Catalytic procedures for multicomponent synthesis of imidazoles: selectivity control during the competitive formation of tri- and tetrasubstituted imidazoles

• Published in: Green chemistry : an international journal and green chemistry resource : GC 2012-01, Vol.14 (7), p.238-249
• Main Authors: Kumar, Dinesh, Kommi, Damodara N, Bollineni, Narendra, Patel, Alpesh R, Chakraborti, Asit K
• Format: Article
• Language: English
• Subjects: Aldehydes; Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; Heterocyclic compounds; Heterocyclic compounds with several n hetero atoms in the same ring, in separated rings or in fused rings; Organic chemistry; Preparations and properties; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 1463-9262; 1463-9270
• DOI: 10.1039/c2gc35277j

The catalytic potential of different fluoroboric acid-derived catalyst systems viz. aq HBF 4 , solid supported HBF 4 , metal tetrafluoroborates (inorganic salts), solid supported metal tetrafluoroborates, and tetrafluoroborate based ionic liquids (organic salts) were investigated for the three component reaction (3-MCR) of 1,2-diketone, aldehyde, and ammonium salts to form 2,4,5-trisubstituted imidazoles and the four component reaction (4-MCR) involving 1,2-diketone, aldehyde, amine and ammonium acetate to form 1,2,4,5-tetrasubstituted imidazoles. The HBF 4 -SiO 2 was found to be the stand out catalyst for both the 3-MCR and 4-MCR processes. The next most effective catalysts are LiBF 4 and Zn(BF 4 ) 2 to form 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles via the 3-MCR and 4-MCR, respectively. This is the first report on the unaddressed issue of competitive formation of 2,4,5-trisubstituted imidazole during the 4-MCR involving 1,2-diketone, aldehyde, amine and ammonium acetate and highlights the influence of the catalyst systems in controlling the selective formation of tetra substituted imidazole. The metal salt of weak protic acids drive selectivity towards tetra substituted imidazole in the order tetrafluoroborates > perchlorates > triflates. The catalytic potency of tetrafluoroborates was in the order Zn(BF 4 ) 2 > Co(BF 4 ) 2 > AgBF 4 Fe(BF 4 ) 2 > NaBF 4 LiBF 4 Cu(BF 4 ) 2 . The developed protocols worked well for different diketones, various aryl, heteroaryl, and alkyl aldehydes and in the case of the preparation of 1,2,4,5-tetrasubstituted imidazoles different amines can be used. The effectiveness of different ammonium salts as nitrogen source has been investigated and ammonium acetate is proved to be the best. The HBF 4 -SiO 2 is recyclable for five consecutive uses without significant loss of catalytic activity. Fluoroboric acid derived catalyst systems for selectivity control in multicomponent reactions for synthesis of tri- and tetrasubstituted imidazoles.