Efficient and versatile catalysis of N-alkylation of heterocyclic amines with alcohols and one-pot synthesis of 2-aryl substituted benzazoles with newly designed ruthenium(ii) complexes of PNS thiosemicarbazones

Abstract

Ruthenium(II) carbonyl complexes with phosphine-functionalized PNS type thiosemicarbazone ligands [RuCl(CO)(EPh3)(L)] (1-6) (E = P or As, L = 2-(2-(diphenylphosphino)benzylidene) thiosemicarbazone (PNS-H), 2-(2-(diphenylphosphino)benzylidene)-N-methylthiosemicarbazone (PNS-Me), 2-(2-(diphenylphosphino)benzylidene)-N-phenylthiosemicarbazone (PNS-Ph)) have been synthesized and characterized by elemental analysis and spectroscopy (IR, UV-Vis, (1)H, (13)C, (31)P-NMR) as well as ESI mass spectrometry. The molecular structures of complexes 1, 2 and 6 were identified by means of single-crystal X-ray diffraction analysis. The analysis revealed that all the complexes possess a distorted octahedral geometry with the ligand coordinating in a uni-negative tridentate PNS fashion. All the ruthenium complexes (1-6) were tested as catalyst for N-alkylation of heteroaromatic amines with alcohols. Notably, complex 2 was found to be a very efficient and versatile catalyst towards N-alkylation of a wide range of heterocyclic amines with alcohols. Complex 2 can also catalyze the direct amination of 2-nitropyridine with benzyl alcohol to the corresponding secondary amine. Furthermore, a preliminary examination of performance for N,N-dialkylation of diamine showed promising results, giving good conversion and high selectivity. In addition, N-alkylation of ortho-substituted anilines (-NH2, -OH and -SH) led to the one-pot synthesis of 2-aryl substituted benzimidazoles, benzoxazoles and benzothiazoles, also revealing the catalytic activity of complex 2.

Enhanced Catalytic Properties of Molybdenum Promoted Mesoporous Cobalt Oxide: Structure‐Surface‐Dependent Activity for Selective Synthesis of 2‐Substituted Benzimidazoles

“…This method is preferred in terms of applications as a wide range of 2‐substituted benzimidazole derivatives could be obtained due to the availability of a high variety of aldehydes . However, these reactions require either stoichiometric amounts of hydrogen acceptors or strong bases in order to achieve efficient and selective benzimidazole yields, which has been an issue of high interest …”

Ruthenium(II) complexes encompassing 2-oxo-1,2-dihydroquinoline-3-carbaldehyde thiosemicarbazone hybrid ligand: A new versatile potential catalyst for dehydrogenative amide synthesis

“…The electronic spectra of the complexes (1-6) have been recorded in dichloromethane and they displayed bands in the region around 246−469 nm. The high-energy absorption in the region 246−328 nm have been assigned to ligand-centered (LC) transitions [38], the lowest energy observed in the region 386-398 nm have been attributed to the ligand to metal charge transfer (LMCT) transitions, and the bands at 418−469 nm has been assigned to a d→d transition [39].…”

Phosphine ligand-free RuCl3-catalyzed reductive N-alkylation of aryl nitro compounds

“…45,46 Viswanathamurthi and co-workers found ruthenium(II) carbonyl complexes with phosphinefunctionalized type thiosemicarbazone ligands such as [RuCl(-CO)(PPh 3 )(PNS-Me)] (PNSeMe¼2-(2-(diphenylphosphino)benzylidene)-N-methylthiosemicarbazone) were efficient and versatile N-alkylation catalysts. 47 Zhang et al reported the synthesis of quinolines from a-2-nitroaryl alcohols and alcohols using Ru 3 (CO) 12 /dppf as the catalyst or 2-nitroanilines and vicinal diols using Ru 3 (CO) 12 /dppp as the catalyst. 48,49 However, despite their versatility as ligands for stabilizing low valent metal transition states and intermediates, phosphines are often toxic, moisture and air sensitive and hard to separate from the organic products.…”