Solar photo-thermochemical syntheses of 4-bromo-2,5-substituted oxazoles from N-arylethylamides

Article abstract of DOI:10.1039/c3ra47603k

Solar photo-thermochemical C(sp³)-H bromination, conducted efficiently in a specially designed reactor, was reported recently. In the present study, the more complex formation of 4-bromo-2,5-substituted oxazoles from N-arylethylamides using this approach was achieved. The one-pot syntheses were carried out with N-bromosuccinimide-dichloroethane over 6 h (10.00 am to 4.00 pm) on sunny days. The isolated yields were in the range 42-82%. Benzylic bromination, followed by O-C bond formation through intramolecular nucleophilic substitution, and a second benzylic bromination followed by HBr elimination, gave the oxazole ring. A third bromination of the ring yielded the final product. The feasibility of synthesizing thiazole derivatives using a similar approach was also demonstrated. During the course of the reactions, succinimide and HBr were co-generated in the aqueous phase. Treatment with NaBrO₃ and additional acid returned the reagent in 57% isolated yield upon chilling. The overall methodology was greener as a result.

Full text of DOI:10.1039/c3ra47603k

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Solar photo-thermochemical syntheses of
4-bromo-2,5-substituted oxazoles from
Cite this: RSC Adv., 2014, 4, 12252
N-arylethylamides†
ab
Milan Dinda,a Supravat Samanta,a Suresh Eringathodiab and Pushpito K. Ghosh
*
Solar photo-thermochemical C(sp3)–H bromination, conducted efficiently in a specially designed reactor,
was reported recently. In the present study, the more complex formation of 4-bromo-2,5-substituted
oxazoles from N-arylethylamides using this approach was achieved. The one-pot syntheses were carried
out with N-bromosuccinimide–dichloroethane over 6 h (10.00 am to 4.00 pm) on sunny days. The
isolated yields were in the range 42–82%. Benzylic bromination, followed by O–C bond formation
through intramolecular nucleophilic substitution, and a second benzylic bromination followed by HBr
elimination, gave the oxazole ring. A third bromination of the ring yielded the final product. The feasibility
of synthesizing thiazole derivatives using a similar approach was also demonstrated. During the course of
the reactions, succinimide and HBr were co-generated in the aqueous phase. Treatment with NaBrO3
and additional acid returned the reagent in 57% isolated yield upon chilling. The overall methodology
was greener as a result.
Received 13th December 2013
Accepted 14th January 2014
DOI: 10.1039/c3ra47603k
www.rsc.org/advances
which undergo facile intramolecular vinylation of the amide
oxygen to provide a broader range of oxazoles.14 A method was
1. Introduction
also developed for the cyclization of enamides to oxazoles, even
without vinylic C–H functionalization. This was achieved using
a metal catalyst and an oxidant.15 Cyclization of activated N-allyl
benzamide with excess N-bromosuccinimide (NBS) without any
metal catalyst has also been reported recently.16
Utilisation of solar radiation as a simultaneous source of
light and heat to drive organic reactions is a desirable objective.
Photo-thermochemical C(sp3)–H bromination reactions, driven
solely by solar energy in a specially designed solar photo-ther-
mochemical reactor (SPTR), were reported recently.17 In our
quest to expand the scope of reactions conducted in a SPTR, the
present study envisaged the assembly of more complex organic
molecules from simple building blocks through multi-step
solar-driven reactions under metal-free conditions. In partic-
ular, the syntheses of 2,5-substituted oxazoles from N-arylethy-
lamides were considered (Scheme 2). The reaction involved two
2,5-Substituted oxazoles were isolated from natural sources by
Belzile.1 It was subsequently established that this functional
moiety acts against diabetes, breast cancer and pancreatic
cancer.2 Besides the naturally occurring derivatives, similar
synthetic oxazoles are important in drug discovery research.3
Such molecules are also of interest in the agrochemical, uo-
rescent dye and polymer industries,4 and serve as ligands in
various metal-catalyzed reactions.5
Some of the classical strategies developed for the syntheses
of substituted oxazoles include the Davidson synthesis, Fischer
¨
synthesis, Japp synthesis, Schollkopf synthesis and the Nier-
enstein reaction.6 Common approaches have been used to
convert acyclic precursors into oxazole rings (Scheme 1). An
effective cyclization process to prepare a range of substituted
and complex oxazoles is the Bronsted- or Lewis acid-catalysed
Robinson–Gabriel condensation.7 Synthesis of oxazole units
from N-propargyl benzamide starting materials using Au,8 Pd,9
Ag,10 Cu,11 and Mo12 metal catalysts, or DBU as a basic catalyst,13
is also of considerable importance. Enamides bearing b-vinylic
C–heteroatom bonds have been generated, or isolated in situ,
aAcSIR–CSMCRI, Bhavnagar 364002, Gujarat, India
bCentral Salt and Marine Chemicals Research Institute, Council of Scientic &
Industrial Research, G. B. Marg, Bhavnagar 364002, Gujarat, India. E-mail:
pkghosh@csmcri.org; Fax: +91-278-2567562
† Electronic supplementary information (ESI) available: Analytical data; NMR,
FT-IR and HR-MS data of some selective compounds. CCDC 964937–964941. See
DOI: 10.1039/c3ra47603k
Scheme 1 Approaches to the synthesis of 2,5-substituted oxazoles.
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solar photo-thermochemical benzylic bromination steps, as
well as solar thermochemical reaction steps, for construction of
the oxazole ring. The amide substrates could be easily synthe-
sized from corresponding acid chlorides and amines. Results of
the study are presented herein.
2. Results and discussion
The reaction proposed in Scheme 2 was attempted employing
4-nitro-N-phenethylbenzamide as the initial substrate. The
studies were conducted with a 36 W compact uorescent lamp
(CFL), exhibiting a similar spectro-radiometric prole as solar
radiation (Fig. 1). An oil bath was used as the source of heat. The
reactions were conducted with 2 equivalents of NBS using
different solvents. 1,2-Dichloroethane (DCE) gave the maximum
oxazole formation among all solvents studied at 70 ꢀC under
illumination (entry 6, Table S1†). A mixture of the free (A) and
4-bromo (B) oxazole derivatives were obtained, their isolated
yields being 27% and 21%, respectively. Both products were
characterized by spectroscopic techniques as well as through
single crystal XRD (Table S2†). B could be obtained in 87%
isolated yield from A through the reaction of eqn (1), conducted
with 1 equivalent of NBS in DCE under reux conditions in the
dark (exposure to light, however, had no detrimental effect).18
The 4-bromo derivatives are important as these afford further
functionalisation of the oxazole moiety (Scheme 3).19 Further
studies were focused on its exclusive formation and, for this
purpose, the reaction with 4-nitro-N-phenethylbenzamide was
repeated with 3 equivalents of NBS (entry 7, Table S1†). B
was obtained selectively in 61% isolated yield, whereas A was
formed only in trace amounts. Oxazole formation did not
proceed to any signicant extent when the above reaction was
conducted at room temperature under CFL illumination (entry
8, Table S1†), in tꢀhe presence of TEMPO (entry 9, Table S1†) or
in the dark at 70 C (entry 10, Table S1†).
Fig. 1 Spectroradiometric plots showing intensity versus wavelength
of 2ꢁ sunlight incident on the SPTR (blue) and a 36 W CFL lamp
(green).
isolated yield (entry 1, Table 1). Data of the prevailing ambient
conditions on the day of the experiment, along with data on the
solar intensity inside the SPTR and the reaction temperature
prole, are presented in Fig. 2. The scope of the reaction was
subsequently expanded, varying R1 and R2. In entries 1–18 R1
was an aryl group and the isolated yields of the 4-bromo-2,5-
substituted oxazoles were in the range 42–82%, the highest and
lowest yields being registered for entries 7 (R1 ¼ p-Cl-C6H4; R2 ¼
H) and 16 (R1 ¼ p-OMe-C6H4; R2 ¼ p-Cl), respectively (Table 1). A
few 4-bromo oxazoles incorporating alkyl and heteroaryl
substituents at the 2-position were also prepared (entries 19–22,
Table 1), and the desired products were obtained in moderate
isolated yields.
To gain insight into the mechanism of the reactions in Table
1, an experiment was conducted with 4-nitro-N-(3-phenylpropyl)
benzamide in the SPTR. The 1,3-oxazine derivative shown in
Scheme 4 was obtained in 23% isolated yield. On the other
hand, there was no evidence of oxazole formation. Formation of
the 1,3-oxazine derivative, despite the less favoured formation
of the 6-membered ring over the oxazole ring, indicates that the
reaction involves direct O–C bond formation through intra-
molecular nucleophilic substitution at the benzylic position
instead of through intermediate formation of N-allyl benza-
mide.18 Accordingly, Scheme 5 is proposed for the reactions in
Table 1.
(1)
Thus, photo-thermochemical conditions were essential and
oxazole formation through benzylic bromination proceeded via
a radical pathway, as expected.17a
The reaction of entry 7, Table S1† was subsequently under-
taken in the SPTR. The desired product was obtained in 63%
Scheme 2 Scheme for the synthesis of a 2,5-substituted oxazole from
an N-arylethylamide through photo-thermochemical benzylic Scheme 3 The application of 4-bromo-2,5-diphenyl oxazole for
bromination.
further functionalisation.
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Table
Paper
1
Syntheses of 4-bromo-2,5-substituted oxazoles from
N-arylethylamides in the solar photo-thermochemical reactor (SPTR)a
Scheme 4 Outcome of the experiment with 4-nitro-N-(3-phenyl-
propyl)benzamide in the SPTR.
Substrate
Reaction temperature
(ꢀC)
Isolated yield
(%)
two substrates which were synthesized through trans-amidation
(ESI†). The data are presented in Table 2. It can be seen that the
isolated yields of the desired 4-bromo thiazoles were moderate
to good.
A limitation of the above methodology for the 4-bromo oxa-
zole and 4-bromo thiazole syntheses was the low bromine uti-
lisation efficiency. Considering an ideal reaction with 100%
conversion, three molecules of succinimide and two of HBr are
generated, which end up in the aqueous effluent aer work up
(Scheme 6).
The reaction shown in eqn (2) was utilised to regenerate and
reuse NBS.20 A reaction was conducted in the SPTR employing
5 mmol (1.3 g) of the substrate in entry 7, Table 1, and 15 mmol
of NBS (reaction time ¼ 6 h, reaction temperature ¼ 60–80 ꢀC).
The desired product was obtained in 68% isolated yield. NBS was
recovered next from the aqueous effluent through the reaction of
eqn (2). NaBrO3 was added to the effluent as required as per the
equation. The regenerated reagent (8.4 mmol, 57% yield) was
utilized in a second batch, with the reagent to substrate ratio
maintained at 3 : 1. The desired product was obtained in 75%
yield, and the somewhat higher yield with the recycled reagent
may possibly be due to the differences in the scale of the reaction
and the day-to-day variations in solar insolation.
Entry
R1
R2
1
2
3
4
5
6
7
8
p-C6H4NO2
p-C6H4NO2
C6H5
C6H5
C6H5
H
61–76
53–69
56–70
64–80
63–79
72–87
58–72
52–78
54–66
57–76
53–65
61–85
57–84
54–79
60–77
55–82
56–76
62–81
63–86
65–89
63
69
65
73
77
73
82
79
63
67
62
64
71
66
47
42
51
52
61b
56
p-Cl
p-Br
H
p-Cl
p-Br
H
p-Cl
p-Br
H
p-Cl
p-Br
H
p-Cl
H
p-Cl
H
p-Cl
H
H
p-C6H4Cl
p-C6H4Cl
p-C6H4Cl
p-C6H4F
p-C6H4F
p-C6H4F
o-C6H4F
o-C6H4F
o-C6H4F
p-C6H4OMe
p-C6H4OMe
p-C6H4CF3
p-C6H4CF3
CH3
9
10
11
12
13
14
15
16
17
18
19
20
CF3
21
H
57–77
62
22
p-Cl
59–78
52
a
All reactions were carried out in the SPTR unit on sunny days between
(2)
10 am and 4 pm, i.e., a reaction time of 6 h. Reaction conditions:
substrate, 0.5 mmol; NBS, 1.5 mmol; DCE, 3 mL. Yield includes ca.
b
10% co-formation of 4-bromo-2-(bromomethyl)-5-phenyloxazole.
3. Experimental
3.1. Materials and methods
General information. All commercially available chemicals
and reagents were used without any further purication unless
Fig. 2 Temperature and solar intensity profiles in the SPTR while
conducting the reaction of entry 1, Table 1.
Scheme 5 Scheme for the synthesis of a 2,5-substituted oxazole from
N-arylethylamide (R ¼ aryl, heteroaryl, alkyl) through photo-thermo-
chemical benzylic bromination. The photo-thermochemical and
It was of interest to explore whether similar reactions with
N-arylethylbenzthiamides might yield the corresponding
4-bromo-2,5-substituted thiazoles. Studies were conducted with thermochemical steps are delineated.
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Table 2 Reaction of N-arylethylbenzthiamide with NBS in the SPTRa
tracked according to the movement of the sun. The reaction was
carried out over 6 h (10.00 am to 4.00 pm) on sunny days, during
which period the reaction temperature varied from 60–80 ꢀC.
Aer cooling to room temperature, 15 mL of water was added,
the mixture was basied with aqueous NaHCO3 solution, and
the contents extracted with DCM. Upon purication of the
crude mixture on a silica gel column using 5% (v/v) ethyl acetate
in hexane, 4-bromo-2-(4-nitrophenyl)-5-phenyloxazole was
Reaction temperature
(ꢀC)
Isolated yield
(%)
ꢀ
obtained as a yellow crystalline solid (mp. 162–165 C) in 63%
Entry
Substrate
isolated yield. A single crystal was obtained through re-crystal-
lization from a CH3CN–hexane solvent mixture. Characteriza-
tion data: 1H NMR (200 MHz, CDCl3) d 8.38 (d, J ¼ 8.8, 2H), 8.28
(d, J ¼ 9.0, 2H), 8.04 (m, 2H), 7.52 (m, 3H). 13C NMR (50 MHz,
CDCl3) d 157.3, 148.4, 147.3, 131.1, 129.0, 128.4, 126.5, 125.9,
125.2, 123.8, 112.9; IR (in KBr, cmꢂ1) 3429, 2925, 2854, 2363,
1744, 1600, 1519, 1480, 1338, 1260, 1090, 1020, 972, 851, 830,
760, 707, 682; anal. (%) found: C, 52.44; H, 2.98 (calc. C, 52.20;
H, 2.63 for C15H9BrN2O3); lmax ¼ 357 nm. Crystallographic data
are provided in Table S2.†
1
2
H
Cl
61–76
54–79
79
68
a
Reaction conditions: substrate, 0.5 mmol; NBS, 1.5 mmol; DCE, 3 mL;
reaction time, 6 h (under sunny conditions).
3.3. Regeneration of NBS from the reaction mixture
The reaction as described in Section 3.2 was carried out on a
5 mmol scale using 4-chloro-N-(3-phenylethyl)benzamide and
15 mmol of NBS. Aer completion of the reaction, 2 ꢁ 30 mL
dilute aqueous NaOH was added and the aqueous phase
subsequently concentrated to a 10 mL volume. 5.5 mmol
Scheme 6 Fate of the reagent.
otherwise indicated. The various substrates were synthesized as
described in the ESI.† 1H and 13C NMR spectra were recorded at
500 MHz and 125 MHz, respectively, on a Model Bruker Avance
II instrument. The spectra were recorded in CDCl3 and DMSO-
d6 solvents. The multiplicity was indicated as follows: s (singlet);
d (doublet); t (triplet); m (multiplet), and coupling constants (J)
were given in Hz. The chemical shis are reported in ppm
relative to TMS as an internal standard. The peaks around the
delta values of 7.2 (1H NMR) and 77.0 (13C NMR) are for CDCl3
while the peaks around 2.5 (1H NMR) and 40.0 (13C NMR)
correspond to DMSO-d6. FT-IR spectra were recorded in KBr
using a Perkin Elmer spectrometer. Melting points were recor-
ded using a Mettler Toledo instrument which was uncorrected.
Mass spectra were obtained by electro spray ionization (ESI)
using a Waters 2695 LC-MS. Elemental (CHNS) analyses were
carried out using an elemental analyzer, Vario Micro Cube. The
crystal structures were obtained on a Bruker Smart Apex CCD.
The progress of the reactions was monitored by thin layer
chromatography (TLC). All products were puried through
column chromatography using silica gel (100–200 mesh).
ꢀ
NaBrO3 was then added and the contents stirred at 10–15 C.
5 mL of 9 M H2SO4 solution was added slowly under stirring for
2.5 h. The white solid was ltered and dried over vacuum at
75 ꢀC for 1 h to ꢀyield 2.37 g (57%) of NBS. Melting point
ꢀ
observed 177–179 C (reported 180–183 C).
4. Conclusions
A new route for the direct synthesis of 4-bromo-2,5-substituted
oxazoles and thiazoles was developed employing N-arylethyla-
mides and N-arylethylthiamides as substrates, respectively, and
NBS as a reagent. Easy access to the substrates and utilisation of
solar energy to drive the reactions without recourse to metal
catalysts were the main motivations behind this approach. Solar
photo-thermochemical benzylic bromination was at the heart of
the strategy but it also had a limitation, as only an aryl
substituent was feasible at the 5-position. Another limitation
was the lack of precise control over the illumination and reac-
tion temperature. Even so, the results obtained were compa-
rable to those achieved using CFL lamp illumination and
controlled heating in an oil bath. A third limitation was the
3.2. Experimental procedure for the synthesis of bromo
oxazoles in the solar photo thermochemical reactor (SPTR)
Reaction of entry 1, Table 1: the experiment in the SPTR was requirement of 3 molar equivalents of NBS even though only
conducted in Bhavnagar, Gujarat, India (location: 21ꢀ 460N, 72ꢀ one bromine atom was incorporated in the product. However, a
110E). A single neck RB ask equipped with a magnet was placed solution was devised to regenerate NBS from the aqueous
in the SPTR with the neck sticking out from the glass lid.17a effluent, but the process requires optimization to improve
0.5 mmol of 4-bromo-2-(4-nitrophenyl)-5-phenyloxazole (see recovery. Another attractive feature was the one pot synthesis of
ESI† for details of the preparation), 1.5 mmol of NBS and 3 mL these multi-step reactions. Reactions up to a gram scale were
of DCE were added into the ask. The contents were stirred with feasible in the present SPTR. Future studies will aim to expand
the built-in magnetic stirrer. The position of the reectors was the scope of the methodology presented.
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Acknowledgements
CSIR is acknowledged for supporting the research as part of a
laboratory project (OLP-0069). Dr S. Maiti is acknowledged for
helpful discussions and access to the SPTR. Ms C. Bhatt is
acknowledged for collecting solar insolation data. The authors
are grateful to the Institute's Centralized Analytical Instru-
mentation Facility and to NCL, Pune for NMR, HRMS and CHNS
analyses. Dr S. Adimurthy is thanked for providing the
substrates for the reactions of entries 4 and 19, Table 1. M.D
and S.S thank CSIR for their fellowships.
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