Mild and Efficient Enamination of β-Dicarbonyl Compounds Catalyzed by Fe₃O₄ nanoparticles under Solvent-Free Conditions

Shivanand G. Sonkamble¹, Radhakrishnan M.Tigote²

¹Department of Chemistry, Pratishthan Mahavidyalaya Paithan, Dist. Aurangabad (M.S.), 431107, India

²Department of Chemistry, Dr. Babasaheb Ambedkar Marathawada University, Subcampus Osmanabad-413 510(MS), India

*Corresponding Author E-mail: sg_sonkamble@yahoo.co.in ; rmtigote@gmail.com

ABSTRACT:

Fe₃O₄ nanoparticle were prepared by chemical co-precipitation method and found to be a mild and effective catalyst for the efficient, one-pot synthesis enamination of β-dicarbonyl compounds with high yields of products and short reaction time at room temperature. This methodology is found to be fairly general and catalyst is easily separated by magnetic devices and can be reused without any apparent loss of activity for the reaction.

KEYWORDS: 1, 3-Diketones, β-enaminones, magnetic nanoparticles of ferroferic oxide (Fe₃O₄), solvent free condition.

INTRODUCTION:

Nanotechnology is now expanding very rapidly, as result of the unique physical and chemical properties that nanoparticles (NPs) exhibit compared to bulk materials. Magnetic iron oxide nanoparticles have attracted much research interest over the recent years because of the inherent properties such as large surface area and fast response under applied external magnetic field , their superparamagnety, high coercivity and low Curie temperature¹ On the other hand, recent studies shows that magnetic nanoparticles are excellent catalysts for organic reaction ² Additionally the magnetic properties make the recovery of the catalysts compete by mean of external magnetic field. The advantages even more attractive if reaction can conduct in aqueous media. Recently, the preparation and application of nanoparticles (NPs) in organic synthesis has been the subject of intense interest ³. Using NPs offers advantages for “clean” chemistry, since, in addition to being readily recovered, they are non-toxic and widely accessible.

Therefore, nanoparticles material are recently attracted the attention of synthetic organic chemist and has been successfully used as a catalyst in organic transformation as effective catalyst as oxidizing agent for several synthetic process in a previous study of  Goosen reported the catalytic decarboxylative cross- ketonisation of aryl- and alkylcarboxylic acids using Fe₃O₄ nanoparticles.^4,5

Among the wide variety of nitrogen heterocycles that have been explored as the synthesis of β -enaminoesters has recently received an increasing interest due to their potentialities in organic synthesis.^6a These compounds constituted significant subunits in some biologically important natural products such as therapeutic agents.^6b and were employed as synthons of different important antibacterial,^6c anticonvulsant,^6d  anti-inflammatory,^6e and antitumour agents.^6f They are also useful for the preparation of β –aminoacid,^6g precursors for protease inhibitors^6h and as building blocks in cryptophycins.⁶ⁱ Due to their biological importance, several approaches for the synthesis of these compounds were developed.^6j-n

The synthesis of β-enaminones are an important class of compounds used for selective alkylation and acylation of carbonyl compounds and are valuable intermediates for the synthesis of biologically active natural products. ⁷ Also these compounds are useful precursors for the synthesis of variety of heterocyclic compounds⁸ which have been used in pharmaceuticals⁹ and are building blocks for the synthesis of amino acids,¹⁰ peptides¹¹ or alkolids^12. In addiation, chiral enaminones obtained from optically active compounds are useful ligand for disteroselective synthesis.¹³These compounds can be obtained via amide enolates to nitriles,¹⁴ tosyl imines¹⁵ and via addiation of enamines to activated carboxylic acid derivatives.¹⁶ These compounds also synthesized by direct condensation of β-ketoesters with amines.¹⁷ Consequently, various modified synthesis pathways have been reported, such as the addition of zinc ester enolates or amide enolates to nitriles. ¹⁸ Some improved procedures have been also reported which use protonic acids such as PTSA,^19a Lewis acids such as BF₃-OEt_2,^19b Mg(ClO₄)_2, ^19c Bi(OTf)_3, ^19d Sc(OTf)_3.^19e Recently, NaAuCl_4,^20a InBr_3,^20b  Zn(OAc)₂-6H₂O,^20c  SnCl_4, ^20d  ZrOCl₂-8H₂O, ^20e K-7 PW₁₁CoO₄₀ ^20f and ceric ammonium nitrate (CAN) ^20g have been employed to promote this reaction. Unfortunately, many of these processes suffer some limitations, such as drastic reaction conditions, long reaction time, costly catalyst, high metal loading ²¹ and tedious work of procedures and co-occurrence of several side reactions. Therefore, the development of rapid and eco-friendly approaches has crucial  importance using the Nanomaterial particle as Ferroferic oxide.  It has some interesting features, because of his low environmental impact, easy prepared catalyst, less costly, more effective, magnetic property and highly chemo-selective catalyst for their organic synthesis.

MATERIAL AND METHODS:

(a) Typical experimental procedure for Synthesis of   Fe₃O₄ ( ferroferric oxide nanoparticles) MNPs:

Take 10 ml (0.5M) ferric nitrate as a precursor into 50 ml round bottom flask and add capping agent of 5 ml of starch (0.5%) into the precursor and stirred at room temperature for 1Hr. Among this reaction mixture add slowly aqueous solution of ammonia to afford the   redious precipitate into liquid media till up to P^H=9. This precipitate were centrifuged at 10 minute to get redious colored precipitated of nanoparticle then isolate these nanoparticle and wash with hot water using filtration method up to P^H=7  Finally collect this material of Fe₃O₄ nanoparticle and their application towards the electronic, optical, magnetic properties as well as  enaminones also.

(b)Synthesis of enaminones using ferroferric oxide nanoparticles (MNPs)

To the mixture of β-Dicarbonyl compound (1 mmol) and amine (1 mmol) ferroferric oxide nanoparticles (2.5 mol %) was added and mixture is stirred at room temperature within 30 minute. The progress of reaction was followed by TLC. After the reaction was completed, the catalyst was separated by an external magnet and reused as such for the next experiment. The reaction mixture was dissolved in dichloromethane and filtered. The filtrate was concentrated on a rotary evaporator under reduced pressure and the solid product obtained was washed with water and recrystallized from ethanol. Pure products β enaminones were obtained in excellent yields, as summarized in Table 2. Most of the products are known and were identified by comparison of their physical and spectral data with those of authentic samples.

Characterization data of some representative compounds:

4-Phenylamino-pent-3-en-2-one (3a): (white solid); MP 51 ^◦C; IR (cm⁻¹) 3452, 3040, 3029, 1631, 1580, 1515,1289, 1194, 1035, 937, 766, 710; ¹H NMR (CDCl₃, 300 MHz) δ 2.01 (s, 3H), 2.15 (s, 3H), 5.22 (s, 1H), 7.21–7.49 (m, 5H). 11.58 (brs, 1H); ¹³C NMR (CDCl₃, 75 MHz) δ 20.1, 30.5, 100.1, 128.7, 130.0, 132.4, 140.5, 161.7, 190.3; Anal. Calcd for C₁₁H₁₃NO: C, 75.40; H, 7.48; N, 7.99. Found: C, 75.40; H, 7.45; N, 7.90

4-(4-Chloro-phenylamino)-pent-3-en-2-one (3b): (White solid); mp 64 ^oC; IR (cm⁻¹) 3342, 3022, 1625, 1598,1523, 1299, 1221, 1036, 777; ¹H NMR (CDCl₃, 300 MHz) δ 2.03 (s, 3H), 2.14 (s, 3H), 5.25 (s, 1H,), 7.19 (d, 2H),7.35 (d, 2H), 11.41 (brs, 1H); ¹³C NMR (CDCl₃, 75 MHz) δ 19.8,30.0, 99.0, 117.4, 125.3, 128.4, 137.4, 160.7,190.4; Anal. Calcd for C₁₁H₁₂ClNO: C, 63.01; H, 5.77; Cl, 16.91; N, 6.68. Found: C, 63.05; H, 5.76; Cl, 16.89; N, 6.70.

3-Benylamino-but-2-enoic acid ethyl ester (3c): (Colourless liquid); IR (cm⁻¹) 3299, 3041, 2997, 1654, 1621, 1464,1291, 1254, 1188, 1098, 768; ¹H NMR (300MHz,CDCl₃) δ 1.77 (t, 3H), 2.09 (s, 3H), 5.11 (q, 2H), 5.35 (d, 2H),5.74 (s, 1H), 7.71–8.08 (m, 5H), 10.47 (brs, 1H); ¹³C NMR (75 MHz,CDCl₃) δ 17.2, 20.1, 50.7, 61.5, 89.4, 127.5,129.0, 130.4, 140.2, 162.9, 175.4; Anal. Calcd for C₁₃H₁₇NO₂: C, 71.21; H, 7.81; N, 6.39. Found: C, 71.18; H, 7.77;N, 6.40.

3-(4-Methoxy-phenylamino)-but-2-enoic acid ethyl ester (3d): (Yellow oil); IR (cm⁻¹) 3291, 2966, 2857, 1678,1629 1535, 1298, 1178, 1054, 786; ¹H NMR (300 MHz, CDCl₃) δ 1.88 (t, 3H), 2.05 (s, 3H,), 3.98 (s, 3H,), 4.21 (q,2H), 4.77 (s, 1H), 6.97 (d, 2H), 7.24 (d, 2H), 11.25 (brs, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 16.5, 19.1, 59.1, 61.3,89.4, 118.5, 128.6, 134.7, 160.4, 163.8, 178.1; Anal. Calcd for C₁₃H₁₇NO₃: C, 66.36; H, 7.28; N, 5.95. Found: C,66.33; H, 7.27; N, 5.92.

4-Phenylamino-pent-3-en-2-one (3h): (White solid); mp 50 ◦C; IR (cm⁻¹) 3481, 3045, 3075, 1621, 1554, 1533,1288, 1198, 1024, 965, 747, 701; ¹H NMR (300 MHz ,CDCl₃,) δ 2.02 (s, 3H), 2.17 (s, 3H), 5.23 (s, 1H), 7.45–7.87(m, 5H). 11.48 (brs, 1H); ¹³C NMR (75MHz, CDCl3) δ 20.1, 29.4, 99.7, 126.2, 127.6, 130.8, 141.3, 161.5, 200.4;Anal. Calcd for C₁₁H₁₃NO: C, 75.40; H, 7.48; N, 7.99. Found: C, 75.41; H, 7.50; N, 7.97.

4-Benzylamino-pent-3-en-2-one (3i): (Yellow viscous oil); IR (cm⁻¹) 3436, 3045, 1674, 1541, 1365, 1288, 1035,775; ¹H NMR (300 MHz ,CDCl₃) δ 1.99 (s, 3H), 2.08 (s, 3H), 4.65 (d, 2H), 5.11 (s, 1H), 7.41–7.65 (m, 5H), 11.26(brs. 1H); ¹³C NMR (75MHz ,CDCl₃,) δ 19.8, 28.8, 50.0, 97.6, 127.9, 129.5, 131.9, 140.7, 166.2, 199.7; Anal. Calcd For C₁₂H₁₅NO: C, 76.16; H, 7.99; N, 7.40. Found: C, 76.19; H, 7.97; N, 7.39.

RESULT AND DISCUSSION:

Here, we report a pertinent method for the preparation of β -enaminoesters by the condensation of β-ketoesters with primary amines under solvent-free conditions at room temperature with Fe₃O₄ as a new catalyst. (Scheme 1)

Scheme 1

In order to optimize the reaction conditions for the synthesis of β-enaminones under mild and solvent-free conditions, we chose the model reaction involving ethylacetoacete and benzyl amine at room temperature. In order to find out the best reaction conditions, a set of experiments was performed using varying amount of ferroferic oxide (Fe₃O₄) MNPs as a catalyst. The results of our optimization study are presented in Table1.

As shown in Table 1; in the absence of the catalyst (entry 1) the reaction gave very low yield of the product which clearly indicates the need for catalyst for the success of the reaction. The catalytic loading as low as 1 mol % (entry 2) was effective to proceed the enamination, however, 2.5 mol % of catalyst was found to be optimal to obtain the highest yield of the enamination product. Interestingly, the enamination could be completed within less than an hour when the amount of the catalyst was doubled (Table 1, entry 5) giving almost similar yield of the desired product. But, in view of our objective to develop relatively cheap and environmentally benign protocol, we preferred the former conditions as an optimal one. Thus the best reaction condition for the present reaction were 1 mmol of β-ketoester, 1 mmol of amine in the presence of 2.5 mol % ferroferic oxide (Fe₃O₄) MNPs  under solvent-free conditions within 30 minute at ambient temperature.

Table 2 (continue)

The results are collected in Table 2. As show in our results, in all the examples studied the reaction proceeded smoothly to afford the corresponding β-enaminones in high yields under present reaction conditions. The reactions were performed under solvent-free conditions and reached to completion within 1 Hr. at room temperature. The present reaction represents the true catalytic process for the synthesis of a wide variety of β-enaminones as a very small amount (2.5 mol %) of the catalyst was found to be effective to obtain the high yields of the products within short reaction times.

With good result being obtained in the reactions with benzyl amine, next in order to gauge the generality and scope of the present method, the structurally diverse amines such as aliphatic and cyclic, primary, as well as secondary amines were subjected to enamination with ethyl acetoacetate using our optimized reaction conditions.

CONCLUSION:

In summary, this method can be a useful addition to the present methodologies for the synthesis of β- enaminones derivatives using magnetically recoverable Fe₃O₄ nanoparticles under room temperature conditions. It is not only provides high yields and purities, but it is also cheap, convenient, quick, and economically friendly. The generality, it is mild, solvent-free conditions, low catalytic requirement, simplicity and  high yields of the products coupled with the use of environmentally benign catalyst makes the present methodology as an attractive synthetic protocols for the synthesis of a wide variety of β- enaminones at room temperature.

ACKNOWLEDGEMENTS:

We are very much thankful to Late Dr. V.G. Dongra, Ex-Professor, Department of Chemistry, Mumbai University, Mumbai and Late Dr.V. Kamble, Deputy Director, Forensic Lab, Aurangabad for his valuable guidance and supports for research work.

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Received on 12.09.2014         Modified on 29.09.2014

Accepted on 07.10.2014         © AJRC All right reserved

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