Facile Synthesis of Nitriles and Amides from Aldehyde over Heterogeneous Reusable Copper Fluorapatite (CuFAP) Catalyst under Neat Reaction Condition

Santosh S. Chavan, Mohsinkhan Y. Pathan, Taufeekaslam M. Y. Shaikh, Shafeek A. R. Mulla^*

Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune, India.

DOI: 10.4236/ojsta.2017.63003                

Abstract

A new robust heterogeneous, versatile, an environmentally benign, eco-friendly, recyclable CuFAP catalyst has been developed for the direct synthesis of nitriles and amides from aldehydes at 100°C for 6 h and 4 h, respectively, under neat reaction condition using hydroxylamine hydrochloride in the presence and the absence of tosyl chloride, respectively. Also the recyclability of catalyst as well as influence of solvents, additives on catalysts performance was investigated. The protocol can be considered as an alternative to conventional method for the synthesis of nitriles and amides in good to excellent yields. A highlight of our protocol is the easy separation of catalyst from reaction mixture, hence the catalyst is reused several times without significant loss of its catalytic activity.

Keywords

Heterogeneous, Recyclable, Nitriles, Amide, Aldehyde, Copper Fluorapatite

1. Introduction

The nitriles and amides are versatile prolific moieties acting as major building blocks in organic chemistry to synthesise numerous nitrogen-containing heterocyclic compounds which are not only integral part of fine chemicals, agrochemicals, polymer but also medicinally and biologically potent compounds. The nitrile bond containing compounds are used in clinical developments as well as for the preparation of angiotensin 1 receptor ligands, losartan and valsartan [1] , 1,2-diarylimidazoles as anti-inflammatory agents [2] , thiazole analogues as inhibitor of superoxides [3] [4] [5] . Owing to the potential applications of nitriles in pharmaceuticals, agricultural chemicals, polymer, dyes [6] , various methods for its synthesis such as classic Sandmeyer reaction [7] [8] [9] [10] [11] , or nucleophilic substitution of aryl halides with metal cyanide [12] [13] [14] [15] , dehydration of aldoximes [16] [17] [18] [19] , dehydration of amide [20] [21] [22] [23] , amine to nitrile [24] [25] [26] , oxidation of alcohols [27] [28] , and direct conversion of aldehydes to nitriles using NH₂OH∙HCl [29] , and/or ammonia over CuCl₂/NaOMe/ O₂ [30] , Pd(OAc)₂ [31] , Oxone [32] , H₂O₂ [33] , I₂ [34] , NBS [35] , IBX [36] , NaICl₂ [37] , p-TSA [38] , KF/Al₂O₃ [39] , catalysts have been well documented. Amides also serve as key precursor for variety chemical transformation/biologically active compounds [40] [41] [42] [43] as well as for the synthesis of peptide, protein, anti-block reagents, colour pigments, detergents and lubricants etc. [44] [45] . Therefore, the various methods for the synthesis of amide using varieties of reagent and catalysts such as enzyme catalyzed hydrolysis of nitriles and or amidation of acids or esters with ammonia [46] [47] , Beckmann rearrangement [48] [49] , iodine and H₂O₂ [50] , Ir [51] , Cu [52] , Ag/Au [53] , Pd [54] , Rh [55] [56] [57] [58] , Ru compounds [59] [60] , anhydrous oxalic acid [61] , Sulfamic acid [62] , cyanuric chloride/DMF [63] , ethyl chloroformate/boron trifluoride etherate [64] , and chlorosulfonic acid [65] have been reported. Moreover, a direct synthesis of nitriles and amides from aldehydes using dry or wet alumina and chitosan supported ionic liquid were reported by Sharghi et al. [66] and Nezhad et al. [67] , respectively. Even though significant progress has been achieved, almost all of these approaches developed so far for the synthesis of nitriles and amides suffer from one or other major limitations and drawbacks such as drastic reaction conditions, high temperature, long reaction time, and low yield to desired products. In addition to these limitations, use of non-recoverable, moisture sensitive, toxic, corrosive, expensive reagents and catalysts such as HCN/metal cyanides, metal/enzyme etc. in excess as well as the prohibitive isolation costs and applicability to a limited range of substrates constitute seriousness for their industrial scale application.

Therefore, addressing limitation and/or issues associated with catalysis is one of the prime important nowadays to develop the green chemical process. Consequently, the development of cost effective, non-toxic, non-corrosive, stable, recyclable, eco-friendly, robust heterogeneous catalysts is still challenging and an active research area. As part of our continuous studies toward development of cost effective, eco-friendly heterogeneous recyclable catalysts for the organic transformation, [68] - [73] herein, we report a simple, novel, solvent free, cost effective protocol using non-toxic, non-corrosive, stable, recyclable, eco-friendly robust heterogeneous CuFAP catalyst for the facile one pot synthesis of a wide range of nitriles and amides from the various aldehydes (Scheme 1), which may be a practical approach for the synthesis of various nitriles and amides.

2. Results and Discussion

To optimize the reaction conditions, a series of experiments were performed for

Scheme 1. CuFAP catalysed synthesis of nitriles and amides from aldehydes.

Table 1. Optimization of the reaction conditions.^a

^aReaction conditions: 1a (1 mmol), hydroxylamine hydrochloride (1.2 mmol), additives (20 mol%), solvent (5 mL), CuFAP (100 mg), 80˚C - 110˚C, 4 - 24 h. ^bThe yields indicated are the isolated yield by column chromatography ^cNo reaction, ^dWithout catalyst no reaction.

the synthesis of nitriles and amides by reacting benzaldehyde with hydroxylamine hydrochloride salt in the presence and absence of additives, respectively. Initially, we perform the reaction of benzaldehyde (1 mmole) by reacting with hydroxylamine hydrochloride salt (1.2 mmole) in the presence of CuFAP (100 mg) catalyst using 20 mol% PTSA as an additive in MeCN (5 mL) solvent under reflux reaction temperature for 10 h. To our surprise, the expected desired product benzonitrile 2a was obtained in 82% yield (entry 1, Table 1). Encouraged by this result, we studied the effect of various additives (entries 1 - 7, Table 1). However, tosyl chloride additive gave higher yield to desire product (entry 6, Table 1) compared to other additives, whereas formation of 2a product was not observed in case sodium sulfate additive (entry 7, Table 1). Moreover, the 10 and 15 mol% loading of TsCl shows inferior yield compared to 20 mol%. The screening the organic solvent such as THF, PhMe, Dioxane and DMF were carried out by taking 20 mol% TsCl (entries 8 - 11, Table 1). However, the reaction proceeded smoothly in the PhMe solvent to furnish corresponding product in 92% yield (entry 9, Table 1). The THF, dioxane and DMF solvents provided trace and 41% yield, respectively, in 24 h (entries 8, 10 and 11, Table 1). In addition, the reaction was also performed in solvent free condition at 100˚C, interestingly; desired product was obtained in 94% yield in 6 h (entry 12, Table 1).

After achieving the optimization of reaction conditions for the synthesis of nitriles, we switch over our studies to optimize the reaction parameter for the synthesis of amide (3a). The one pot transformation of benzaldehyde was performed in the presence of solvents (PhMe, MeCN, DCE) and in the absence of solvent by reacting hydroxylamine hydrochloride salt without an additives using CuFAP catalyst (entries 13 - 16, Table 1). To our delight, the neat reaction condition provided desired product 3a in 88% yield in 4 h. The formation of the desired product either 2a or 3a was not observed in the absence of catalyst and/or additive.

Having optimized reaction conditions in hand, we examine the scope and limitation of our protocol for the synthesis of various nitriles and amides. As shown in Table 2, various benzaldehydes were reacted efficiently with hydroxylamine hydrochloride in the presence of additive, and provided the corresponding nitrile product in good to excellent yield (entries 2a - 2v, Table 2).

Amazingly, the neutral, saturated, unsaturated benzaldehyde as well as benzaldehydes having electron rich and electron poor groups such as such as methyl, methoxy, trimethoxy, chloro, and nitro were efficiently converted into corresponding nitrile product in excellent (entries 2a-2n, 2p, 2r, and 2s, Table 2) except ortho-nitrobenzaldehyde (entry 2t, Table 2) gave desire nitrile in good yields. Due to promising results, the scope of protocol was extended to 1-3 benzodioxole-4-carbaldehyde, 3-phenoxybenzaldehyde and heterocyclic aldehydes, miraculously all these aldehydes were smoothly converted into corresponding nitrile in excellent yield (entries 2o, 2q, 2u and 2v, Table 2).

Having succeeded the application of a robust heterogeneous recyclable CuFAP catalyst for the synthesis of various nitriles, we further elaborated the scope of protocol for the synthesis of amides. As predicted in Table 3, various benzaldehydes were reacted well with hydroxylamine hydrochloride, and delivered the corresponding amide product in good to excellent yield (entries 3a - 3o, Table 3). The neutral benzaldehyde as well as benzaldehyde having electron rich and electron poor groups such as methyl, methoxy, trimethoxy, chloro, nitro and hydroxy were easily converted into corresponding amides in good to excellent

Table 2. Substrate scopes of various aromatic nitriles.^a

^aReactions conditions: 1a - 1v (1 mmol), hydroxylamine hydrochloride (1.2 mmol), TsCl (20 Mol%) CuFAP (100 mg), 100˚C, 6 h. The yields indicated are the isolated yield after purification.

Table 3. Substrate scopes of various aromatic primary amides.^a

^aReactions conditions: 1a - 1o (1 mmol), hydroxylamine hydrochloride (1.2 mmol), TsCl (20 Mol%), CuFAP (100 mg) at 100˚C for 4 h. The yields indicated are the isolated yield after purification.

yield (entries 3a - 3j, Table 3). Unexpectedly, 1-3 benzodioxole-4-carbaldehyde (entry 3k, Table 3), saturated, unsaturated (entry 3l and 3m, Table 3) and heterocyclic aldehydes (entry 3n and 3o, Table 3) also willing deliver desired corresponding amide in good to excellent yields. The results in Table 2 and Table 3 clearly reveal that developed solvent free protocol over a robust heterogeneous recyclable CuFAP catalyst is facile, efficient, general, and applicable to variety of substrates having different functional groups. Nevertheless, yield achieved is dependent on the functional groups/substituent on the aldehydes.

As per our [68] - [73] and previous [74] [75] research work reported in the literature over copper fluoroapatite catalyst, a possible reaction pathway for one pot synthesis of nitriles and amides from benzaldehyde is outlined in Scheme 2. Initially benzaldehyde and hydroxylamine hydrochloride reacts to form aldoxime (I) which on coordination with CuFAP catalyst provide intermediate complex (II). The complex II on hydrolysis with water generate the complex III which is instantaneously undergo rearrangement to eliminate of CuFAP catalyst as well as amide, whereas the complex II reacts with tosyl chloride additive to release the CuFAP and nitrile.

The recyclability and reusability of the copper fluorapatite catalyst for one-pot synthesis of nitriles and amides from benzaldehyde was investigated under optimized reaction conditions, and results are summarized in Table 4. The CuFAP catalyst was recovered quantitatively from the reaction mixture by simple filtration and reused several times without loss of catalytic activity. The isolated yield obtained for desired products (2a and 3a) even after forth recycle is comparable with fresh CuFAP catalyst.

3. Conclusion

In conclusion, the CuFAP is an efficient, versatile, ecofriendly, inexpensive, nontoxic, reusable heterogeneous and green catalyst has been developed for a

Scheme 2. Plausible mechanism for the nitrile and amide synthesis over CuFAP catalyst.

Table 4. Recyclability of catalyst.

^aReaction conditions: benzaldehyde (1 mmol), hydroxyl amine hydrochloride (1.2 mmol), except TsCl (20 mol%) (used in case of nitrile) at 100˚C. ^bThe yields indicated are the isolated yields by column chromatography.

one-pot synthesis of various nitriles and amides via aldoximes formation from various aldehydes at 100˚C for 6 h and 4 h, respectively, under neat reaction condition using hydroxylamine hydrochloride in the presence and the absence of tosyl chloride, respectively. The following features make this methodology attractive: a) its simplicity, clean, efficient, rapid, mild, and neat reactions conditions, b) the protocol is very general and it works well with variety of aldehydes affording good to excellent yields, c) the CuFAP catalyst is recovered and reused without further purifications and gave 90% - 94% yield to the nitriles and 82% - 88% yield to the amides without loss of catalytic activity. The further studies are expected for an industrial scale application of CuFAP catalyst in our research group.

4. Experimental Section

All chemicals and reagents were procured from suppliers and used without further purification. The products were characterized using ¹H NMR, ¹³C NMR spectra. NMR spectrums of product were obtained using Bruker AC-200 MHz spectrometer with TMS as the internal standard.

4.1. Typical Experimental Procedure for the Synthesis of Nitriles

In a 10 mL round bottom flask equipped with aldehydes (1 mmol), hydroxylamine hydrochloride (1.2 mmol), TsCl (20 mol%) and CuFAP catalyst (100 mg) were taken. Then the resulting reaction mixture was refluxed at 100˚C for 6 h (Table 2) and the progress of the reaction was monitored by TLC. Upon completion of the reaction, the reaction mixture was diluted with 15 mL Ethyl acetate followed by filtration to recover the catalyst. The filtrate was washed with 10 mL solution of 10% NaHCO₃ and organic layer was dried over anhydrous Na₂SO₄ and concentrated by rotary evaporation to get the crude nitrile compounds, all compounds were isolated without column chromatography.

4.2. Typical Experimental Procedure for the Synthesis of Amides

In a 10 mL round bottom flask equipped with aldehydes (1 mmol), hydroxylamine hydrochloride (1.2 mmol), and CuFAP catalyst (100 mg) were taken. Then the reaction mixture was refluxed at 100˚C for 4 h (Table 3) and the progress of the reaction was monitored by TLC. Upon completion of the reaction, the reaction mixture was diluted with 15 mL Ethyl acetate followed by filtration to recover the catalyst. The filtrate was washed with 10 mL solution of 10% NaHCO₃ and organic layer was dried over anhydrous Na₂SO₄ and concentrated by rotary evaporation to get the crude amide, all compounds 3a - 3o were isolated by column chromatography (ethyl acetate: Pet ether 3:7).

Acknowledgements

TMYS thanks to UGC New Delhi for a SRF. The authors also thank to Dr. S. S. Tambe, Chair, CE-PD for their encouragement and support.

Conflicts of Interest

The authors declare no conflicts of interest.

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