Metal-Ion-Promoted Dehydrogenation and Elimination Reactions of a 2,3-Dihydro-anthraimidazole Derivative: An Example of an Imidazole to Quinoxaline Ring Expansion

Abstract Metal ions act as precise chemical switches, inducing diverse transformations of a dihydro-anthraimidazole derivative, including a unique imidazole-to-quinoxaline ring expansion. Specifically, zinc(II) and cadmium(II) salts trigger 1,2-dehydrogenation (aromatization) and ring expansion reactions, while ruthenium(II) and osmium(II) precursors drive cleavage reactions. An intermolecular Cannizzaro-type reaction of 1,2-diaminoanthraquinone with pyridine-2-aldehyde in the presence of p-TsOH yields 2-(pyridin-2-yl)-1-(pyridin-2-ylmethyl)-2,3-dihydro-1H-anthra[2,3-d]imidazole-5,10-dione (H2Ima) along with 2-methyl-3H-anthra[1,2-d]imidazole-6,11-dione (ImMe). In the presence of MCl2 (M = Zn, Cd), H2Ima undergoes 1,2-dehydrogenation reaction affording H2O2 and Ima complexes, [ZnII(Ima)Cl2] (1) and [CdII(Ima)Cl2] (2). Notably, in the presence of zinc(II) acetate/perchlorate and cadmium(II) acetate, H2Ima undergoes transformation to Ima and finally to 2,3-di(pyridin-2-yl)naphtho[2,3-f]quinoxaline-7,12-dione (Qox) derivative. The unprecedented imidazole → quinoxaline ring expansion reaction has also been achieved using 1 as a precursor, inferring that 1,2-dehydrogenation of H2Ima is an essential step for this transformation. ESI mass spectrometry confirmed that a methoxide complex as an intermediate led to the ring expansion reaction. Surprisingly, the reactions of H2Ima with ruthenium(II) and osmium(II) precursors containing PPh3 and CO as co-ligands promote a 1,2-elimination reaction furnishing 2-(pyridin-2-yl)-3H-anthra[1,2-d]imidazole-6,11-dione (HImb), where 2-methylpyridine is eliminated. Following this protocol, HImb complexes of ruthenium(II) and osmium(II) of types, [RuII(HImb)(PPh3)2Cl2] (3), [OsII(Imb-)(PPh3)2(CO)H] (4), [RuII(Imb-)(PPh3)2(CO)H] (5) and [(PPh3)2(CO)(H)RuII(μ-Imb-)RuII(PPh3)(CO)Cl2] (6) were isolated and authenticated.