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Direct Preparation of Primary Amides from Carboxylic Acids and Urea Using Imidazole Under Microwave Irradiation

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Direct Preparation of Primary Amides from Carboxylic Acids and Urea Using Imidazole Under Microwave Irradiation TETRAHEDRON LETTERS Pergamon Tetrahedron Letters 44 (2003) 7325–7328 Direct preparation of primary amides from carboxylic acids and urea using imidazole under microwave irradiation Ali Khalafi-Nezhad,* Babak Mokhtari and Mohammad Navid Soltani Rad Department of Chemistry, College of Science, Shiraz University, Shiraz 71454, Iran Received 24 June 2003; revised 20 July 2003; accepted 31 July 2003 Abstract—A very simple and efficient solvent-free procedure for the preparation of primary amides is described from carboxylic acids and urea using imidazole under microwave irradiation. Various aliphatic and aromatic primary amides were prepared in good yields by this direct amidation method. © 2003 Elsevier Ltd. All rights reserved. The preparation of primary amides from the corre- amides under microwave assisted solvent free condi- sponding carboxylic acids is an important and well- tions.17–19 However, there have been no reports of the known transformation in organic synthesis.1,2 In direct synthesis of primary amides from carboxylic general, the formation of carboxamides from carboxylic acids by this means, although primary amides have acids requires activation of the carboxyl group. Car- been prepared by hydrolysis of the corresponding boxylic acid activation can be achieved by conversion nitriles under microwave irradiation in the presence or of the carboxyl group to a more reactive functional absence of solvents.20,21 group such as acyl halide, mixed anhydride, acyl azide, active ester or in situ activation by coupling reagents, Herein, as an extension of our previous studies on the the most common of which is N,N-dicyclohexylcar- application of microwave irradiation in organic synthe- bodiimide (DCC).3 Other coupling reagents that have sis,22 we describe the first procedure for the synthesis of been used for this purpose include TiCl ,4 activated 4 primary amides by direct reaction of carboxylic acid phosphate,5 Sn [N(TMS) ] ,6 equivalent amounts of 2 2 and urea in the presence of imidazole under microwave triphenylphosphine and N-halosuccinimide,7 triphenyl- 8 9 irradiation (Scheme 1). phosphine and trichloroacetonitrile, ArB(OH)2, Lawesson’s reagent,10 tert-butyl-3-(3,4-dihydrobenzotri- 11 12 Imidazole was used in this reaction because previously azine-4-on)yl carbonate (Boc-Odhbt), (R2N)2Mg, and 2-mercaptopyridone-1-oxide based uronium salts.13 it has been demonstrated that this compound shows useful promotion ability23 and forms polar carboxylic The application of microwave technology in organic acid salts for efficient microwave energy absorption.24 chemistry has been explored extensively within the last The promotion ability of imidazole was attributed to decade. Microwave irradiation often leads to a remark- the low melting point of imidazolium carboxylate salts able decrease in reaction time, increased yields, easier which melt after heating at low microwave power and work up matching with green chemistry protocols, and short irradiation time and homogenize the reaction may enhance the regio- and stereoselectivity of mixture in dry media.25 Instead of imidazole we used reactions.14–16 other bases such as 4-dimethylaminopyridine (DMAP), More recently, there have been some reports in which are described the preparation of secondary and tertiary Keywords: primary amide; microwave irradiation; urea; imidazole. * Corresponding author: Tel.: +98-711-2282380; fax: +98-711- 2280926; e-mail: khalafi@chem.susc.ac.ir Scheme 1. 0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0040-4039(03)01866-5 7326 A. Khalafi-Nezhad et al. / Tetrahedron Letters 44 (2003) 7325–7328 triethylamine and 1,8-diazabicyclo[5,4,0]undec-7-en To investigate the generality and scope of this method, (DBU), but none of them acted as well as imidazole. the reaction was examined with various structurally The results for amidation of various carboxylic acids diverse carboxylic acids. As is clear from Table 1 the are summarized in Table 1. reactions proceed cleanly, and the desired carboxamides Table 1. Direct preparation of primary amides from carboxylic acids and urea by using imidazole under microwave irradiation A. Khalafi-Nezhad et al. / Tetrahedron Letters 44 (2003) 7325–7328 7327 were obtained in good yields but the presence of a References hydroxyl group on the aromatic ring (entry g) lowered the yield. The aromatic carboxylic acids undergo reac- 1. (a) March, J. Advanced Organic Chemistry; John Wiley & tion relative slowly than aliphatic ones. In the case of Sons: New York, 1992; pp. 416–425; (b) Comprehensive aromatic carboxylic acids, the presence of electron- Organic Synthesis; Trost, B. M., Ed.; Pergamon Press: withdrawing groups on the aromatic ring accelerated Oxford, 1991; Vol. 6, pp. 381–417. the reaction rate and increasing the reaction yields 2. Brown, W. Idrugs 1999, 2, 1059–1068. (entries i and n). 3. Sheehan, J. C.; Hess, G. P. J. Am. Chem. Soc. 1955, 77, 1067–1068. To explore the formation of acid anhydrides during 4. Wilson, J. D.; Hobbs, C. F; Wengaten, H. J. Org. Chem. reaction progress a mixture of benzoic acid, imidazole 1970, 15, 1542–1545. and urea was exposed to microwave irradiation. TLC 5. Yasuhara, T.; Nagaka, Y.; Tomioka, K. J. Chem. Soc., monitoring of this reaction mixture after each 30 s for Perkin Trans. 1 2000, 2901–2901. 6 min showed us that benzoic anhydride was not 6. Burnell-Curty, C.; Roskamp, E. J. Tetrahedron Lett. 1993, 34, 5193–5196. formed in this conditions. Similarly an experiment with 7. Froyen, P. Synth. Commun. 1995, 25, 959–968. a mixture of benzoic acid and imidazole also confirmed 8. Jang, D. O.; Park, D. J.; Kim, J. Tetrahedron Lett. 1999, the above result. 40, 5323–5326. 9. Ishihara, K.; Ohara, S.; Yamamoto, H. J. Org. Chem. In addition, the effect of different ammonia sources on 1996, 61, 4196–4199. the reaction progress was studied by using several 10. Thorsen, J. D.; Andersen, T. P.; Pedersen, U.; Yde, B.; ammonium salts such as ammonium acetate, ammo- Leweson, S. Tetrahedron 1985, 41, 5633–5636. nium chloride, ammonium sulfate, and ammonium car- 11. Basel, Y.; Hassner, A. Tetrahedron Lett. 2002, 43, 2529– bonate. The results obtained showed that urea is the 2533. most suitable source for in situ generation of ammonia. 12. Sanchez, R.; Vest, G.; Depres, L. Synth. Commun. 1989, 19, 2909–2913. As previously reported17,29 we suggest that this reaction 13. Bailen, M. A.; Chinchilla, R.; Dodsworth, D. J.; Najera, might be proceeding firstly by formation of an imida- C. Tetrahedron Lett. 2000, 41, 9809–9813. zolium carboxylate salt. This salt increases the absorp- 14. Varma, R. S. Green Chem. 1999, 43–55. tion of the microwave energy and this energy 15. Loupy, A.; Petit, A.; Hamelin, J.; Texier-Boullet, F.; absorption increment causes the pyrolysis of urea and Francoise, P.; Mathe, D. Synthesis 1998, 1213–1234. liberation of ammonia. In the second step the imidazole 16. Lidstron, P.; Tierney, J.; Wathey, B.; Westman, J. Tetra- is exchanged by ammonia to form the ammonium hedron 2001, 57, 9225–9283. carboxylate salt, and finally strong heating of the 17. Baldwin, B. W.; Hirose, T.; Wang, Z. H. Chem. Commun. ammonium carboxylate salt gives the corresponding 1996, 2669–2670. carboxamide. 18. Varma, R. S.; Naicker, K. P. Tetrahedron Lett. 1999, 40, 6177–6180. In summary, the present microwave-assisted procedure 19. Perreux, L.; Loupy, A.; Volatron, F. Tetrahedron 2002, provides an efficient and very simple methodology for 58, 2155–2162. the preparation of primary amides using urea as a very 20. Sharifi, A.; Mohsenzadeh, M.; Mojtahedi, M.; Saidi, M. cheap and safe ammonia source with good yields under R.; Balalaie, S. Synth. Commun. 2001, 31, 431–434. solvent free conditions. 21. Khadikar, B. M.; Madyar, V. R. Synth. Commun. 2002, 32, 1731–1734. 22. (a) Khalafi-Nezhad, A.; Hashemi, A. J. Chem. Res. (S) 1999, 12, 720–721; (b) Khalafi-Nezhad, A.; Fareghi Alamdari, R.; Zekri, N. Tetrahedron 2000, 56, 7503–7506; General procedure (c) Khalafi-Nezhad, A.; Hashemi, A. J. Chem. Chem. Eng. 2001, 20,9–12; (d) Khalafi-Nezhad, A.; Soltani Rad, Our procedure entails the grinding of the mixture of M. N.; Hakimelahi, G. H. Helv. Chem. Acta 2003, 86, carboxylic acid (1 mmol), urea (2 mmol), imidazole (1 2396–2403. mmol). The mixture was exposed to microwave irradia- 23. Kingston, B. H. M.; Carey, J. J.; Hellwig, B. Anal. Chem. tion in domestic MW oven for appropriate time and 1969, 41, 86–89. power of MW oven (300 W). The resulting crude 24. Loupy, A.; Pigeon, P.; Ramdani, M. Tetrahedron 1996, product extracted was purified by column chromatogra- 52, 6705–6712. phy using silica gel and EtOAc/n-hexane as eluent. 25. Garrigues, B.; Laporte, C.; Laurent, R.; Laporterie, A.; Dubac, J. Liebigs Ann. 1996, 739–742. 26. CRC, Handbook of Tables for Organic Compound Iden- Acknowledgements tification, 54th and 80th. 27. Chen, S. T.; Wu, S. H.; Wang, K. T. Synthesis 1989, 37–38. 1 We thank Shiraz University research council for the 28. Selected H NMR data (DMSO-d6, 250 MHz) for prod- financial support for this work. We also thank Mr ucts c, d, k, o. c: l 3.72 (s, 2H, CH2), 6.30 (s, 2H, NH2), Sajedian Fard for running the NMR spectra. 7.13–7.27 (m, 5H, Ar). d: l 1.61 (s, 9H, 3CH3), 3.83 (s, 7328 A. Khalafi-Nezhad et al. / Tetrahedron Letters 44 (2003) 7325–7328 2H, CH2), 6.02 (s, 2H, NH2), 8.01 (s, 1H, NH). k: l 5.97 1H, J=4.5 Hz, Ar), 8.68–8.69 (d, 1H, J=4.5 Hz, Ar), (s, 2H, NH2), 6.78–6.80 (d, 1H, J=15.5 Hz, olefinic), 9.06 (s, 1H, Ar). 7.15–7.43 (m, 6H, Ar and olefinic). o: l 7.44–7.49 (dd, 29.
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