Stereoselective bromination of 2-vinyl chromones using NBS

Article abstract of DOI:10.1080/00397910903534007

A simple one-step synthetic methodology for stereoselective synthesis of E- and Z-3-bromo-2-vinyl chromones in quantitative yield in polar solvents under ambient conditions without the use of catalysts is reported. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903534007

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Stereoselective Bromination of 2-Vinyl
Chromones Using NBS
Seema Bhatnagar a , Swati Kaushik a & Shakti Sahi b
a Amity Institute of Biotechnology, Amity University , Noida, India
b School of Biotechnology, Gautum Buddha University , Noida, India
Published online: 15 Dec 2010.
To cite this article: Seema Bhatnagar , Swati Kaushik & Shakti Sahi (2010) Stereoselective
Bromination of 2-Vinyl Chromones Using NBS, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:2, 219-226, DOI:
10.1080/00397910903534007
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Synthetic Communications1, 41: 219–226, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903534007
STEREOSELECTIVE BROMINATION OF 2-VINYL
CHROMONES USING NBS
Seema Bhatnagar,1 Swati Kaushik,1 and Shakti Sahi2
1Amity Institute of Biotechnology, Amity University, Noida, India
2School of Biotechnology, Gautum Buddha University, Noida, India
A simple one-step synthetic methodology for stereoselective synthesis of E- and
Z-3-bromo-2-vinyl chromones in quantitative yield in polar solvents under ambient
conditions without the use of catalysts is reported.
Keywords: N-Bromosuccinimide; stereoselective bromination; vinyl chromones
INTRODUCTION
Vinyl analogs of chromones, a rare class of oxygen heterocycles, possess a wide
spectrum of biological activity. The natural and synthetic analogs of vinyl chromones
have been reported to exhibit antineoplastic, antiallergic, antihepatoprotective, and
estrogenic activity.[1] In view of our ongoing study to evaluate the biological activity
of these compounds, we were interested in the preparation of 3-halogenated-2-vinyl
chromones. Although synthetic methodologies for 3-halogenated chromones that
lack substitution at C-2 have been reported,[2] efficient methodologies for preparation
of 3-halogenated-2-vinyl chromones were not available.
Literature reports indicated that the chemical reactivity of vinyl chromones
had been reasonably explored.[3] The reactive center exploited in these reactions
=
was the acidic Ca Cb of vinyl chromone. The challenge in carrying out electro-
=
philic=nucleophilic reactions on vinyl chromones was the acidic Ca Cb bond, which
was more susceptible to attack by various electron-acceptor and electron-donor sys-
tems. Electrophilic attack on the pyran ring is influenced by ring substituents, and
electron donors at C-2 and C-3 were reported to activate the adjacent ring position.[4]
Based on these reports, it was considered worthwhile to explore the reactivity of
3-hydroxy-2-vinyl chromones in the presence of brominating agents. Herein, we
report a one-step stereoselective bromination reaction of E-3-hydroxy-2-vinyl chro-
mones to obtain quantitative yield of E- and Z-3-bromo-2-vinyl chromones using
N-bromosuccinimide (NBS) as the brominating agent.
Received August 22, 2009.
Address correspondence to Seema Bhatnagar, Amity Institute of Biotechnology, Amity University,
Sector 125, Noida, Pincode 201303, UP, India. E-mail: sbhatnagar1@amity.edu
219

220
S. BHATNAGAR, S. KAUSHIK, AND S. SAHI
RESULTS
Our studies were carried out on E-3-hydroxy-2-vinyl chromones, which were
prepared by methods reported earlier in literature.[5] According to literature reports,
E-3-hydroxy-2-vinyl chromones reacted with pyridinium tribromide to furnish
dibromo and tribromo products. It was also reported that the type of substituent
on ring B of vinyl chromone affected the product that was isolated.[6a] There were
also reports that 3-bromo-2-vinyl chromones can be produced through cyclization
of intermediate 2-bromo-1,3-diones using Br2, dioxane, and as NBS=CCl4. The pro-
ducts were obtained as mixtures that could not be separated even after repeated crys-
tallization.[6b] The lack of an efficient one-step procedure with quantitative yields of
product prompted us to explore this reaction further. In our initial studies, we
attempted bromination with Br2=CH2Cl2. In both cases, we obtained a mixture of
products that was difficult to separate by column chromatography. The presence
of more than one reactive center in the molecule prompted us to attempt bromin-
ation under milder conditions. In the light of reagents that had been used to carry
out 3-halogenation in benzopyranones, it was concluded that NBS in the presence
of various aprotic solvents had been used to get quantitative yield of 3-halogenated
product. Therefore, as an alternative strategy, the bromination reactions were
attempted using N-bromosuccinimide in the presence of a,a-azobisisobutyronitrile
(AIBN) in benzene. Compounds 2 and 3(a–c) were obtained in poor yield. A signifi-
cant observation made during the course of this study: compounds 2 and 3(a–c) were
formed even in the absence of free radical initiator AIBN. Further, as a modification;
the reaction was attempted with aprotic and dipolar aprotic solvents such as
CH3CN, dimethylformamide (DMF), and CH2Cl2. It was observed that if reaction
was carried out in CH2Cl2 for 1 h, it led exclusively to the formation of compounds
2(a–c). Further exploration of the reaction conditions revealed that if compounds
2(a–c) were suspended overnight in methanol, the products isolated were 3(a–c).
Therefore, as a modification, the reaction of compounds 1(a–c) was attempted in
protic solvents for 1 h, and 3(a–c) were obtained in good yield. A summary of the
different reaction conditions explored is presented in Scheme 1.
The structure elucidations of the compounds were done spectroscopically.
Compounds 2a and 3a were chosen as representative compounds, and detailed spec-
troscopic studies were carried out. The mass spectra of compound 2a and 3a indi-
cated a molecular ion peak corresponding to the presence of a monobromo
derivative. However, the possibility of bromination at the Cb-position had to be
ruled out. The infrared (IR) spectrum of compounds 2a and 3a exhibit peaks at
1738 cmꢀ1, 1628 cmꢀ1, and 1738 cmꢀ1, and 1633 cmꢀ1 respectively for the C-4 car-
bonyl group and vinyl group along with similar fingerprint region. In comparison,
compound 1a exhibited corresponding group peaks at 1633 cmꢀ1 and 1596 cmꢀ1
.
An increase in the absorption frequency of the carbonyl group suggests field effect
in its vicinity. Also, if bromination had occurred at the Cb-position it would have
=
resulted in a decrease in absorption frequency of the Ca Cb bond because of the
presence of bulky substituent[7] as compared to the starting compound 1a, which
was not observed. In our case, we have observed an increase in the absorption fre-
=
quency of the Ca Cb bond in the product. Furthermore, the similarity in the finger-
print region of the spectrum suggests similar electronic environments in 2a and 3a.

BROMINATION OF 2-VINYL CHROMONES
221
Scheme 1. (i) C6H6, NBS, AIBN; (ii) DMF NBS, AIBN; (iii) CH2Cl2, NBS, AIBN; (iv) DMF, NBS, dark;
(v) CH2Cl2, NBS, Dark; (vi) CH2Cl2, NBS, hn; (vii) CH2Cl2, NBS, 1 h; (viii) NBS, MeOH,1 h; (ix) MeOH,
overnight.
The NMR of compounds 2a and 3a characteristically depicted the absence of a
hydroxyl group at 12.2 ppm and a downfield shift in the position of Ha to d 7.47,
3
7.45 and d 7.51, 7.47, respectively as compared to d 6.80 in 1a. The JCH coupling
constants for Ha and Hb protons were 9.9 Hz for 2a and 18.9 Hz for 3a suggesting
them to be Z- and E-isomers, respectively. Long-range coupling with C-20 in 2a and
C-20 and C-60 in 3a was also observed, leading to a difference in multiplicity pattern
of Hb. 13C experiments on 2a and 3a revealed that Ca and Cb appeared at 129 ppm,
147 ppm and 138 ppm, 146 ppm, respectively. An unusual downfield shift in the pos-
ition of C-8 was also observed at 141 ppm and 128 ppm for 2a and 3a, respectively.
Compound 2a can exist in two possible conformations: I and II. Earlier reports[5]
had concluded conformations I based on energy calculations and upfield shift of
H-8, which was contrary to results obtained in our case. In our case, the energy mini-
mizations for 2a and 3a were conducted with Macromodel software[8] using Opti-
mized Potentials for Liquid Simulations 2005 (OPLS 2005) force field. The
convergence threshold used was the rms gradient of 0.01. The energy calculation
using truncated Newton conjugate gradient (TNCG) reveals that conformation I
has energy ꢀ8781 kJ=mol, and II has ꢀ8799 kJ=mol, while 3a has energy
ꢀ8788.15 kJ=mol. Our preference for conformation II is based on the spectroscopic
data and energy calculations. The energy-minimized structures using TNCG of 2a
and 3a are depicted in Fig. 1.
Detailed correlation spectroscopy (COSY) experiments were also performed on
2a and 3a. In the case of compound 2a, correlation peaks were observed between
H-20 at d 7.15 and H b at 7.75; however, no correlation peaks were observed between
H-60 at d 7.53 and Hb at d 7.73. Correlation peaks were also observed between d 7.50
and d 7.70, indicating correlation between Ha and Hb protons. In the case of com-
pound 3a, correlation peaks were observed between H-20 at d 6.90 and Hb at d 7.70;
H-60 at d 7.04 and H b at d 7.69; and Ha at d 7.44 and Hb at d 7.76.

222
S. BHATNAGAR, S. KAUSHIK, AND S. SAHI
Figure 1. Energy-minimized structures of compounds 2a and 3a. I and II are the energy-minimized con-
formations of compound 2a.
DISCUSSION
The two important observations made during the course of the study were that
reactions invariably occurred at the C-3 position, however, there was a difference in
the stereochemical outcome of products 2 and 3. Bromination reactions using NBS
occur by a free radical mechanism involving allylic or benzylic protons and electro-
philic addition. Our observation of the occurrence of a reaction in the absence of
light or free radical initiator (although slight variation in the reaction time was
=
observed) and the presence of an intact Ca Cb bond in the products suggested that
the reaction was not occurring by free radical or electrophilic mechanism. Based on
these facts, the question arises with regard to the reactive species involved in bromin-
ation. The mechanism of NBS decomposition has been studied under different reac-
tion conditions in detail, and several mechanisms have been proposed.[9] It has been
observed that the results of bromination reactions using NBS were influenced by rad-
ical intermediates other than the bromine atoms.[10] Goldfinger had earlier suggested
that NBS reacted rapidly with HBr to generate a low concentration of bromine to
facilitate chain propagation but prevent ionic addition to olefin. It has also been
suggested that a succinimidyl radical may be a possible chain carrier. Competitive
brominations in methylene chloride, a solvent in which NBS has minimal solubility,
has been known to consume bromine as bromodichloromethane.[11] These observa-
tions lead us to conclude that the when dichloromethane is being used as solvent, the
possible intermediate was bromodichloromethane formed by hydrogen abstraction
from dichloromethane, further leading to the formation of HBr, which was the
reactive species leading to the bromination product. The change in stereochemistry
from E ! Z may be due to a second attack of HBr on the vinylic carbon. It has been
reported that vinylic systems usually undergo nucleophilic addition elimination
reaction, which leads to products of opposite stereochemistry because of olefin
inversion. Olefin inversion is known to occur by various mechanisms, which include
thermal–chemical, photoisomerization, and addition elimination reactions.[12]

BROMINATION OF 2-VINYL CHROMONES
223
Oxidative addition to an olefin involving nucleophilic substitution followed by
reductive elimination usually leads to an olefin with inverted geometry. Several
mechanisms have been proposed that involve syn=anti addition and elimination,
leading to different stereochemical outcomes. The addition of electrophilic reagents
to olefins is usually anti.[12] Literature reports indicate that bromine and modified
brominating reagents have been used for controlled anti addition of halogens olefins.
It is proposed that HBr was first undergoing anti addition followed by syn elimin-
ation, leading to inversion in stereochemistry from E ! Z in the product.
Analogously, when methanol was being used as solvent, the possible inter-
mediate leading to the formation of HBr could be bromomethanol by hydrogen
abstraction. However, in this case, because of the presence of protic solvents, the
reaction was probably proceeding through a carbocation intermediate. The retention
in stereochemistry further suggests possible neighboring group participation could
be operational. The attack of the bromide ion led to the product with an overall
retention in configuration.
EXPERIMENTAL
All the melting points were determined on a hot-stage apparatus and are uncor-
rected. Infrared (IR) spectra were recorded on a Bruker instrument as KBr discs or neat.
1H NMR spectra were recorded on a Bruker Spectrospin 300-MHz instrument. The
solvent used was CDCl3 unless mentioned otherwise. Compounds have been analyzed
using 2D correlation spectroscopy unless mentioned otherwise. 4H-1-Benzopyran-
4-one-3-hydroxy-2-(2-phenylethenyl) 1(a–c) were prepared by reported procedures.[5]
Synthesis of Z-3-Bromo-2-(phenyl ethenyl) Benzopyran-4-ones (2a)
N-Bromosuccinimide (6.75 g, 0.038 mol) was added to the compound
3-hydroxy-2-(2-phenyl ethenyl) benzopyran-4-one (10.0 g, 0.038 mol) in dry CH2Cl2
(50 ml),and the mixture was stirred at room temperature for an hour. The reaction
was monitored by thin-layer chromatography (TLC), and thereafter the excess
CH2Cl2 was distilled off in vacuo. The reaction mixture was quenched with water
and extracted with ethyl acetate (3 ꢁ 25 ml). The organic layers were pooled, washed
with brine, and dried over anhydrous sodium sulfate to obtain the product. Yield
68.26%, mp 144–146 ꢂC, C17H11O2Br 326 (Mþ), IR (cmꢀ1) 1738 cmꢀ1, 1644 cmꢀ1
;
NMR (CDCl3) 7.85 (d, 1H, H-8), 7.80 (d, 1H, H5), 7.75, 7.69 (dd, J ¼ 9.6 Hz,
¼Cb H) 7.47, 7.45 (d, 2H, ¼CaH, H-60 (J ¼ 7.8 Hz) merged), 7.33 (m, 3H, H40,
H50, H7), 7.00–7.15 (m, 3H, H-20, H-6, H-4); C13 (CDCl3) 162.38 (C2), 122.64
(C3), 194.6 (C4), 130.96 (C5), 127.6 (C7), 141.02 (C8), 159.12 (C9), 110.20 (C10),
129.88 (Ca), 147.54 (Cb), 135.57 (C10), 126.23 (C20), 121.99 (C-30), 121.46 (C40),
128.93 (C50,6), 144.53 (C60).
E-3-Bromo2-(2-phenyl ethenyl) Benzopyran-4-one (3a)
N-Bromosuccinimide (6.75 g, 0.038 mol) was added to the compound
3-hydroxy-2-(2-phenyl ethenyl) benzopyran-4-one (10.0 g, 0.038 mol) in methanol
(50 ml), and the mixture was stirred at room temperature for an hour. The reaction

224
S. BHATNAGAR, S. KAUSHIK, AND S. SAHI
was monitored by TLC, and thereafter the insoluble solid was crystallized using
hexane–ether mixture to obtain an insoluble solid, which was filtered off. Yield
(88.63%), mp 152–153 ꢂC, C17H11O2Br 326 (Mþ), IR (cmꢀ1) 1738cmꢀ1, 1633 cmꢀ1
;
NMR (CDCl3) 7.84 (d, 1H, H-5), 7.72 (m, 1H, Hb), 7.51, 7.47 (m3H, Ha, H-6,
H-8), 7.39 (m, 3H, Ar-H), 7–7.07 (m, 2H, Ar-H), 6.93 (t, 1H, Ar-H); C13 (CDCl3)
159.12 (C2), 122.64 (C3), 193.723 (C4), 131.69 (C5), 129.67 (C6,50), 127.6 (C7),
128.93 (C8), 162.38 (C9), 110.34 (C10), 138.7 (Ca), 146.47 (Cb), 135.71 (C10),
120.57 (C20), 121.17 (C30), 126.40 (C40), 128.93 (C8), 143.71 (C60).
Synthesis of Z-3-Bromo-2-[2-phenyl(-2’-methoxy) ethenyl]
Benzopyran-4-ones (2b)
To the compound 3-hydroxy-2-(phenyl ethenyl)20-methoxy benzopyran-4-one
(10.0 g, 0.034 mol) in dry CH2Cl2 (50 ml), NBS (9.081 g, 0.051 mol) was slowly
added. The reaction mixture was stirred at room temperature for 1.5 h. Reaction
was monitored by TLC, and the excess CH2CL2 was distilled off in vacuo. The reac-
tion mixture was quenched with water and extracted with ethyl acetate (3 ꢁ 25 ml).
The organic layers were pooled, washed with brine, and dried over anhydrous
sodium sulfate to obtain the product. Yield (76%) mp ¼ 128–130 ꢂC.
Synthesis of E-3-Bromo-2-[2-phenyl(-2’-methoxy) ethenyl]
Benzopyran-4-ones (3b)
N-Bromosuccinimide (6.75 g, 0.038 mol) was added to the compound
3-hydroxy-2-(2-phenyl ethenyl) benzopyran-4-one (10.0 g, 0.038 mol) in methanol
(50 ml) and the mixture was stirred at room temperature for 30 min. The reaction
was monitored by TLC, and thereafter the insoluble solid was crystallized using
hexane–CH3OH to obtain an insoluble solid, which was filtered off. Yield (75%).
E,Z mass 356 (Mþ); IR 1736 cmꢀ1; 1640; NMR (CDCl3) d 7.86, 7.78 (dd, J ¼ 8.1 Hz,
b H, 7.49, 7.46 (d, 2H, aH, Ar-H merged), 7.43–6.84 (m, 8H, Ar-H).
Synthesis of Z-3-Bromo-2-(2-methyl ethenyl) Benzopyran-4-one (2c)
N-Bromosuccinimide (8.81 g, 0.038 mol) in CH2CL2 (50 ml) was slowly added
to compound 3-hydroxy-2-(methyl) benzopyran-4-one (10.0 g, 0.038 mol) in dry
CH2CL2. The mixture was stirred at room temperature for 1 h. Reaction was mon-
itored by TLC, and thereafter the excess CH2CL2 was distilled off in vacuo. The
reaction mixture was quenched with water and extracted with ethyl acetate
(3 ꢁ 25 ml). The organic layers were pooled, washed with brine, and dried over anhy-
drous sodium sulfate to obtain the product. Yield (78%), oil.
Synthesis of E-3-Bromo-2-(2-methyl ethenyl) Benzopyran-4-one (3c)
N-Bromosuccinimide (6.75 g, 0.038 mol) was added to the compound
3-hydroxy-2-(2-phenyl ethenyl) benzopyran-4-one (10.0 g, 0.038 mol) in methanol
(50 ml), and the mixture was stirred at room temperature for an hour. The reaction

BROMINATION OF 2-VINYL CHROMONES
225
was monitored by TLC, and thereafter the insoluble solid was crystallized using
hexane–MeOH to obtain an insoluble solid, which was filtered off. Yield (76%), oil.
E,Z mass 265 (Mþ); IR 1708 cmꢀ1, 1644 cmꢀ1; NMR (CDCl3) 7.82, 7.77 (dd,
J ¼ 8.1 Hz, ¼Cb H), 7.48, 7.46 (d, 2H, aH, Ar-H merged), 7.02–6.70 (m, 3H, Ar-H),
3.38 (s, 3H, CH3).
ACKNOWLEDGMENTS
S. B. gratefully acknowledges the financial assistance provided by the
Department of Science and Technology for the study. The invaluable contributions
by Dr. Shakti Sahi in carrying out energy calculations using Macromodel software
are gratefully acknowledged. The assistance provided by the Indian Institute of
Technology, New Delhi, and Central Drug Research Institute, Lucknow, for analyti-
cal analysis of the samples are gratefully acknowledged. All authors gratefully
acknowledge infrastructure facilities provided by Amity Institute of Biotechnology,
Amity University, Noida.
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