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

Abstract

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.

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1. Introduction

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, fluorescent 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, Schöllkopf synthesis and the Nierenstein 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 β-vinylic C–heteroatom bonds have been generated, or isolated in situ, which undergo facile intramolecular vinylation of the amide oxygen to provide a broader range of oxazoles.¹⁴ A method was 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

Scheme 1 Approaches to the synthesis of 2,5-substituted oxazoles.

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-thermochemical 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 particular, the syntheses of 2,5-substituted oxazoles from N-arylethylamides were considered (Scheme 2). The reaction involved two 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 synthesized from corresponding acid chlorides and amines. Results of the study are presented herein.

Scheme 2 Scheme for the synthesis of a 2,5-substituted oxazole from an N-arylethylamide through photo-thermochemical benzylic bromination.

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 fluorescent lamp (CFL), exhibiting a similar spectro-radiometric profile 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 reflux 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 significant extent when the above reaction was conducted at room temperature under CFL illumination (entry 8, Table S1†), in the 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 2× sunlight incident on the SPTR (blue) and a 36 W CFL lamp (green).

Scheme 3 The application of 4-bromo-2,5-diphenyl oxazole for further functionalisation.

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 undertaken in the SPTR. The desired product was obtained in 63% 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 profile, 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 = p-Cl-C₆H₄; R₂ = H) and 16 (R₁ = p-OMe-C₆H₄; R₂ = 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.

Table 1 Syntheses of 4-bromo-2,5-substituted oxazoles from N-arylethylamides in the solar photo-thermochemical reactor (SPTR)^a

Entry	Substrate		Reaction temperature (°C)	Isolated yield (%)
	R1	R2
a All reactions were carried out in the SPTR unit on sunny days between 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.b Yield includes ca. 10% co-formation of 4-bromo-2-(bromomethyl)-5-phenyloxazole.
1	p-C6H4NO2	H	61–76	63
2	p-C6H4NO2	p-Cl	53–69	69
3	C6H5	p-Br	56–70	65
4	C6H5	H	64–80	73
5	C6H5	p-Cl	63–79	77
6	p-C6H4Cl	p-Br	72–87	73
7	p-C6H4Cl	H	58–72	82
8	p-C6H4Cl	p-Cl	52–78	79
9	p-C6H4F	p-Br	54–66	63
10	p-C6H4F	H	57–76	67
11	p-C6H4F	p-Cl	53–65	62
12	o-C6H4F	p-Br	61–85	64
13	o-C6H4F	H	57–84	71
14	o-C6H4F	p-Cl	54–79	66
15	p-C6H4OMe	H	60–77	47
16	p-C6H4OMe	p-Cl	55–82	42
17	p-C6H4CF3	H	56–76	51
18	p-C6H4CF3	p-Cl	62–81	52
19	CH3	H	63–86	61^b
20	CF3	H	65–89	56
21		H	57–77	62
22		p-Cl	59–78	52

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Fig. 2 Temperature and solar intensity profiles in the SPTR while conducting the reaction of entry 1, Table 1.

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 intramolecular nucleophilic substitution at the benzylic position instead of through intermediate formation of N-allyl benzamide.¹⁸ Accordingly, Scheme 5 is proposed for the reactions in Table 1.

Scheme 4 Outcome of the experiment with 4-nitro-N-(3-phenylpropyl)benzamide in the SPTR.

Scheme 5 Scheme for the synthesis of a 2,5-substituted oxazole from N-arylethylamide (R = aryl, heteroaryl, alkyl) through photo-thermochemical benzylic bromination. The photo-thermochemical and thermochemical steps are delineated.

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 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.

Table 2 Reaction of N-arylethylbenzthiamide with NBS in the SPTR^a

Entry	Substrate	Reaction temperature (°C)	Isolated yield (%)
a Reaction conditions: substrate, 0.5 mmol; NBS, 1.5 mmol; DCE, 3 mL; reaction time, 6 h (under sunny conditions).
1	H	61–76	79
2	Cl	54–79	68

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A limitation of the above methodology for the 4-bromo oxazole and 4-bromo thiazole syntheses was the low bromine utilisation 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 after work up (Scheme 6).

Scheme 6 Fate of the reagent.

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 = 6 h, reaction temperature = 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.

3. Experimental

3.1. Materials and methods

General information. All commercially available chemicals and reagents were used without any further purification unless 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 shifts 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 recorded 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 purified through column chromatography using silica gel (100–200 mesh).

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 conducted in Bhavnagar, Gujarat, India (location: 21° 46′N, 72° 11′E). A single neck RB flask equipped with a magnet was placed in the SPTR with the neck sticking out from the glass lid.^17a 0.5 mmol of 4-bromo-2-(4-nitrophenyl)-5-phenyloxazole (see ESI† for details of the preparation), 1.5 mmol of NBS and 3 mL of DCE were added into the flask. The contents were stirred with the built-in magnetic stirrer. The position of the reflectors was 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. After cooling to room temperature, 15 mL of water was added, the mixture was basified with aqueous NaHCO₃ solution, and the contents extracted with DCM. Upon purification 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 obtained as a yellow crystalline solid (mp. 162–165 °C) in 63% isolated yield. A single crystal was obtained through re-crystallization from a CH₃CN–hexane solvent mixture. Characterization data: ¹H NMR (200 MHz, CDCl₃) δ 8.38 (d, J = 8.8, 2H), 8.28 (d, J = 9.0, 2H), 8.04 (m, 2H), 7.52 (m, 3H). ¹³C NMR (50 MHz, CDCl₃) δ 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, cm⁻¹) 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); λ_max = 357 nm. Crystallographic data are provided in Table S2.†

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. After 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 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 filtered 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-arylethylamides 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 reaction temperature. Even so, the results obtained were comparable to those achieved using CFL lamp illumination and controlled heating in an oil bath. A third limitation was the requirement of 3 molar equivalents of NBS even though only one bromine atom was incorporated in the product. However, a solution was devised to regenerate NBS from the aqueous effluent, but the process requires optimization to improve recovery. Another attractive feature was the one pot synthesis of these multi-step reactions. Reactions up to a gram scale were feasible in the present SPTR. Future studies will aim to expand the scope of the methodology presented.

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 Instrumentation 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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Footnote

† 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

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