Niobium Based Catalytic Systems for Organic Transformations

Abstract

Niobium-based catalytic systems have been developed for the synthesis of Nheterocycle newlinederivatives. Commercially sourced ammonium niobium oxalate (ANO) newlinewas used as a recyclable catalyst for the Friedel-Crafts alkylation of indoles with newlinenitroalkenes to yield C3-alkylated indoles in a green solvent mixture. The catalytic newlinesystem was compatible with various indoles and and#61538;-nitroalkenes, including aromatic, newlineheterocyclic, and organometallic substrates, and the desired products were obtained in newlinegood yields. Ruthenium-polyoxoniobate complexes were found to be efficient catalysts newlinefor the selective synthesis of 2-substituted benzimidazoles and 1,2-disubstituted newlinebenzimidazoles by solvent switching via dehydrogenative coupling reaction between ophenylenediamines newlineand aryl alcohols. 2-substituted benzimidazoles were synthesized in newlinewater, whereas 1,2-disubstituted benzimidazoles were obtained when toluene was used newlineas the solvent. Control experiments showed the involvement of both acceptorless newlinedehydrogenation and oxidative coupling pathways. Niobium pentoxide with a deformed newlineorthorhombic structure was used as a recyclable catalyst for the synthesis of 2- newlinesubstituted benzimidazoles and quinazolinones via dehydrogenative coupling reaction newlinein an aqueous medium. The initial mechanistic studies suggested a free radical newlinemechanism for the oxidative dehydrogenative coupling reaction. The synthesis of newlineamides by the hydration of nitriles and cascade synthesis of 2-substituted newlinequinazolinones from 2-aminobenzonitriles and aryl alcohols were done using a newlinepolyoxoniobate (K7HNb6O19·13H2O) catalyst. The reaction proceeded efficiently in an newlineaqueous medium and was found to be compatible with various substrates. A plausible newlinefree radical mechanism was discussed for the cascade synthesis of 2-substituted newlinequinazolinones. newline