Ferrocene Based Ruthenium II Arene Catalytic Systems for Acceptorless Dehydrogenative Coupling Reactions

Abstract

A series of ferrocene (Fc)-based Ru(II)-arene complexes (1-12) were investigated for their catalytic activity towards acceptorless dehydrogenative coupling (ADC) reactions. The Ru(II)-arene complexes containing different ferrocene ligands [Fc-thiosemicarbazone, Fc-hydrazone, Fc-thioamide or Fc-acylthiourea (L1-L14)] were synthesized, and characterized by employing spectroscopic and analytical techniques. The crystal structures of ligands L3, L5, L6 and L14, and complexes 4, 7 and 9 were confirmed using a single crystal X-ray diffraction technique. The complexes exhibited a pseudo-octahedral geometry around the Ru(II) ion, with monodentate/bidentate coordination of the Fc ligands. Initially, Ru(II)-p-cymene complexes (1-5) containing ferrocene-based thiosemicarbazone (Fc-TSC) ligands (L1-L4) were investigated for the selective synthesis of 1,2-disubstituted benzimidazoles via ADC of diamine with primary alcohols. The catalysis was extended to aromatic, aliphatic and heterocyclic alcohols. Similarly, Ru(II)-p-cymene complexes (6-8) containing heterocyclic Fc-hydrazone ligands (L5-L7) were studied for the solvent-free synthesis of 1,2-disubstituted benzimidazoles and quinazolinones, and they exhibited an improved catalytic activity. The quinolines were synthesized via mono/double dehydrogenative coupling of 2-aminobenzyl alcohol with ketones/secondary alcohols, which was accomplished using the in situ catalytic activity of [RuCl2(p-cymene)]2 with ferrocene thioamide (Fc-TA) ligands (L8-L10). The catalysis was extended to aromatic and heterocyclic ketones, and secondary alcohols, with unsubstituted and chloro-substituted 2-aminobenzyl alcohols. Furthermore, the synthesis of quinazolines from 2-aminobenzyl alcohol and benzonitriles using Ru(II)-p-cymene catalysts containing Fc-acylthiourea (Fc-Tu) ligands (9-12) was discussed. Several substrates (electron-donating and -withdrawing benzonitriles) were utilized for extending the substrate scope. A plausible mechanism for the reactions was discussed and overall