = 8.6 Hz, 2H), 7.67 (d, J = 8.6 Hz, 2H), 5.33 (dd, J
= 10.7, 4.1 Hz, 1H), 4.21 (dd, J = 10.7, 9.8 Hz, 1H), 3.79 (dd, J = 9.8, 4.1 Hz, 1H) ppm; 13C
NMR (126 MHz, CDCl3) δ 189.9, 133.0, 132.6, 130.6, 129.9, 41.3, 28.7 ppm; IR(CHCl3)
79
1691 cm-1 (C=O); HRMS (FAB+): m/z calcd for C9H8 Br3O [M+H]+: 368.8125, found:
368.8122.
2,3-dibromo-1-(4-chlorophenyl)propan-1-one (2d)
1H NMR (500 MHz, CDCl3) δ 7.97 (d, J = 8.7 Hz, 2H), 7.50 (d, J = 8.7 Hz, 2H), 5.34 (dd, J
13
= 10.8, 4.1 Hz, 1H), 4.21(dd, J = 10.8, 9.8 Hz, 1H), 3.79 (dd, J = 9.8, 4.1 Hz, 1H) ppm; C
NMR (126 MHz, CDCl3) δ 189.7, 141.1, 132.6, 130.5, 129.6, 41.4, 28.7 ppm; IR(CHCl3)
79
35
1690 cm-1 (C=O); HRMS (FAB+): m/z calcd for C9H8 Br2 ClO [M+H]+: 324.8630, found:
364.8633.
9

2,3-dibromo-1-(2-methoxyphenyl)propan-1-one (2e)
1H NMR (500 MHz, CDCl3) δ 7.86 (dd, J = 7.8, 1.8 Hz, 1H), 7.65 – 7.48 (m, 1H), 7.13-7.04
(m, 1H), 7.00 (d, J = 8.4 Hz, 1H), 5.72 (dd, J = 10.4, 4.5 Hz, 1H), 4.20 (dd, J = 10.4, 9.8 Hz,
1H), 3.96 (s, 3H), 3.74 (dd, J = 9.8, 4.5 Hz, 1H) ppm; 13C NMR (126 MHz, CDCl3) δ 192.6,
159.0, 135.2, 132.3, 125.0, 121.4, 112.1, 56.0, 47.8, 29.8 ppm; IR(CHCl3) 1674 cm-1 (C=O);
HRMS (FAB+): m/z calcd for C10H1179Br2O2 [M+H]+: 320.9126, found: 320.9127.
2,3-dibromo-1-(3-bromophenyl)propan-1-one (2f)
1H NMR (500 MHz, CDCl3) δ 8.15 (s, 1H), 7.94 (d, J = 7.8 Hz, 1H), 7.76 (d, J = 6.2 Hz,
1H), 7.41 (dd, J = 7.8, 6.2 Hz, 1H), 5.33 (dd, J = 10.8, 4.0 Hz, 1H), 4.20 (dd, J = 10.8, 9.8 Hz,
1H), 3.79 (dd, J = 9.8, 4.0 Hz, 1H) ppm; 13C NMR (126 MHz, CDCl3) δ 189.6, 137.3, 136.0,
132.1, 130.7, 127.6, 123.5, 41.3, 28.6 ppm; IR(CHCl3) 1692 cm-1 (C=O); HRMS (FAB+):
79
m/z calcd for C9H8 Br3O [M+H]+: 368.8125, found: 368.8122.
2,3-dibromo-1-(3-chlorophenyl)propan-1-one (2g)
1H NMR (500 MHz, CDCl3) δ 7.99 (s, 1H), 7.89 (d, J = 7.7 Hz, 1H), 7.61 (d, J = 7.7 Hz,
1H), 7.47 (t, J = 7.7 Hz, 1H), 5.33 (dd, J = 10.7, 3.8 Hz, 1H), 4.20 (dd, J = 10.7, 9.6 Hz, 1H),
3.80 (dd, J = 9.6, 3.8 Hz, 1H) ppm; 13C NMR (126 MHz, CDCl3) δ 189.7, 135.8, 135.6, 134.4,
130.5, 129.2, 127.1, 41.3, 28.6 ppm; IR(CHCl3) 1689 cm-1 (C=O); HRMS (FAB+): m/z calcd
for C9H8 Br2 ClO [M+H]+: 324.8630, found: 324.8633.
79
35
2,3-dibromo-1-phenylbutan-1-one (2k)22
1H NMR (500 MHz, CDCl3) δ 8.02 (dd, J = 8.3, 1.1 Hz, 2H), 7.64 (t, J = 7.4 Hz, 1H), 7.52
(t, J = 7.8 Hz, 2H), 5.38 (d, J = 10.6 Hz, 1H), 4.75 (dq, J = 10.6, 6.7 Hz, 1H), 2.03 (d, J = 6.7
13
Hz, 3H) ppm; C NMR (126 MHz, CDCl3) δ 191.7, 134.5, 134.3, 129.2, 129.1, 49.6, 45.1,
24.6 ppm.
2,3-dibromo-3-methyl-1-phenylbutan-1-one (2m)
1H NMR (500 MHz, CDCl3) δ 8.00 (d, J = 7.3 Hz, 2H), 7.62 (t, J = 7.4 Hz, 1H), 7.51 (t, J =
13
7.8 Hz, 2H), 5.65 (s, 1H), 2.15 (s, 2H), 2.09 (s, 2H) ppm; C NMR (126 MHz, CDCl3) δ
192.5, 135.7, 134.0, 129.1, 128.9, 63.2, 53.6, 34.8, 31.2 ppm; IR(CHCl3) 1687 cm-1 (C=O);
HRMS (FAB+): m/z calcd for C11H1379Br2O [M+H]+: 318.9333, found: 318.9331.
trans-2,3-dibromo-1,3-diphenylpropan-1-one (2n)23
1H NMR (500 MHz, CDCl3) δ 8.11 (d, J = 8.3, 1.1 Hz, 2H), 7.67 (t, J = 7.4 Hz, 1H), 7.60-
7.50 (m, 4H), 7.47 – 7.35 (m, 3H), 5.84 (d, J = 11.3 Hz, 1H), 5.65 (d, J = 11.3 Hz, 1H) ppm;
13C NMR (126 MHz, CDCl3) δ 191.4, 138.5, 134.6, 134.4, 129.5, 129.2, 129.1, 129.1, 128.6,
50.0, 47.0 ppm.
2,3-dibromo-2-methyl-1-phenylpropan-1-one (2o)
1H NMR (500 MHz, CDCl3) δ 8.03 (d, J = 7.9 Hz, 2H), 7.55 (t, J = 6.9 Hz, 1H), 7.45 (t, J =
7.7 Hz, 2H), 4.35 (d, J = 10.1 Hz, 1H), 3.90 (d, J = 10.1 Hz, 1H), 2.15 (s, 3H) ppm; 13C NMR
(126 MHz, CDCl3) δ 194.9, 135.5, 132.7, 129.6, 128.6, 59.5, 38.9, 27.8 ppm; IR(CHCl3)
1687 cm-1 (C=O); HRMS (FAB+): m/z calcd for C10H1179Br2O [M+H]+: 304.9176, found:
304.9172.
Conflicts of interest
10

There are no conflicts to declare.
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*Photochemical β-bromination of α-bromopropiophenones.
*Simple and eco-friendly reaction procedure
*No hazardous chemical such as bromine is used

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: