Iodine-catalysed oxidative cyclisation of acylhydrazones to 2,5-substituted 1,3,4-oxadiazoles

Abstract

An environmentally benign synthesis of 2,5-disubstituted 1,3,4-oxadiazoles has been developed starting from N-aroylhydrazones and N-acetylhydrazones at room or ambient temperature using a catalytic quantity of iodine in the presence of an aqueous hydrogen peroxide oxidant.

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Introduction

1,3,4-Oxadiazoles, π-conjugated heterocycles, are an interesting class of compound since molecules possessing this skeleton have been widely used in materials science, particularly in organic light emitting diodes (OLEDs), as electron conducting and hole blocking materials. Due to their electron deficient and electron transporting abilities they are utilised in an energy efficient, full-colour, flat panel display.¹ From the point of view of medicinal chemistry, 1,3,4-oxadiazoles display various biological activities such as anticancer, benzodiazepine receptor agonists, antimicrobial, analgesic, diuretic and tyrosinase inhibitors etc.² Besides these, 2,5-disubstituted 1,3,4-oxadiazoles have been used as a bioisosteric replacement of ester, amide and acid functionalities in pharmaceutical chemistry^3a–c as in furamizole,^3d raltegravir,^3e nesapidil^3f and zibotentan^3g,h as shown in Fig. 1. Some other significant molecules possessing 1,3,4-oxadiazole as the core unit include butyl PBD or b-PBD which is used in the liquid scintillator neutrino detector (LSND) (Fig. 1).⁴

Fig. 1 Representative examples of bioactive compounds possessing the 1,3,4-oxadiazole motif.

Due to myriad of applications of the 1,3,4-oxadiazole motif several methodologies are described in the literature for their synthesis. These scaffolds are synthesised by the electrophilic substitution of 2-substituted-5-trimethylsilyl-1,3,4-oxadiazole toward various electrophiles such as Cl₂, Br₂, acyl chlorides or isocyanates (path I, Scheme 1).⁵ An alternative route to 2,5-disubstituted 1,3,4-oxadiazoles is via the dehydrative cyclisation of 1,2-diacylhydrazines. Typically, a dehydrative cyclisation is carried out using reagents such as phosphorus oxychloride, sulphuric acid, thionyl chloride, PPA or [Et₂NSF₂]BF₄ (path II, Scheme 1).⁶ Besides the traditional synthesis of 2,5-disubstituted 1,3,4-oxadiazoles, involving acyl chlorides or carboxylic acids with acid hydrazines or hydrazides,⁷ some elegant protocols have also been developed. Oxidative cyclisation of N-acylhydrazones is another convenient method for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles (path III, Scheme 1).

Scheme 1 Various routes to 2,5-substituted 1,3,4-oxadiazoles.

However the later methods use stoichiometric amounts of oxidising agents such as KMnO₄,^8a ceric ammonium nitrate,^8b Br₂,^8c chloramineT,^8d FeCl₃,^8e HgO/I₂,^8f tetravalent lead reagents,^8g or hypervalent iodine reagents.^8h–m Miura’s group⁹ has developed a copper mediated C–H arylation of a preformed 2-substituted 1,3,4-oxadiazole with an aryl halide (path IV, Scheme 1). Recently we have achieved direct access to the synthesis of both symmetrical and unsymmetrical 2,5-disubstituted 1,3,4-oxadiazoles via the imine C–H bond functionalisation of N-arylidenearoylhydrazides or aroylhydrazone using a catalytic quantity of Cu(OTf)₂ (path V, Scheme 1).¹⁰ Although the above method is superior to existing methods the requirement of an expensive catalyst and the need to perform the reaction at a high temperature in DMF makes it unattractive for a large scale synthesis. Further, the method is not successful with N-acetylhydrazone in the preparation of alkyl-substituted 1,3,4-oxadiazoles.¹⁰

Usually iodine mediated reactions are preferred over metal catalysed oxidation in pharmaceutical industries due to avoiding the need for removal of metal impurities, low cost, being environmental benign and operational simplicity. In recent years, several organic transformations have been achieved by our group, and several others, using molecular iodine.¹¹ The combination of TBHP and I₂ has been widely used in many oxidative transformations to overcome the drawbacks of the metal catalysed reactions.^11a,11g,12 For any of the iodine catalysed reactions the reduced iodide species needs to be reoxidised to iodine with an external oxidant to maintain the catalytic cycle. If this process can be achieved at the expense of a cheap and environmentally benign oxidant such as aqueous hydrogen peroxide the method would be much more attractive. Although the transformation of N-acylhydrazones to 2,5-substituted 1,3,4-oxadiazoles has been reported using 20 mol% of iodine in the absence of any base¹³ the result could not be reproduced by us. The failure to give any product using 20 mol% of the iodine could possibly be because I₂ gets converted to I⁻ during the reaction and in the absence of any suitable oxidant the reduced I⁻ cannot be oxidised back to I₂ to make the reaction catalytic. Further from our subsequent findings it is clear that a base is a must for deprotonation of the starting material and neutralisation of HI generated in the medium. Hence we aspired to report an iodine catalysed oxidative cyclisation of acylhydrazones for the synthesis of densely functionalised 2,5-substituted 1,3,4-oxadiazoles (path VI, Scheme 1).

Results and discussion

Initially N-benzylidenebenzohydrazide (1 mmol) was treated with iodine (10 mol%), aq. TBHP (4 equiv.) and Cs₂CO₃ (2 equiv.) in DMF at room temperature, which afforded the product (1a) in 77% isolated yield (entry 1, Table 1). Replacement of DMF with DMSO gave an improved yield of 86% (entry 2, Table 1). Interestingly, when the same reaction was carried out with aq. H₂O₂ (4 equiv.) and K₂CO₃ (2 equiv.) instead of aq. TBHP (4 equiv.) and Cs₂CO₃ (2 equiv.) in DMSO the yield improved marginally (88%, entry 3, Table 1). Since the latter combination was better both from cost and yield considerations the reaction condition was preset but the pursuit to improve the yield continued. In a quest to improve the yield other solvents such as toluene, THF, CH₃CN and CH₃OH (entries 4–7, Table 1) were screened but all were found to be less effective compared to the use of DMSO. The use of organic bases such as Et₃N, DBU (entries 8 and 9, Table 1) were found to be inferior to the inorganic carbonate bases. No substantial improvement in the yield (90%) was observed upon increasing the amount of iodine from 10 to 20 mol%. In principle one equivalent of aq. H₂O₂ is sufficient to reoxidise I⁻ to I₂. A decrease in the quantity of H₂O₂ below 4 equiv. lowered the yield of the product. This is because iodine is also known to decompose H₂O₂ thereby effectively making it less available for the reoxidation of I⁻ to I₂ in the catalytic cycle.¹⁴ In the absence of any external oxidant the desired product was obtained in a mere yield of 9% (entry 11, Table 1), thereby suggesting the requirement of an added oxidant for the reoxidation of I⁻ to I₂. The transformation was completely unproductive in the absence of either iodine or base; thus suggesting the essential requirement of iodine, oxidant and base for this transformation.

Table 1 Screening of reaction conditions^a

Entry	Catalyst	Oxidant	Solvent	Base	Yield^b (%)
a The reactions were carried out on the same scale under identical reaction conditions.b Isolated yields.
1	10% I2	Aq. TBHP	DMF	Cs2CO3	77
2	10% I2	Aq. TBHP	DMSO	Cs2CO3	86
3	10% I2	Aq. H2O2	DMSO	K2CO3	88
4	10% I2	Aq. H2O2	Toluene	K2CO3	5
5	10% I2	Aq. H2O2	THF	K2CO3	10
6	10% I2	Aq. H2O2	CH3CN	K2CO3	10
7	10% I2	Aq. H2O2	CH3OH	K2CO3	15
8	10% I2	Aq. H2O2	DMSO	Et3N	0
9	10% I2	Aq. H2O2	DMSO	DBU	20
10	20% I2	Aq. H2O2	DMSO	K2CO3	90
11	10% I2	Nil	DMSO	K2CO3	09

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Having the above optimised conditions in hand various N-benzoylhydrazones derived from benzoylhydrazide and various aldehydes were subjected to the present reaction conditions and the results are summarised in Scheme 2. This methodology gave better yields of the products for substrates possessing electron donating groups such as p-Me (2a), p-OMe (3a) and o-OH (8a) in the aryl ring of the precursor aldehyde. However, N-benzoylhydrazones derived from an aldehydic moiety possessing electron-withdrawing groups such as p-F (4a), p-Cl (5a), p-CHO (6a) and p-Ph (7a) gave lower yields of the products compared to the substrates possessing electron donating groups as shown in Scheme 2. This observation is quite contrary to our recent observation during the Cu(II) catalysed synthesis of 2,5-disubstituted 1,3,4-oxiadiazoles via an imine C–H functionalisation.¹⁰ During the oxadiazole synthesis via the C–H functionalisation, substrates possessing electron withdrawing groups in arylaldehydes gave better yields. This differential observation in the trends of yields is due to the difference in their reaction mechanisms. During the C–H functionalisation strategy¹⁰ the acidity of the imine C–H proton is enhanced when electron withdrawing substituents are present in the aryl ring thereby facilitating the reactions and improving the yields.

Scheme 2 Formation of 2,5-substituted 1,3,4-oxadiazoles via oxidative cyclisation of N-benzoylhydrazones. Reactions were monitored by TLC. Confirmed by spectroscopic analysis. The yield of the isolated pure product is reported.

Mono N-benzoylhydrazone derived from phthalaldehyde, gave oxadiazole (6a) without affecting the CHO functionality under the reaction conditions. N-Benzoylhydrazone derived from cyclohexane carboxaldehyde and butyraldehyde gave lower yields of the products (9a) and (10a) respectively. It may be mentioned here that the benzoylhydrazones obtained from aliphatic aldehydes were found to be inert in the Cu(II) catalysed synthesis of 2,5-substituted 1,3,4-oxadiazoles¹⁰ suggesting the superiority of this method.

This methodology is equally successful with various other N-benzoylhydrazones derived from substituted aroylhydrazide possessing an electron donating group (p-OMe) and an aromatic aldehyde possessing an electron neutral –H, electron donating p-Me and electron withdrawing p-Cl giving corresponding products (3a), (11a) and (12a) respectively as shown in Scheme 3. The heteroaromatic aldehyde derived N-aroylhydrazone underwent smooth transformations giving the products (13a), (14a) and (15a) in excellent yields. It may be mentioned here that the synthesis of 2,5-substituted 1,3,4-oxadiazoles possessing heterocyclic units required a longer reaction time, giving lower yields of the products by the C–H functionalisation strategy,¹⁰ delineating the superiority of this iodine catalysed reaction. Further, N-aroylhydrazone derived from aroylhydrazide possessing the electron withdrawing group (p-Cl) and various aromatic aldehydes underwent oxidative cyclisation to furnish 2,5-substituted 1,3,4-oxadiazoles (5a), (16a), (17a) and (18a) in moderate to good yields. Unlike in Scheme 3, no correlation between the yields of the products obtained and electronic effects of the substituents present in the aryl rings of the aldehydes and aroylhydrazides could be derived. The present reaction conditions when applied to various N-acetylhydrazones failed to give good yields of the products. Interestingly increasing the reaction temperature to 50 °C and the use of the stronger base Cs₂CO₃ gave satisfactory yields of various 2,5-substituted 1,3,4-oxadiazoles 19a–26a as shown in Scheme 4 demonstrating the power of this methodology.

Scheme 3 Formation of 2,5-substituted 1,3,4-oxadiazoles via oxidative cyclisation of substituted N-aroylhydrazones. Reactions were monitored by TLC. Confirmed by spectroscopic analysis. Yields of isolated pure products are reported.

Scheme 4 Formation of 2,5-substituted 1,3,4-oxadiazoles from oxidative cyclisation of N-acetylhydrazones. Reactions were monitored by TLC. Confirmed by spectroscopic analysis. Yield of isolated pure product reported.

As the results shown in Schemes 2–4 demonstrate, this protocol is compatible with a diverse range of electron donating and withdrawing groups. This method was however not successful for acylhydrazones derived from aliphatic aldehydes and aliphatic hydrazides as no desired products were observed. These substrates decomposed to an aldehyde along with numerous other inseparable products. If both of the counterparts are aliphatic, for example an aliphatic aldehyde and aliphatic hydrazide, the reaction does not work. If any of the counterparts are aromatic, for example an aromatic aldehyde and aliphatic hydrazide, or an aliphatic aldehyde and aromatic hydrazide then the reaction is successful. Products (9a) and (10a) (Scheme 2) are derived from aliphatic aldehydes and aromatic hydrazides.

Some of the heterocyclics (3a) and (5a) can be prepared by two alternative paths as shown in Scheme 5. For compound (3a) path-a gave a better yield, while for (5a) path-b was superior. No correlation between the substituent present in the aryl ring of the aldehyde or hydrazide could be found.

Scheme 5 Dual routes for the formation of 2,5-substituted 1,3,4-oxadiazoles (3a) and (5a).

Based on literature reports^8k,11g and the control experiments carried out a possible mechanism has been proposed for the formation of 2,5-substituted 1,3,4-oxadiazoles. Initially the imine N of (A) attacks I₂ to form an iminium iodide (B). Subsequently, the iminium carbon in (B) is attacked intramolecularly by the amidic oxygen to afford the intermediate (C). A base promoted elimination (aromatisation) releases hydroiodic acid (HI) giving the desired product (D) (Scheme 6).

Scheme 6 Proposed mechanism for the formation of 2,5-substituted 1,3,4-oxadiazoles.

Conclusion

In summary, we have developed an oxidative cyclisation method using an I₂/H₂O₂ catalytic system for the synthesis of 2,5-substituted 1,3,4-oxadiazoles. An inexpensive, environmental friendly catalyst, mild reaction conditions and compatibility with a wide range of substrates make this method superior to all other methods documented in the literature.

General procedure for the preparation of 2,5-diphenyl-1,3,4-oxadiazole (1a)

A mixture of N-benzylidenebenzohydrazide 1 (1 mmol, 224 mg), K₂CO₃ (2 mmol, 276 mg) and I₂ (10 mol%, 26 mg) in DMSO (2 mL) was stirred at room temperature. To this reaction mixture H₂O₂ (30% solution in water, 0.45 mL, 4 mmol) was added in nine equal lots over a period of 3 h. The progress of the reaction was monitored by TLC. After completion of the reaction, the reaction mixture was treated with a 5% hypo solution (5 mL). The reaction mixture was then extracted with ethylacetate (2 × 15 mL) and the combined ethylacetate layer was washed with water (1 × 3 mL), dried over anhydrous Na₂SO₄ and concentrated under reduced pressure. The product was purified over a column of silica gel and eluted with (15% EtOAc/hexane) to give 2,5-diphenyl-1,3,4-oxadiazole 1a (195 mg, 88% yield).

Acknowledgements

B. K. P acknowledges the support of this research by the Department of Science and Technology (DST) (SR/S1/OC-79/2009), New Delhi, and the Council of Scientific and Industrial Research (CSIR) (02(0096)/12/EMR-II). MG thanks UGC for fellowship. Thanks are due to Central Instruments Facility (CIF) IIT Guwahati for NMR spectra and DST-FIST for XRD facility.

References and notes

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Footnote

† Electronic supplementary information (ESI) available. CCDC 953478. For ESI and crystallographic data in CIF or other electronic formats see DOI: 10.1039/c3ra44897e

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