Solvent free, light induced 1,2-bromine shift reaction of α-bromo ketones

Article abstract of DOI:10.1016/j.tet.2018.10.015

Photolysis of α-bromopropiophenones in acetonitrile results in formation of β-bromopropiophenones with good product selectivity, which can be coined as 1,2-Br shift reaction. The product selectivity increases when the reaction is done in neat or solid state, where only the 1,2-Br shift product is formed in some cases. The reaction is suggested to proceed by C–Br bond homolysis to give a radical pair, followed by disproportionation and conjugate addition of HBr to the α,β-unsaturated ketone intermediate. When the unsaturated intermediate is stabilized by an extra conjugation, the reaction stops at the stage, in which the unsaturated ketone becomes a major product. The synthetic method described in this research fits in a category of eco-friendly organic synthesis nicely since the reaction does not use volatile organic solvents and any other additives such as acid, base or metal catalysts, etc. Besides, the method fits into perfect atom economy, which does not give any side products. The synthetic method should find much advantage over other alternative methods to obtain β-bromo carbonyl compounds.

Full text of DOI:10.1016/j.tet.2018.10.015

Tetrahedron xxx (2018) 1e7
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Solvent free, light induced 1,2-bromine shift reaction of
ketones
a-bromo
Sejin An, Da Yoon Moon, Bong Ser Park*
Department of Chemistry, Dongguk University(Seoul Campus), Seoul 04620, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Photolysis of a-bromopropiophenones in acetonitrile results in formation of b-bromopropiophenones
Received 27 July 2018
Received in revised form
8 October 2018
Accepted 10 October 2018
Available online xxx
with good product selectivity, which can be coined as 1,2-Br shift reaction. The product selectivity in-
creases when the reaction is done in neat or solid state, where only the 1,2-Br shift product is formed in
some cases. The reaction is suggested to proceed by CeBr bond homolysis to give a radical pair, followed
by disproportionation and conjugate addition of HBr to the a,b-unsaturated ketone intermediate. When
the unsaturated intermediate is stabilized by an extra conjugation, the reaction stops at the stage, in
which the unsaturated ketone becomes a major product. The synthetic method described in this research
fits in a category of eco-friendly organic synthesis nicely since the reaction does not use volatile organic
solvents and any other additives such as acid, base or metal catalysts, etc. Besides, the method fits into
perfect atom economy, which does not give any side products. The synthetic method should find much
Keywords:
No solvent reaction
Photochemistry
1,2-Br shift
advantage over other alternative methods to obtain
b
-bromo carbonyl compounds.
© 2018 Elsevier Ltd. All rights reserved.
Green chemistry
Solid state reaction
1. Introduction
starting materials such as
a-bromo ketones.
Photoinduced CeBr bond cleavage reactions of
a
-bromo ketones
For a long time
b
-bromo ketones have been utilized as very
have received much attention in mechanistic studies of organic
reactions as well as in synthetic application [4]. The reaction has
also been exploited in polymer chemistry as a radical initiator [5].
More recently, we have witnessed a surge of reports describing the
important precursors to a broad range of intermediates in organic
synthesis: enones, heterocyclic derivatives, 1,3-dicarbonyl com-
pounds, etc. [1] Recently, the
to synthesize -azido carbonyl compounds which themselves have
a variety of useful applications including DNA cleaving agents [2].
Even though synthetic methods for -bromo ketones are
numerous, only a limited number of methodologies to synthesize
the -bromo ketones can be found in the literature [3]. One reason
for the relatively less known examples of the -bromo ketone
synthesis is that any drastic reaction conditions such as high tem-
perature can easily lead to dehydrobromination of the -bromo
ketones to form -unsaturated ketones. In order to obtain high
yield of the -bromo ketones, very mild reaction conditions would
b-bromo ketones have also been used
b
debromination of
years ago, we reported that UV irradiation of
ophenone in benzene gave a mixture of -bromo valerophenone
a-bromo ketones using visible light [6]. Several
a
-bromo valer-
a
b
and valerophenone in ca. 3 to 2 ratio, respectively [7] (Scheme 1).
It was suggested that the former product came from HBr addi-
b
b
tion to an initially formed
a,
b-unsaturated ketone, a mixture of (E)-
and (Z)-1-phenylpent-2-en-1-one, which the HBr and the
a,b
-un-
b
saturated ketone were formed by photoinduced CeBr homolysis
followed by disproportionation of the resulting radical pairs. The
a,
b
b
reaction can be considered as a formal 1,2-Br shift reaction of
a-
be required. Even the methods developed for operating under mild
condition are still unsatisfactory because they involve the use of
rather exotic starting materials such as cyclopropanols or cyclo-
propyl ethers which are not readily available. It would be nice to get
bromo ketones. Even though photochemistry of -bromo phenyl
a
alkyl ketones has been extensively studied for a long time [8], only
one other report except our work describing such conversion could
be found in the literature up to now, in which they observed for-
access to the
b-bromo ketones directly from easily obtainable
mation of
ation of
b
a
-bromoindanone as one of many products from irradi-
-bromoindanone [9]. They explained that the b-
bromoindanone came from benzylic bromination of the initially
formed indanone. Since the 1,2-Br shift reaction can be very valu-
able in organic synthesis, we decided to investigate the reaction
* Corresponding author.
E-mail address: parkbs@dongguk.edu (B.S. Park).
https://doi.org/10.1016/j.tet.2018.10.015
0040-4020/© 2018 Elsevier Ltd. All rights reserved.
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2
S. An et al. / Tetrahedron xxx (2018) 1e7
Table 2
Product distribution in photolysis of
solid state condition.
a
-bromopropiophenones in solutiona and neat/
Scheme 1. Photochemical reaction of a-bromovalerophenone in benzene.
more closely.
Compounds
Reaction condition
Product distribution (%)b
Major goals to achieve in modern organic synthesis would be to
obtain high efficiency and selectivity of each reaction using eco-
friendly methods. This article describes the 1,2-Br shift reaction of
2
3
4
5
In solution
neat
90
89
10
11
0
0
0
0
a
-bromo ketones was explored in solution, neat and in the solid
state. Because the neat and the solid state reactions are highly se-
lective, the 1,2-Br shift fits nicely into the concept of ‘atom econ-
omy’ as originally coined by Trost [10]. Additional advantage of this
reaction is that it can be carried out under mild conditions such as
ambient temperature. Here we would like to report our results on
the scope and limitation of the reaction.
In solution
solid
89
100
11
0
0
0
0
0
2. Results and discussion
In solution
solid
91
86
9
14
0
0
0
0
Our research started with the investigation of the photochem-
istry of a-bromopropiophenone, 1a. Photolysis of the a-bromo ke-
tone in benzene using the Pyrex filtered light of a 450 W Hanovia
medium pressure mercury arc lamp gave the formal 1,2-Br shift
reaction product, 2a, as a major product with a small amount of the
debrominated ketone, 3a. Several different solvents were used to
find the best solvent to obtain the best yields of 2a, and the results
are shown in Table 1.
Photolysis in other solvents also showed that 2a was the major
product, but in 2-propanol only the debrominated product, 3a, was
formed. When the photolysis was made in 9 to 1 mixture of
acetonitrile and H2O, less amount of 2a than in pure acetonitrile
was formed, in which 2a and 3a were formed in comparable
amounts. It appears that amount of the debrominated product 3a
increases in the presence of hydrogen atom donors. It is also
notable that the reaction occurs in acetone as efficiently as in other
solvents, where the triplet sensitized reaction can be assumed. As
shown in the above table, the reaction provided the highest yield of
2a in acetonitrile among other solvents. Thus, acetonitrile was
chosen as the solvent for photolysis studies of other ketones. The
In solution
neat
87
91
13
9
0
0
0
0
In solutionc
solid
44
100
3
0
9
0
0
0
In solution
solid
90
100
10
0
0
0
0
0
In solution
solid
94
100
6
0
0
0
0
0
In solution
neat
89
100
11
0
0
0
0
0
Table 1
Product distribution obtained from irradiating 1a in various solvents.
In solutionc
neat
40
100
4
0
10
0
0
0
Solvent
Product
Product selectivity (2a/3a)
distribution (%)a
In solution
neat
96
94
4
6
0
0
0
0
2a
3a
acetone
76
96
95
51
82
72
0
24
4
5
49
18
28
100
45
3.2
24
19
1.0
4.6
2.6
e
acetonitrile
chloroform
ether
hexane
benzene
a
In acetonitrile.
b
Values obtained by integration of 1H NMR signals of the starting material and
the photoproducts.
c
Secondary photoreaction products such as
formed. (See the text for more information.)
b-bromo propiophenone were also
2-propanol
aq. acetonitrileb
55
1.2
Yields were measured by integration of 1H NMR signals for 1a, 2a and 3a at 5.30,
3.75 and 3.01 ppm, respectively. Mass balances were 100% within experimental
errors as judged by the 1H NMR spectra of reaction samples which contained in-
ternal standard, methyl benzoate.
a
results of photolysis of ring substituted
are summarized below in Table 2.
a-bromo propiophenones
b
Photolysis of
a-bromopropiophenone derivatives 1a to 1j
9 to 1 mixture of acetonitrile and H2O.
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S. An et al. / Tetrahedron xxx (2018) 1e7
3
resulted in formation of product 2 as the major product in all in-
stances. To our delight, the 1,2-Br shift reaction occurred even in
neat or in the solid state in high chemical yield, even though it took
much longer to complete the reaction than in solution. Photolysis in
no solvent condition was performed simply by hanging a glass vial
containing a small amount of liquid or solid sample by the
photolysis setup. The solid state reaction times could be shortened
significantly by irradiating the samples on a thin glass plate,
without changing the product ratio. The most encouraging result
was that product selectivity of 2 over other products increased
significantly in the solid state irradiation compared to the selec-
which turned out to be photo-oxidation product, benzoic acid.
There were a few reports describing photo-oxidation of -bromo
ketones in the presence of oxygen [13]. The amount of the photo-
oxidation product was sensitive to the degree of deaerating the
samples, so the deaerating procedure was essential for high yield of
the 1,2-Br shift reaction.
The data given in Table 2 were taken after the reaction pro-
ceeded until no starting ketones were remained. When the reaction
stopped before completion, the a,b-dibrominated ketone, 4, shown
in Table 2 was also present in the product mixture in all the cases.
Thus, we decided to monitor the reaction more closely by irradi-
ating 1a in CD3CN at regular intervals. The result is shown in Fig. 1.
At early stage of the reaction, formation of 4a was evident together
with 2a and 3a, but the relative amount of 4a started to diminish
and eventually disappeared as irradiation continued. It appears that
photochemical reaction of 4a is competing with that of 1a under
the given reaction condition.
a
tivity in solution for most cases that we tried. In the case of
a-
bromo ketones 1b, 1e, 1f, and 1g, the corresponding -bromo ke-
b
tones 2b, 2e, 2f, and 2g are the sole solid state photoproducts
observed. Thus, the solid-state photoreactions for these ketones are
both selective and environmentally friendly. Only one exception to
the selectivity increase in the solid state was the case of 1c which
has an OH group in the phenyl ring. We think that the OH group
plays a role as a hydrogen atom donor and increases the relative
amount of the photoreduction product 3 in such a condensed
phase, vide infra.
We have also tested photochemistry of several other a-bromo
phenyl alkyl ketones besides the above propiophenone derivatives.
The results are summarized in Table 3.
For all the compounds except 1q shown in Table 3 which have an
extra alkyl/aryl group or a hetero atom at either alpha or beta po-
sition to the carbonyl group, the product selectivity of 2 over other
products decreased mainly due to an increase in the amount of the
elimination product 5. As shown in Scheme 3, the elimination
products 5 have structures where the alkenyl moiety can be sta-
bilized by the extra alkyl/aryl group or the hetero atom. It seems
that the stabilization of the elimination product makes addition of
HBr to the enone intermediates reluctant and reduces the relative
amount of 2 in product distribution. 1p shows such an extreme
example, where only 5p was formed both in solution and solid state
photolysis. Steric factors may also be important in the product
distribution because putting an extra methyl group to the beta
position to the carbonyl as in the case of 1k decreases the relative
amount of the 1,2-Br shift product 2k both in solution and neat
condition whereas putting it to the alpha position as in the case of
1o does not reduce the relative amount of the 1,2-Br shift product.
Interestingly 2o was the only product in photolysis of 1o in neat
condition.
In cases of 1e and 1i which have an extra Br on their phenyl
rings, product analysis becomes more complicated than others
because the phenyl CeBr bond cleavage can also occur. Seminal
work by Wagner [11] showed that bromophenyl ketones cleaved
the ring CeBr bonds very efficiently from their triplet excited states.
The cleavage rate constants for p-bromoacetophenone and m-
bromoacetophenone are 7 ꢀ 107 sꢁ1 and 2 ꢀ 108
s
ꢁ1, respectively
[11]. When photoreactions of 1e and 1i were monitored at regular
time intervals, it showed that both 2 and 3 still maintaining the
phenyl CeBr bond appeared first as major photoproducts and later
the cleavage of the phenyl CeBr bond of primary photoproducts
started to occur eventually. The result demonstrates that the
cleavage of CeBr bonds at
than the cleavage of CeBr bonds of phenyl rings, which means the
rate constant of the former is [108 ꢁ1. Scaiano and coworkers
reported that the rate constant of CeBr bond cleavage of -bro-
a position to carbonyls are even faster
s
a
moacetophenone is above 1 ꢀ 108 sꢁ1, which is consistent with our
current observation [12]. We were very excited to observe that only
2e and 2i were formed in photolysis of 1e and 1i in no solvent
condition. In the solid state, both CeBr and Ph-Br bond cleavage can
occur with the former being much faster rate, but the radical pairs
resulting from Ph-Br bond cleavage recombine in the restricted
environment while the 1,2-Br shift reaction still proceeds with no
difficulty (Scheme 2).
For 1k and 1l, the Norrish/Yang reaction [14] did not occur even
though prolonged irradiation showed the Norrish/Yang reaction of
primary photoproducts. Once again, the results demonstrate that
the 1,2-Br shift reaction rates are extremely fast since rates of the
Norrish/Yang reaction of
a
-substituted phenyl alkyl ketones are
known to be in the order of 108
s
ꢁ1 [15].
In cases of 1f and 1j, which has a phenyl CeCl bond, the CeCl
bond cleavage did not occur even at prolonged irradiation. The
result is understandable by the fact that the CeCl bond dissociation
energy (84 kcal/mol) is much higher than triplet energies of phenyl
alkyl ketones, which is ca. 68 kcal/mol.
It is worthwhile to mention that photolysis of 1n in the solid
state resulted in formation of [2 þ 2] photodimer of 5n together
When the photolysis in solution was performed without dea-
erating the sample, another product became a major product,
Scheme 2. Product formation from photolysis of 1e in solution and the solid-state.
Fig. 1. Plot of product distribution vs. irradiation time in photolysis of 1a in CD3CN.
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4
S. An et al. / Tetrahedron xxx (2018) 1e7
Table 3
with 2n and 3n while no such dimers were produced in solution.
Product distribution in photolysis of other
a-bromo phenyl alkyl ketones than
The chalcone skeleton of 5n is well known to give [2 þ 2] photo-
dimers upon photolysis [16]. It seems that the dimerization
pathway in the photolysis of 1n is only competent in the solid state.
Interestingly, only trans-head-to-head cycloaddition product was
formed stereoselectively.
Our proposed reaction mechanism is shown in Scheme 4. We
think the reaction starts from photoinduced CeBr bond cleavage of
excited states of the ketones to give radical pairs since there is no
propiophenones in solutiona and neat/solid state condition.
Compounds
Reaction condition
Product distribution (%)b
2
3
4
5
In solution
neat
66
78
7
7
0
7
27
8
reason to believe these ketones behave differently from
moacetophenones whose photochemistry has been thoroughly
studied mainly by Scaiano and coworkers [4].
a-bro-
In solution
neat
75
40
13
16
12
21
0
23
The initially formed radical pair can either disproportionate to
give the elimination product, 5, and HBr, or abstract a hydrogen
atom from any H atom donors to give the reduction product, 3.
While the rate of H atom abstraction of bromine atom to give HBr is
known to be very fast [17], that of phenacyl radical may not be fast
enough to compete with other paths of this reaction unless a large
excess of highly active H donors are present [18]. In fact, the
photoreduction is known to occur via rather complicated chain
reaction [19], whose detailed mechanism will not be explained here
except just noting that the photoreduction product can also be
formed from CeBr bond cleavage of ketyl radicals that is formed by
H atom abstraction of phenacyl bromides [20]. In a highly hydrogen
donating solvent such as 2-propanol the reduction path pre-
dominates and 3 becomes a sole product. Along the same line,
addition of a small amount of water to acetonitrile solvent was
enough to produce a significant amount of the photoreduction
product. It is also understandable that 1c having p-hydroxy group
gives non-negligible amount of reduction product while 1b having
p-methoxy group gives no such product in photolysis in the solid
state, since the phenol moiety of 1c can be a good source of
hydrogen donors.
The fate of the elimination product 5 can be split into two
events: Conjugate addition of HBr to 5 will form the 1,2-Br shift
product 2, and addition of Br2 to 5 will give the dibrominated
product 4. The origin of Br2 may vary depending upon reaction
condition. One of them would be oxidative formation from HBr by
any oxidants present in the solution such as oxygen. Even though
the reaction was done in deaerated condition, a possibility of a
small amount of oxygen that may still be present in the solution
cannot be excluded rigorously. The other source of Br2 would be the
reaction between HBr and the starting bromo ketone, as shown in
Scheme 5. The reaction may proceed by either ionic or radical
mechanism. The ionic version has recently been described in the
debromination of bromo ketones by bromide ion in the literature
[21].
In solution
solid
44
30
24
35
0
0
31
35
In solution
solidc
12
35
11
22
15
0
62
0
In solution
neat
71
100
0
0
13
0
16
0
In solution
solid
0
0
0
0
0
0
100
100
In solutiond
neat
46
75
35
25
0
0
0
0
a
In acetonitrile.
Values obtained by integration of 1H NMR.
b
c
A [2 þ 2] photodimer of 5 was also formed in 43%.
d
Unidentified products were also formed in 19%.
3. Conclusions
Scheme 3. Structures of photoproducts 5 formed from photolysis of other
phenyl alkyl ketones than propiophenones.
a
-bromo
In summary, we have demonstrated that
b-bromo-
Scheme
4. Proposed
mechanism
of
photochemical
reactions
of
a-
bromopropiophenones.
Scheme 5. Origin of Br2 in photolysis of 1.
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S. An et al. / Tetrahedron xxx (2018) 1e7
5
propiophenones can be obtained in good yield simply by irradiating
the -bromopropiophenones in neat or solid state. The 1,2-Br shift
(d, J ¼ 8.7 Hz, 2H), 5.22 (q, J ¼ 6.6 Hz, 1H), 1.90 (d, J ¼ 6.6 Hz, 3H), 13
C
a
NMR (126 MHz, CDCl3) d (ppm) 192.4, 140.4, 132.6, 130.6, 129.3,
reaction does not require volatile organic solvents and any other
reagents such as acid, base or metal catalysts, etc. Besides, the
method fits into perfect atom economy, which does not waste any
atoms in the conversion. Thus, we have developed a synthetic
41.5, 20.2.
4.1.7. 2-Bromo-1-(p-tolyl)propan-1-one (1g) [27]
1H NMR (500 MHz, CDCl3)
(ppm) 7.93 (d, J ¼ 8.0 Hz, 2H), 7.29
d
method to form
b
-bromo arylketones in high yields, and it is
(d, J ¼ 8.0 Hz, 2H), 5.28 (q, J ¼ 6.6 Hz, 1H), 2.43 (s, 3H), 1.90 (d,
environmentally friendly as it does not require any solvents.
J ¼ 6.6 Hz, 3H), 13C NMR (126 MHz, CDCl3)
d (ppm) 193.2, 144.9,
131.7, 129.7, 129.3, 41.7, 22.0, 20.4.
4. Experimental section
4.1.8. 2-Bromo-1-(2-methoxyphenyl)propan-1-one (1h) [28]
1H NMR (500 MHz, CDCl3)
4.1. Typical experimental procedure
d
(ppm) 7.73 (dd, J ¼ 7.7, 1.8 Hz, 1H),
7.51e7.44 (m,1H), 7.05e7.01 (m, 1H), 6.97 (d, J ¼ 8.4 Hz,1H). 5.52 (q,
All the a-bromo ketones tested for the photolysis experiment
J ¼ 6.7 Hz, 1H), 3.93 (s, 3H), 1.86 (d, J ¼ 6.7 Hz, 3H),13C NMR
were prepared by bromination of the corresponding ketones using
CuBr2. The synthetic procedure has been described in our previous
paper [7]. Photolysis in solution phase was done with 0.01 M so-
lution of the ketones after purging Ar gas through the solution for a
few minutes using a typical photolysis setup whose light source
was the Pyrex filtered light of a 450 W Hanovia medium pressure
mercury arc lamp. Photolysis in no solvent condition was done in
much simpler way. A sample vial containing either liquid or sold
sample was hung next to the photolysis setup and irradiated until
completion. For solid samples, the reaction times could be short-
ened significantly by irradiating the sample doped on a thin glass
plate which was simply prepared by dropping acetone solution of a
given compound onto any thin glass plate and evaporating the
solvent. Structural identification of photoproducts was done either
by comparing them with authentic samples when they were
available, or by typical spectroscopic identification procedures.
(126 MHz, CDCl3)
d (ppm) 196.7, 158.2, 134.2, 131.8, 126.0, 121.2,
111.7, 56.0, 48.2, 20.5.
4.1.9. 2-Bromo-1-(3-bromophenyl)propan-1-one (1i) [29]
1H NMR (500 MHz, CDCl3)
d
(ppm) 8.15 (t, J ¼ 1.8 Hz, 1H),
7.97e7.91 (m, 1H), 7.71 (ddd, J ¼ 7.9, 1.8, 0.9 Hz, 1H), 7.37 (t,
J ¼ 7.9 Hz, 1H). 5.21 (q, J ¼ 6.6 Hz, 1H), 1.90 (d, J ¼ 6.6 Hz, 3H), 13C
NMR (126 MHz, CDCl3)
127.6, 123.3, 41.4, 20.2.
d (ppm) 192.2, 136.7, 136.0, 132.1, 130.5,
4.1.10. 2-Bromo-1-(3-chlorophenyl)propan-1-one (1j)
1H NMR (500 MHz, CDCl3)
d
(ppm) 7.99 (t, J ¼ 1.8 Hz, 1H),
7.93e7.86 (m, 1H), 7.56 (ddd, J ¼ 8.0, 1.8, 1.0 Hz, 1H), 7.44 (t,
J ¼ 8.0 Hz, 1H). 5.22 (q, J ¼ 6.6 Hz, 1H), 1.90 (d, J ¼ 6.6 Hz, 3H), 13C
NMR (126 MHz, CDCl3)
d (ppm) 192.3, 135.9, 135.3, 133.8, 130.3,
129.2, 127.2, 41.5, 20.2. IR(CCl4) 1688 cmꢁ1 (C]O). HRMS
(C9H799BrClO, MþH) calcd. 246.9525, found 246.9529.
4.1.1. 2-Bromo-1-phenylpropan-1-one (1a) [22]
1H NMR (500 MHz, CDCl3)
d
(ppm) 8.03 (d, J ¼ 7.7 Hz, 2H), 7.60
4.1.11. 2-Bromo-1-phenylbutan-1-one (1k) [30]
(t, J ¼ 7.7 Hz, 1H), 7.50 (t, J ¼ 7.7 Hz, 2H), 5.30 (q, J ¼ 6.6 Hz, 1H), 1.91
1H NMR (500 MHz, CDCl3)
d
(ppm) 8.02 (d, J ¼ 7.3 Hz, 2H), 7.60
(d, J ¼ 6.6 Hz, 3H), 13C NMR (126 MHz, CDCl3)
d
(ppm) 193.6, 134.2,
(t, J ¼ 7.4 Hz, 1H), 7.49 (t, J ¼ 7.8 Hz, 2H), 5.08 (t, J ¼ 7.3 Hz, 1H),
133.9, 129.1, 129.0, 41.7, 20.4.
2.30e2.20 (m, 1H), 2.19e2.09 (m, 1H), 1.09 (t, J ¼ 7.3 Hz, 3H), 13C
NMR (126 MHz, CDCl3)
27.1, 12.4.
d (ppm) 193.5,134.7,133.9,129.1,129.0, 49.3,
4.1.2. 2-Bromo-1-(4-methoxyphenyl)propan-1-one (1b) [23]
1H NMR (500 MHz, CDCl3)
(ppm) 8.02 (d, J ¼ 8.7 Hz, 2H), 6.96
d
(d, J ¼ 8.7 Hz, 2H), 5.26 (q, J ¼ 6.6 Hz, 1H), 3.89 (s, 3H), 1.89 (d,
4.1.12. 2-Bromo-1-phenylpentan-1-one (1l) [7]
1H NMR (500 MHz, CDCl3)
J ¼ 6.6 Hz, 3H), 13C NMR (126 MHz, CDCl3)
d
(ppm) 192.2, 164.2,
d
(ppm) 8.02 (d, J ¼ 7.2 Hz, 2H), 7.60
131.5, 127.1, 114.2, 55.8, 41.7, 20.5.
(t, J ¼ 7.4 Hz, 1H), 7.49 (t, J ¼ 7.8 Hz, 2H), 5.15 (dd, J ¼ 7.8, 6.5 Hz, 1H),
2.26e2.00 (m, 2H), 1.67e1.52 (m, 1H), 1.52e1.37 (m, 1H), 0.99 (t,
4.1.3. 2-Bromo-1-(4-hydroxyphenyl)propan-1-one (1c) [24]
1H NMR (500 MHz, CDCl3)
(ppm) 7.98 (d, J ¼ 8.8 Hz, 2H), 6.91
J ¼ 7.4 Hz, 3H), 13C NMR (126 MHz, CDCl3)
d (ppm) 193.5, 134.7,
d
133.9, 129.1, 129.0, 47.2, 35.6, 21.0, 13.8.
(d, J ¼ 8.8 Hz, 2H), 5.77 (s, 1H), 5.26 (q, J ¼ 6.6 Hz, 1H), 1.89 (d,
J ¼ 6.6 Hz, 3H). 13C NMR (126 MHz, CDCl3)
d
(ppm) 192.3, 160.6,
4.1.13. 2-Bromo-3-methyl-1-phenylbutan-1-one (1m)
1H NMR (500 MHz, CDCl3)
(ppm) 8.00 (d, J ¼ 7.2 Hz, 2H), 7.60
131.9, 127.3, 115.8, 41.6, 20.5.
d
(t, J ¼ 7.4 Hz, 1H), 7.49 (t, J ¼ 7.7 Hz, 2H), 4.94 (d, J ¼ 8.6 Hz, 1H),
4.1.4. 2-Bromo-1-(2,5-dimethylphenyl)propan-1-one (1d)
1H NMR (500 MHz, CDCl3)
(ppm) 7.40 (s, 1H), 7.21 (d, J ¼ 7.7 Hz,
2.54e2.43 (m,1H),1.22 (d, J ¼ 6.6 Hz, 3H),1.03 (d, J ¼ 6.6 Hz, 3H), 13
C
d
NMR (126 MHz, CDCl3) d (ppm) 193.9, 135.2, 133.8, 129.0, 129.0,
79
1H), 7.17 (d, J ¼ 7.7 Hz, 1H), 5.21 (q, J ¼ 6.6 Hz, 1H), 2.44 (s, 3H), 2.37
56.1, 31.3, 21.0, 20.7. IR(CCl4) 1684 cmꢁ1 (C]O). HRMS (C11
MþH) calcd. 241.0228, found 241.0231.
H BrO,
14
(s, 3H), 1.88 (d, J ¼ 6.6 Hz, 3H), 13C NMR (126 MHz, CDCl3)
d (ppm)
197.2, 136.3, 135.8, 135.4, 132.8, 132.1, 128.4, 45.3, 21.2, 20.7, 20.5.
IR(CCl4) 1686 cmꢁ1 (C]O). HRMS (C11
H
BrO, MþH) calcd.
4.1.14. 2-Bromo-1,3-diphenylpropan-1-one (1n) [31]
79
14
241.0228, found 241.0232.
1H NMR (500 MHz, CDCl3)
d
(ppm) 7.95 (dd, J ¼ 8.3, 1.1 Hz, 2H),
7.55 (t, J ¼ 6.2 Hz,1H), 7.44 (t, J ¼ 7.8 Hz, 2H), 7.32e7.02 (m, 6H), 5.30
(dd, J ¼ 7.6, 7.0 Hz, 1H), 3.65 (dd, J ¼ 14.3, 7.6 Hz, 1H), 3.34 (dd,
4.1.5. 2-Bromo-1-(4-bromophenyl)propan-1-one (1e) [25]
1H NMR (500 MHz, CDCl3)
d
(ppm) 7.89 (d, J ¼ 8.7 Hz, 2H), 7.63
J ¼ 14.3, 7.0 Hz, 1H), 13C NMR (126 MHz, CDCl3)
d (ppm) 193.0, 137.7,
(d, J ¼ 8.7 Hz, 2H), 5.22 (q, J ¼ 6.6 Hz, 1H), 1.90 (d, J ¼ 6.6 Hz, 3H), 13
C
134.6, 134.0, 129.7, 129.1, 129.0, 128.8, 127.3, 46.8, 39.7.
NMR (126 MHz, CDCl3)
41.4, 20.2.
d (ppm) 192.6, 133.0, 132.3, 130.6, 129.2,
4.1.15. 2-Bromo-2-methyl-1-phenylpropan-1-one (1o) [26]
1H NMR (500 MHz, CDCl3)
d
(ppm) 8.14 (d, J ¼ 8.6 Hz, 2H), 7.53 (t,
4.1.6. 2-Bromo-1-(4-chlorophenyl)propan-1-one (1f) [26]
1H NMR (500 MHz, CDCl3)
(ppm) 7.97 (d, J ¼ 8.7 Hz, 2H), 7.47
J ¼ 7.4 Hz, 1H), 7.44 (t, J ¼ 7.7 Hz, 2H), 2.04 (s, 6H),13C NMR
d
(126 MHz, CDCl3) d (ppm) 197.2, 135.1, 132.6, 130.3, 128.4, 60.6, 31.8.
Please cite this article in press as: S. An, et al., Solvent free, light induced 1,2-bromine shift reaction of
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a-bromo ketones, Tetrahedron (2018),

6
S. An et al. / Tetrahedron xxx (2018) 1e7
4.1.16. 2-Bromo-3,4-dihydronaphthalen-1(2H)-one (1p) [32]
1H NMR (500 MHz, CDCl3)
(ppm) 7.94 (dd, J ¼ 7.9, 1.5 Hz, 1H),
4.1.26. 3-Bromo-1-(3-bromophenyl)propan-1-one (2i)
1H NMR (500 MHz, CDCl3)
(ppm) 8.08 (s, 1H), 7.88 (d,
d
d
7.54 (t, J ¼ 7.0 Hz, 1H), 7.09 (t, J ¼ 7.5 Hz, 1H), 7.03 (d, J ¼ 8.4 Hz, 1H),
J ¼ 7.8 Hz, 1H), 7.72 (d, J ¼ 8.9 Hz, 1H), 7.37 (t, J ¼ 7.9 Hz, 1H), 3.73 (t,
4.72e4.52 (m, 3H), 13C NMR (126 MHz, CDCl3)
d (ppm) 185.5, 160.9,
137.0, 128.5, 122.6, 119.0, 118.2, 71.5, 45.6.
J ¼ 6.8 Hz, 2H), 3.55 (t, J ¼ 6.8 Hz, 2H), 13C NMR (126 MHz, CDCl3)
d (ppm) 195.9, 136.7, 131.4, 130.6, 129.0, 126.8, 123.4, 41.8, 25.5.
IR(CCl4) 1690 cmꢁ1 (C]O). HRMS (C9H799Br2O, MþH) calcd.
290.9020, found 290.9027.
4.1.17. 2-Bromo-2,3-dihydro-1H-inden-1-one (1q) [33]
1H NMR (500 MHz, CDCl3)
(ppm) 7.83 (d, J ¼ 7.5 Hz,1H), 7.66 (t,
d
4.1.27. 3-Bromo-1-(3-chlorophenyl)propan-1-one (2j)
J ¼ 7.5 Hz, 1H), 7.44 (m, J ¼ 11.8, 7.3 Hz, 2H), 4.65 (dd, J ¼ 7.5, 3.1 Hz,
1H NMR (500 MHz, CDCl3)
d (ppm) 7.92 (s, 1H), 7.83 (d,
1H), 3.84 (dd, J ¼ 18.1, 7.5 Hz, 1H), 3.43 (dd, J ¼ 18.1, 3.1 Hz, 1H), 13
C
J ¼ 8.8 Hz, 1H), 7.57 (d, J ¼ 8.0 Hz, 1H), 7.43 (t, J ¼ 7.9 Hz, 1H), 3.73 (t,
NMR (126 MHz, CDCl3)
126.6, 125.3, 44.3, 38.2.
d (ppm) 199.8, 151.3, 136.2, 133.7, 128.5,
J ¼ 6.8 Hz, 2H), 3.55 (t, J ¼ 6.8 Hz, 2H),13C NMR (126 MHz, CDCl3)
d
(ppm) 195.9, 137.9, 135.4, 133.7, 130.3, 128.4, 126.3, 41.8, 25.5.
IR(CCl4) 1691 cmꢁ1 (C]O). HRMS (C9H799BrClO, MþH) calcd.
4.1.18. 3-Bromo-1-phenylpropan-1-one (2a) [34]
1H NMR (500 MHz, CDCl3)
(ppm) 7.96 (d, J ¼ 7.1 Hz, 2H), 7.60 (t,
246.9525, found 246.9519.
d
J ¼ 6.9 Hz, 1H), 7.49 (t, J ¼ 7.7 Hz, 2H), 3.75 (t, J ¼ 6.9 Hz, 2H), 3.59 (t,
J ¼ 6.9 Hz, 2H), 13C NMR (126 MHz, CDCl3)
d(ppm) 197.2, 136.4,
4.1.28. 3-Bromo-1-phenylbutan-1-one (2k) [37]
1H NMR (500 MHz, CDCl3)
d
(ppm) 7.96 (d, J ¼ 7.1 Hz, 2H), 7.59 (t,
133.8, 129.0, 128.3, 41.7, 25.9.
J ¼ 7.4 Hz, 1H), 7.49 (t, J ¼ 7.8 Hz, 2H), 4.75e4.67 (m, 1H), 3.73 (dd,
J ¼ 17.3, 6.9 Hz, 1H), 3.40 (dd, J ¼ 17.3, 6.7 Hz, 1H), 1.82 (d, J ¼ 6.7 Hz,
4.1.19. 3-Bromo-1-(4-methoxyphenyl)propan-1-one (2b) [31]
1H NMR (500 MHz, CDCl3)
(ppm) 7.94 (d, J ¼ 8.9 Hz, 2H), 6.95
3H), 13C NMR (126 MHz, CDCl3)
49.6, 43.6, 26.7.
d 196.9, 136.6, 133.7, 129.0, 128.3,
d
(d, J ¼ 8.9 Hz, 2H), 3.88 (s, 3H), 3.74 (t, J ¼ 7.0 Hz, 2H), 3.52 (t,
J ¼ 7.0 Hz, 2H), 13C NMR (126 MHz, CDCl3)
d 195.7, 164.1, 130.6,
4.1.29. 3-Bromo-1-phenylpentan-1-one (2l) [7]
1H NMR (CDCl3, 200 MHz)
129.6, 114.1, 55.7, 41.4, 26.4.
d
7.97 (d, 2H, J ¼ 7.5 Hz), 7.53 (m, 3H),
4.58 (m,1H), 3.74, 3.42 (doublets of AB quartets, 2H, J ¼ 17.2, 7.2 Hz),
4.1.20. 3-Bromo-1-(4-hydroxyphenyl)propan-1-one (2c) [35]
1.97 (m, 2H), 1.11 (t, 3H, J ¼ 7.2 Hz), 13C NMR (CDCl3, 50 MHz)
1H NMR (500 MHz, CDCl3)
d
(ppm) 7.90 (d, J ¼ 8.7 Hz, 2H), 6.91
d
196.9, 136.7, 133.5, 128.8, 128.2, 51.8, 47.4, 32.2, 12.2.
(d, J ¼ 8.7 Hz, 2H), 3.73 (t, J ¼ 7.0 Hz, 2H), 3.52 (t, J ¼ 7.0 Hz, 2H),13
C
NMR (126 MHz, CDCl3)
26.2.
d(ppm) 196.3, 160.9, 131.0, 129.6, 115.8, 41.4,
4.1.30. 3-Bromo-2-methyl-1-phenylpropan-1-one (2o) [38]
1H NMR (500 MHz,CDCl3)
d
(ppm) 7.96 (d, J ¼ 7.1 Hz, 2H), 7.60 (t,
J ¼ 7.4 Hz, 1H), 7.50 (t, J ¼ 7.7 Hz, 1H), 3.95e3.87 (m, 1H), 3.80 (dd,
4.1.21. 3-Bromo-1-(2,5-dimethylphenyl)propan-1-one (2d)
1H NMR (500 MHz,CDCl3)
(ppm) 7.44 (s, 1H), 7.21 (d, J ¼ 7.7 Hz,
1H), 7.15 (d, J ¼ 7.7 Hz, 1H), 3.72 (t, J ¼ 6.8 Hz, 2H), 3.50 (t, J ¼ 6.8 Hz,
2H), 2.47 (s, 3H), 2.37 (s, 3H),13C NMR (126 MHz, CDCl3)
(ppm)
J ¼ 9.9, 7.0 Hz, 1H), 3.44 (dd, J ¼ 9.9, 6.4 Hz, 1H), 1.33 (d, J ¼ 7.0 Hz,
d
3H), 13C NMR (126 MHz, CDCl3)
128.6, 43.8, 34.1, 17.9.
d(ppm) 201.5, 136.0, 133.7, 129.0,
d
200.9, 137.0, 135.7, 135.5, 132.8, 132.3, 129.4, 44.2, 26.4, 21.2, 21.1.
4.1.31. 4H-Chromen-4-one (5p) [39]
1H NMR (500 MHz, CDCl3)
(ppm) 8.26 (dd, J ¼ 8.0, 1.5 Hz, 1H),
79
IR(CCl4) 1683 cmꢁ1 (C]O). HRMS (C11
H
14
BrO, MþH) calcd.
d
241.0228, found 241.0234.
7.99 (d, J ¼ 5.9 Hz, 1H), 7.75 (ddd, J ¼ 8.6, 7.2, 1.6 Hz, 1H), 7.53 (d,
J ¼ 8.4 Hz, 1H), 7.48 (t, J ¼ 7.2 Hz, 1H), 6.68 (d, J ¼ 5.8 Hz, 1H),13
C
4.1.22. 3-Bromo-1-(4-bromophenyl)propan-1-one (2e) [34]
NMR (126 MHz, CDCl3)
125.5, 125.1, 118.4, 113.2.
d(ppm) 177.9, 156.7, 155.5, 134.0, 126.0,
1H NMR (500 MHz, CDCl3)
d
(ppm) 7.82 (d, J ¼ 8.6 Hz, 2H), 7.63
(d, J ¼ 8.6 Hz, 2H), 3.73 (t, J ¼ 6.8 Hz, 2H), 3.54 (t, J ¼ 6.8 Hz, 2H),13
C
NMR (126 MHz, CDCl3)
25.6.
d(ppm) 196.2, 135.2, 132.3, 129.8, 129.1, 41.7,
Conflicts of interest
There are no conflicts to declare.
References
4.1.23. 3-Bromo-1-(4-chlorophenyl)propan-1-one (2f) [36]
1H NMR (500 MHz, CDCl3)
d
(ppm) 7.90 (d, J ¼ 8.6 Hz, 2H), 7.46
(d, J ¼ 8.6 Hz, 2H), 3.73 (t, J ¼ 6.8 Hz, 2H), 3.55 (t, J ¼ 6.8 Hz, 2H),13
C
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Please cite this article in press as: S. An, et al., Solvent free, light induced 1,2-bromine shift reaction of
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a-bromo ketones, Tetrahedron (2018),

S. An et al. / Tetrahedron xxx (2018) 1e7
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Please cite this article in press as: S. An, et al., Solvent free, light induced 1,2-bromine shift reaction of
https://doi.org/10.1016/j.tet.2018.10.015
a-bromo ketones, Tetrahedron (2018),