RESUMEN
Nitrogen heterocycles are found in the structures of many biologically important compounds, as well as materials used in the synthesis of fine chemicals. Notably, ~59% of US Food and Drug Administration-approved small-molecule drugs contain nitrogen heterocycles. It is therefore meaningful to explore greener or more sustainable methods for their synthesis. The use of alcohols as reagents is attractive as they can be readily obtained from biomass derived natural resources. In the last two decades, alcohol dehydrogenative coupling reaction to synthesize various heterocycles were extensively explored which furnished hydrogen (H2) and water (H2O) as the two greener byproducts. In this protocol, we describe several efficient catalytic transformations to synthesize quinolines, 1,8-naphthyridines, quinoxalines, quinazolines, pyrimidines, benzimidazoles, pyrroles and pyridines, using alcohol as starting materials. We also describe the synthesis of several homogeneous iridium/ruthenium catalysts and heterogeneous cobalt/copper catalysts that can be used in these transformations. The reaction setup is simple; in a Schlenk/reaction tube with magnetic stir-bar, alcohol, corresponding coupling reagents (nucleophiles), catalyst, base and solvent (water or organic solvent such as toluene, dioxane or p-xylene) are added. The reaction mixture is refluxed at the specified temperature (110-150 °C)-either in air or under argon-to furnish these heterocycles. Synthesis of the catalysts takes 3-5 h and the coupling reactions take 4-5 h depending on the target product. The cobalt- and copper-based heterogeneous catalytic systems displayed an good catalyst recyclability.
RESUMEN
Herein, control transfer hydrogenation (TH) of azoarenes to hydrazo compounds is established employing easy-to-synthesize reusable cobalt catalyst using lower amounts of N2H4·H2O under mild conditions. With this effective methodology, a library of symmetrical and unsymmetrical azoarene derivatives was successfully converted to their corresponding hydrazo derivatives. Further, this protocol was extended to the TH of nitroarenes to amines with good-to-excellent yields. Several kinetic studies along with Hammett studies were carried out to understand the plausible mechanism and the electronic effects in this transformation. This inexpensive catalyst can be recycled up to five times without considerable loss of catalytic activity.
RESUMEN
The inexpensive and simple NiBr2/1,10-phenanthroline system-catalyzed synthesis of a series of quinoxalines from both 2-nitroanilines and 1,2-diamines is demonstrated. The reusability test for this system was performed up to the seventh cycle, which afforded good yields of the desired product without losing its reactivity significantly. Notably, during the catalytic reaction, the formation of the heterogeneous Ni-particle was observed, which was characterized by PXRD, XPS, and TEM techniques.
RESUMEN
An efficient synthesis of N-methylated amides using methanol in the presence of a ruthenium(II) catalyst is realized. Notably, applying this process, tandem C-methylation and N-methylation were achieved to synthesize α-methyl N-methylated amides. In addition, several kinetic studies and control experiments with the plausible intermediates were performed to understand this novel protocol. Furthermore, detailed computational studies were carried out to understand the mechanism of this transformation.
RESUMEN
The synthesis and reactivity of Mn(i) complexes bearing bifunctional ligands comprising both the amine N-H and benzimidazole fragments are reported. Among the various ligands, the N-((1H-benzimidazol-2-yl)methyl)aniline ligand containing Mn(i) complex presented higher reactivity in the transfer hydrogenation (TH) of ketones in 2-propanol. Experimentally, it was established that both the benzimidazole and amine N-H proton played a vital role in the enhancement of the catalytic activity. Utilizing this system a wide range of aldehydes and ketones were reduced efficiently. Notably, the TH of several imines, as well as chemoselective reduction of unsaturated ketones, was achieved in the presence of this catalyst. DFT calculations were carried out to understand the plausible reaction mechanism which disclosed that the transfer hydrogenation reaction followed a concerted outer-sphere mechanism.
RESUMEN
Ru(II) catalyzed tandem synthesis of α-branched methylated ketones via multicomponent reactions following the hydrogen borrowing process is described. This nonphosphine-based air and moisture stable catalyst efficiently produced various methylated ketones using methanol as a methylating agent. This system was found to be highly effective in three-component coupling between methanol, primary alcohols, and methyl ketones. A proposed catalytic cycle for the α-methylation is supported by DFT calculations as well as kinetic experiments.