Solvent-Controlled α-Monobromination, α,α-Dibromination or Imidation of 1,3-Diketones with N-Bromosuccinimide

Article abstract of DOI:10.1002/ejoc.201800994

In this work, we present a solvent-controlled regioselective method for α-monobromination, dibromination or imidation of 1,3-diketones with N-bromosuccinimide under simple reaction conditions. The employment of solvents plays a key role on the reaction selectivity providing α-monobrominated, dibrominated and imidated products. Visible light irradiation accelerates the dibromination reaction of 1,3-diketones. In particular, one important solvent was found to be highly effective for the imidation of 1,3-diketones under base-free condition.

Full text of DOI:10.1002/ejoc.201800994

A Journal of
Accepted Article
Title: Solvent-Controlled α-Monobromination, α,α-Dibromination or
Imidationof 1,3-Diketones with N-Bromosuccinimide
Authors: Lianghua Zou, Yanchun Li, Pinggui Li, Jing Zhou, and
Zhimeng Wu
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800994
Link to VoR: http://dx.doi.org/10.1002/ejoc.201800994
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10.1002/ejoc.201800994
European Journal of Organic Chemistry
COMMUNICATION
Solvent-Controlled α-Monobromination, α,α-Dibromination or
Imidationof 1,3-Diketones with N-Bromosuccinimide
Liang-Hua Zou,*,a Yan-Chun Li,a Ping-Gui Li,b Jing Zhou,a and Zhimeng Wu*,a
peptide mimetics,17 -amino acids.18 Classical methods for the
Abstract: In this work, we present
a
solvent-controlled
synthesis of -amido -dicarbonylcompounds include strong base
mediated acylation of the ketimine derivatives of -aminoesters19
and the reduction of -hydroxyimino20 and phenylazo21 -
dicarbonyl compounds; the hydrolysis of oxazole-4-carboxylate
derivatives;22 and N–H insertion of metal carbenes.23 In addition,
direct -amination of the readily available -dicarbonylcompounds
has been investigated, including electrophilic amination of -
dicarbonylcompounds to azodicarboxylates24 and the N-selective
nitrosoaldol reaction.25 Despite the significant advances made in
the field,26 it is still highly desirable to develop new and efficient
methods for the direct -amination of the readily available -
dicarbonylcompounds.
regioselective method for -monobromination, dibromination or
imidation of 1,3-diketones with N-bromosuccinimide under simple
reaction conditions. The employment of solvents plays a key role on
the reaction selectivity providing -monobrominated, dibrominated
and imidated products. Visible light irradiation accelerates the
dibromination reaction of 1,3-diketones. In particular, one important
solvent was found to be highly effective for the imidation of 1,3-
diketones under base-free condition.
Introduction
Recently, NBS has been extensively investigated for the
imidation of various ketone compounds such as 1,3-dicarbonyl
compounds, phenyl ketones and chalcones,27 in which an organic
base [1,8-diazabicyclo(5.4.1)undec-7-ene] was required for the
activation of NBS. Herein, we report a solvent-controlled α-
monobromination, dibromination or imidation of 1,3-diketones
with NBS under simple base-free reaction conditions.28
Over the past years, extensive attention has been paid to
bromination reactions of carbonyl compounds due to their
importance in the synthesis of brominated carbonyl compounds
which are known as useful building blocks in the synthesis of
natural products and pharmaceuticals.1 Particularly, bromo-
functionalities exist widely in many industrially valuable products
such as pesticides, herbicides, fire retardants and various new
materials.2 In addition, related research shows that bromination at
the reactive position in
a 1,3-ketone compound enhances
bioactivity like cytotoxicity against breast cancer 1A9 cells
Results and Discussion
compared with unsubstituted compound.3
At first, we initiated the study on the bromination reaction of 1,3-
diphenyl-1,3-propanedione (1a) in the presence of 2.0 equivalents
of NBS by heating at 110oC under dioxygen atmosphere (Table 1).
Preliminary experiments with various solvents were screened for
their influence on the reaction behaviour (Table 1, entries 1-5). No
dibrominated product 2a was observed when DMSO or dioxane
was used in the reaction. Instead, only low yields of
monobrominated product 3a was obtained (Table 1, entries 1 and
2). The reaction in DMF and toluene provided product 2a in 36%
and 41% yields with a little product 3a, respectively (Table 1,
entries 3 and 4). Later, we focused on the transformation affording
dibrominated product 2a. The yield of 2a was increased to 51%
when acetonitrile was used as the medium (Table 1, entry 5).
Replacement of dioxygen with air or argon led to a minor change
of yield (Table 1, entries 6 and 7). Lower temperature was better
for the reaction, yielding product 2a in 60% yield (Table 1, entry 8).
Finally, the yield of 2a was enhanced to 82% yield by introducing
visible light irradiation to the reaction and increasing the loading of
1a to 2.5 equiv (Table 1, entry 9). It is noteworthy that the
monobrominated product 3a was formed in very low yields in all
the above entries. To our delight, 1,3-diketone was exclusively
monobrominated to afford product 3a in 95% yield at 25oC when
the reaction was performed using triethylorthoformate (TOF) as a
solvent under air atmosphere (Table 1, entry 10). An imidated
product 4a was afforded in 74% yield when the reaction was
performed at 130oC and the yield increased to 95% under argon
atmosphere instead (Table 1, entry 11). Lowering the reaction
temperature influenced the reaction obviously (Table 1, entry 12).
Other tested solvents such DMSO, dioxane, DMF, toluene or
CH3CN were all ineffective for this reaction (Table 1, entry 13).
Other imdation reagents N-iodosuccinimide (NIS) and N-
chlorosuccinimide (NCS) were also investigated in the reaction,
The traditional reagents for -bromination of 1,3-diketones
include molecular bromine,4 NBS5 and cupric bromide.6 In terms of
accessibility and ease of handing, NBS possesses many advantages.
For example, the byproduct succinimide can be easily recovered
and reconverted to NBS for subsequent reactions. Normally, some
radical initiators such as azobisisobutyronitrile (AIBN) or
dibenzoyl peroxide (BPO),7 or other additives,8 were required for
α-bromination of ketones with NBS. Various improved variants of
such reaction system have recently been developed. Typical
examples are NBS-NH4OAc,8b NBS-photochemical,9 NBS-
PTSA,10 NBS-silica supported NaHCO3,11 NBS-Amberlyst-15,12
NBS-Lewis acids,8a NBS-ionic liquids,13 among others.14 Although
notable advances have been addressed to the bromination of 1,3-
diketones,15 however, selective -mono or dibromination still
remains as a big challenge, especially for the substrates without α-
substituents.1d,8a
Importantly, -amido -dicarbonylcompounds are widely used
as organic intermediates in the synthesis of various heterocycles,16
[a]
[b]
The Key Laboratory of Carbohydrate Chemistry and
Biotechnology, Ministry of Education, School of Biotechnology,
School of Pharmaceutical Sciences, Jiangnan University, Lihu
Avenue 1800, Wuxi 214122, P.R. China
E-mail: zoulianghua@jiangnan.edu.cn; zwu@jiangnan.edu.cn
State Key Laboratory of Coordination Chemistry, Collaborative
Innovation Center of Chemistry for Life Sciences, School
ofChemistry and Chemical Engineering, Nanjing University,
Xianlin Avenue 163, Nanjing 210093, P. R. China
Supporting information for this article is available on the WWW
under https://www.eurjoc.com.
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10.1002/ejoc.201800994
European Journal of Organic Chemistry
COMMUNICATION
albeit provided product 4a in lower yields (Table 1, entries 14 and
Table 2. Dibronomination of 1,3-diketones in acetonitrile.a
15).
Table 1. Optimization on reaction conditions.a
Yield
(%)
Entry
R1
R2
Prod.
1
p-Tol
p-MeO-C6H4
p-Cl-C6H4
p-F-C6H4
p-CF3-C6H4
Ph
p-Tol
p-MeO-C6H4
p-Cl-C6H4
p-F-C6H4
p-CF3-C6H4
p-Tol
80
79
85
75
60
75
82
85
84
82
75
75
76
80
55
81
2b
2c
2d
2e
2f
2
3
4
5
6
2g
2h
2i
7
p-Tol
p-MeO-C6H4
p-MeO-C6H4
2-Naphth
Me
8
p-Cl-C6H4
p-Tol
9
2j
10
11
12
13
14
15
16
p-Tol
2k
2l
p-MeO-C6H4
2-Naphth
p-Br-C6H4
p-CF3-C6H4
EtO
Me
Me
2m
2n
2o
2p
2q
Me
Me
EtO
aReaction conditions: 1a (0.5 mmol), NBS (1 mmol), solvent
(2 mL), stirred for 12 h under dioxygen atmosphere. bIsolated yield.
cAir atmosphere. dArgon atmosphere. eUsing 40 W fluorescent
2-Thienyl
Me
aReaction conditions: Diketone 1b-q (0.5 mmol), NBS (1.25
mmol), CH3CN (2 mL), stirred with a 40 W fluorescent lamp at
room temperature for 12 h.
f
lamp and 2.5 equivalents of NBS. Using DMSO, dioxane, DMF,
toluene or CH3CN as solvent instead of TOF. gReplacement of
NBS with NIS. hReplacement of NBS with NCS. TOF:
triethylorthoformate. NBS: N-Bromosuccinimide. NIS: N-
Iodosuccinimide. NCS: N-Chlorosuccinimide.
Next the scope of the monobromination of 1,3-diketones was
investigated under the optimized reaction conditions (see
Table 1,entry 10) and the results were summarized in Table 3.
Symmetrical 1,3-diaryl-1,3-propanediones 1b and 1c were
efficiently employed in the procedure, providing the corresponding
With the optimized reaction conditions in hand (see entry 9,
Table 1), the scope of the dibromination of 1,3-diketones was
investigated and the results were summarized in Table 2. At first,
various symmetrical 1,3-diaryl-1,3-propanediones 1b-1f bearing
various electron-rich or electron-deficient aromatic groups were
employed. For example, Me-, MeO-, Cl-, F- and CF3- groups were
well tolerated in the reaction, providing products 2b-2f in yields
ranging from 60% to 85% (Table 2, entries 1-5). Apparently,
diketones containing electron-deficient substituents such as -F and
–CF3 gave lower yields. Subsequently some unsymmetrical 1,3-
diaryl-1,3-propanediones 1g-1j were also treated under the
conditions, affording products 2g-2j in good yields (Table 2,
entries 6-9). To further extend the substrate scope, alkyl
substituents were introduced to 1,3-diketones. Initially, substrates
1k-1o containing an aryl group and an alkyl group were reacted
under the optimized reaction conditions, providing products 2k-2o
in good yields (Table 2, entries 10-14). One example 1p with two
alkyl groups was also tried in the reaction, giving the desired
product 2p in a fair yield of 55% (Table 2, entry 15). Furthermore,
the methodology was effectively applied to a heterocycle 1,3-
diketone 1q as well, yielding product 2q in good yield of 81%
(Table 2, entry 16).
Table 3. Monobronomination of 1,3-diketones in TOF.a
Yield
Entry
R1
R2
Prod.
(%)
80
81
90
90
75
75
1
2
3
4
5
6
p-Tol
p-MeO-C6H4
Ph
p-Tol
p-MeO-C6H4
p-Tol
3b
3c
3d
3e
3f
p-Tol
p-MeO-C6H4
p-OMe-C6H4
Me
p-Cl-C6H4
p-MeO-C6H4
3g
aReaction conditions: Diketone 1 (0.5 mmol), NBS (1 mmol), TOF
(2 mL), stirred at room temperature for 12 h. TOF:
triethylorthoformate.
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10.1002/ejoc.201800994
European Journal of Organic Chemistry
COMMUNICATION
products 3b and 3c in 80% and 81% yields, respectively (Table 3,
entries 1-2). Unsymmetrical substrates 1g, 1h and 1i were also
reacted well, affording products 3d, 3e and 3f in good to excellent
yields (Table 3,entries 3-5). In addition, one substrate containing
an aryl group and an alkyl group was tried under the optimized
reaction conditions as well, providing product 3g in 75% yield
(Table 3, entry 6).
changed reaction conditions. Considering the monobrominated
product 3a might be the intermediate for the synthesis of the
imidated product 4a, one experiment was carried out using 3a as
substrate under optimized reaction conditions (Scheme 1). Indeed,
the reaction time could be shortened to 2 hours, providing the
desired product 4a in 98% yield, which demonstrated the excellent
selectivity of the reactions in TOF.
Finally, the scope of 1,3-diketones imidation was investigated
(Table 4). Symmetrical 1,3-diaryl-1,3-propanedioneswith Me-,
MeO- and F- substituents reacted well with N-bromosuccinimide to
provide the corresponding products 4b, 4c and 4d in 98%, 86% and
85% yields, respectively (Table 4, entries 1-3). Unsymmetrical
substrates were also employed in the procedure, affording products
4e, 4f and 4g in good to excellent yields (Table 4, entries 4-6). In
all these cases, the products were obtained in ketone form.
Interestingly, when substrate containing an aryl group and an alkyl
group was utilized under the optimized reaction conditions, enol
product 4h (in complete enol from) was produced in 70% yield
(Table 4, entry 7). In addition, another enol product 4i (in complete
enol from) was also obtained in a good yield of 82% when one
substrate containing two alkyl groups was used (Table 4, entry 8).
Scheme 1. Imidation of 3a with NBS.
We assumed that α-monobromination or dibromination of 1,3-
diketones mainly depended on the solvent effect. A possible
mechanism for the imidation reaction was proposed as shown in
Scheme 2. In order to elaborate easily, take the synthesis of 4a as
one example. The mechanism may include the promotion of
enolate formation29 and the activation of NBS30 via hydrogen
bonds with the aid of the solvent TOF, simultaneously. The latter
involves the formation of an active intermediate I, which reacts
with 3a to produce the final product 4a.
Table 4. The scope of imidation of 1,3-diketones.a
Scheme 2. Proposed mechanism.
Conclusions
In summary, we have developed a solvent-controlled switchable
method for α-monobromination, dibrominationor imidation of 1,3-
diketones with NBS under simple reaction conditions, providing α-
monobrominated, dibrominated or imidated products by adjusting
the reaction medium. Visible light could efficiently promote the
dibromination reaction. No base was required for the imidation
reaction with the aid of a special solvent. The reaction has a broad
substrate scope and is easy to handle, providing practical routes to
these divergent diketone derivatives.
Experimental Section
aReaction conditions: Diketone 1 (0.5 mmol), NBS (1 mmol), TOF
Representative procedure for the synthesis of product 2a starting
from propanedione 1a: A 10 mL sealed tube equipped with a
magnetic stirring bar was charged with all solid components (if
liquild, added after flushed with argon) including 1a (112.0 mg,
0.5 mmol), NBS (222.5 mg, 1.25 mmol). The aperture of the tube
(2 mL), stirred at 130oC for 12 h. TOF: triethylorthoformate.
In this work, solvent effect played an important role in the
transformations between 1,3-diketones and NBS under minor
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10.1002/ejoc.201800994
European Journal of Organic Chemistry
COMMUNICATION
was then covered with a rubber septum, and purged with argon
flow for 5 minutes. After the addition of acetonitrile (2 mL) by
syringe, the septum was quickly replaced by a teflon-coated screw
cap, and the reaction vessel was placed under visible light (40 W
fluorescent lamp) at room temperature and stirred for 12 h, and
then diluted with ethyl acetate. The resulting solution was directly
concentrated under reduced pressure. Purification by flash
chromatography (petroleum ether/ethyl acetate = 50:1) gave 2a as a
white solid in 82% yield (156.6. mg, 0.41 mmol).
was then covered with a rubber septum, and purged with argon
flow for 5 minutes. After the addition of triethylorthoformate
(2 mL) by syringe, the septum was quickly replaced by a teflon-
coated screw cap, and the reaction vessel was moved to a pre-
heated device at 130oC and stirred for 12 h, and then diluted with
ethyl acetate. The resulting solution was directly concentrated
under reduced pressure. Purification by flash chromatography
(petroleum ether/ethyl acetate = 2:1) gave 4a as a white solid in 95%
yield (152.6. mg, 0.475 mmol).
Representative procedure for the synthesis of product 3a starting
from propanedione 1a: A 10 mL sealed tube equipped with a
magnetic stirring bar was charged with all solid or liquid
components including 1a (112.0 mg, 0.5 mmol), NBS (177.8 mg,
1.0 mmol). After the addition of triethylorthoformate (2 mL) by
syringe, the reaction vessel was placed under air at room
temperature and stirred for 12 h, and then diluted with ethyl acetate.
The resulting solution was directly concentrated under reduced
pressure. Purification by flash chromatography (petroleum
ether/ethyl acetate = 10:1) gave 3a as a white solid in 95% yield
(144.0 mg, 0.48 mmol).
Acknowledgements
This work is financially supported by NSF of Jiangsu Province
(BK20150129), Top-notch Academic Programs Project of Jiangsu
Higher Education Institutions (No. PPZY2015B146), the Project
for Jiangsu Scientific and Technological Innovation Team, Fund
for Jiangsu Distinguished Professorship and the 111 Project (No.
111-2-06).
Representative procedure for the synthesis of product 4a starting
from propanedione 1a: A 10 mL sealed tube equipped with a
magnetic stirring bar was charged with all solid components (if
liquild, added after flushed with argon) including 1a (112.0 mg,
0.5 mmol), NBS (177.8 mg, 1.0 mmol). The aperture of the tube
Keywords: 1,3-Diketones • Monobromination • Dibromination •
Base-free • Selectivity
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10.1002/ejoc.201800994
European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
Selective Functionization
Liang-Hua Zou,* Yan-Chun Li, Ping-Gui
Li, Jing Zhou, and Zhimeng Wu*
Page No. – Page No.
Solvent-Controlled α-Monobromination,
α,α-Dibromination or Imidationof 1,3-
Diketones with N-Bromosuccinimide
A
solvent-controlled regioselective method for α-monobromination, α,α-
dibromination or imidation of 1,3-diketones was developed with N-
bromosuccinimide. Dibromination of 1,3-diketones took place efficiently in
acetonitrile with the aid of visible light irradiation. One important solvent
triethylorthoformate was found to be highly effective for the monobromination of 1,3-
diketones, as well as an ideal medium for the imidation reaction under base-free
condition.
This article is protected by copyright. All rights reserved.