Selective C- and N‑Methylation of Secondary Alcohols and Nitroaromatics with Methanol Catalyzed by a New Ru(II)-Azo Complex

Using MeOH as the methylating agent, selective methylation of secondary alcohols and nitroaromatics is achieved using a newly prepared Ru(II)-catalyst 1a of a tridentate azo-aromatic ligand. Two new organometallic Ru(II)-complexes [Ru(Cp)(PPh3)(La)]Cl (1a) and [Ru(Cp)(PPh3)(Lb)]Cl (1b) of tridentate redox-active ligands 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (La) and 2-(phenyldiazenyl)-1,10-phenanthroline (Lb) were prepared and characterized by different spectroscopic methods. 1a and 1b demonstrated good catalytic activity in the methylation of a diverse array of 1-aryl ethanols and nitroarenes. 1a showed encouraging catalytic efficiency with various naturally occurring steroids, including 4-cholestene-3-one, progesterone, estrone, and β-sitosterol, highlighting its capability to modify natural product scaffolds. Mechanistic studies revealed that 1a-catalyzed methylation reactions advance through a hydrogen borrowing pathway involving cooperative participation of both Ru(II) and the aryl azo scaffold.