Formation of the amides

Formation of the amides plays a vital role in the synthesis of an organic compound or peptides as the amides are present in proteins, peptides and many other naturally occurring as well as synthetic materials.1 2 Moreover an analysis on manufacturing of various drug candidates carried out jointly by Pfizer Global R&D, U.K., Astrazeneca PR&D, U.K., GlaxoSmithKline PharmaceuticalsLtd., U.K., revealed the formation of amide bond was necessary in manufacturing of 84 out of 128 drug candidates surveyed.3 This makes the formation of amides a key process for majority of the pharmaceutical companies

In 1948 Ritter and Minieri studied the reaction between nitriles and alkenes in the presence of sulphuric acid which on further hydrolysis gave amide and water. 4 Later in 1949 Ritter and Benson suggested the use of alcohols with nitrile in presence of sulphuric acid as a catalyst.5 Mukhopadhyay and Iqbal in 1997 also reported the use of alcohols instead of alkenes.6 The formation of amide can be achieved by condensation reactions of amines and carboxylic acids using different coupling agents, however the reaction conditions were not suitable for scale up due to highly acidic medium.3 Many alternative methods involving the use of different acid based catalyst,7, 8 bismuth triflate,9 boron trifluoride ehterate,10 metal complexes,11 trifluoromethanesulfonic anhydride12 have been developed. However the above mentioned methods cannot be employed on large scale production since these methods have at least one of the below mentioned problems: Cost of the reagent, lower yields, higher waste generation, reagents availability, faster reaction conditions, longer reaction time, toxicological issues, corrosion of materials being used and therefore there is a need to develop more sustainable and greener catalyst for amide formation.13 In general the Ritter Reaction is a well-known reaction in wherein the amide is produces from an insitu generated carbocation and nitrile compound.4

The use of Solid-supported catalyst presents an alternative to this problem and various forms of solid-supported catalyst such as silica-sulphuric acid,14 P2O5/SiO2,15 Cs2.5H0.5PW12O40, 16. Despite these methods being quite promising the scale up using the above mentioned catalyst had many issues such as reuse of the catalyst. 17

Bibliography

1. Albericio, F., Developments in peptide and amide synthesis. Current Opinion in Chemical Biology 2004, 8, (3), 211-221.

2. Singh, G. S., Recent progress in the synthesis and chemistry of azetidinones. Tetrahedron 2003, 59, (39), 7631-7649.

3. Gelens, E.; Smeets, L.; Sliedregt, L. A. J. M.; van Steen, B. J.; Kruse, C. G.; Leurs, R.; Orru, R. V. A., An atom efficient and solvent-free synthesis of structurally diverse amides using microwaves. Tetrahedron Letters 2005, 46, (21), 3751-3754.

4. Ritter, J. J.; Minieri, P. P., A New Reaction of Nitriles. I. Amides from Alkenes and Mononitriles1. Journal of the American Chemical Society 1948, 70, (12), 4045-4048.

5. Benson, F. R.; Ritter, J. J., A New Reaction of Nitriles. III. Amides from Dinitriles. Journal of the American Chemical Society 1949, 71, (12), 4128-4129.

6. Mukhopadhyay, M.; Iqbal, J., Co(III)DMG-Catalyzed Synthesis of Allylamides from Allyl Alcohols and Acetonitrile. The Journal of Organic Chemistry 1997, 62, (6), 1843-1845.

7. Lebedev, M. Y.; Erman, M. B., Lower primary alkanols and their esters in a Ritter-type reaction with nitriles. An efficient method for obtaining N-primary-alkyl amides. Tetrahedron Letters 2002, 43, (8), 1397-1399.

8. Justribó, V.; Colombo, M. I., An unexpected result in an intramolecular Ritter reaction induced by triflic anhydride. Tetrahedron Letters 2003, 44, (43), 8023-8024.

9. Callens, E.; Burton, A. J.; Barrett, A. G. M., Synthesis of amides using the Ritter reaction with bismuth triflate catalysis. Tetrahedron Letters 2006, 47, (49), 8699-8701.

10. Reddy, K. L., An efficient method for the conversion of aromatic and aliphatic nitriles to the corresponding N-tert-butyl amides: a modified Ritter reaction. Tetrahedron Letters 2003, 44, (7), 1453-1455.

11. Mukhopadhyay, M.; Reddy, M. M.; Maikap, G. C.; Iqbal, J., Cobalt(II)-Catalyzed Conversion of Allylic Alcohols/Acetates to Allylic Amides in the Presence of Nitriles. The Journal of Organic Chemistry 1995, 60, (9), 2670-2676.

12. Chen, X.; Okuhara, T., A Ritter-Type Reaction over H-ZSM-5: Synthesis of N-Isopropylacrylamide from Acrylonitrile and Isopropyl Alcohol. Journal of Catalysis 2002, 207, (2), 194-201.

13. Lakouraj, M.; Mokhtary, M., Polyvinylpolypyrrolidone-supported boron trifluoride: a high-loaded, polymer-supported Lewis acid for the Ritter reaction. Monatshefte für Chemie / Chemical Monthly 2009, 140, (1), 53-56.

14. Bi Bi Fatemeh Mirjalili, B. S., Silica sulfuric acid: an eco-friendly and reusable catalyst for synthesis of amides via Ritter reaction. Iranian Journal of Organic Chemistry 2009, 2, 76 - 79.

15. Tamaddon, F.; Khoobi, M.; Keshavarz, E., (P2O5/SiO2): a useful heterogeneous alternative for the Ritter reaction. Tetrahedron Letters 2007, 48, (21), 3643-3646.

16. Okuhara, T.; Chen, X., Ritter-type reactions catalyzed by high-silica MFI zeolites. Microporous and Mesoporous Materials 2001, 48, (1-3), 293-299.

17. Polshettiwar, V.; Varma, R. S., Nafion®-catalyzed microwave-assisted Ritter reaction: an atom-economic solvent-free synthesis of amides. Tetrahedron Letters 2008, 49, (16), 2661-2664.

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