Copper (II) as catalyst for intramolecular cyclization and oxidation of (1,4-phenylene)bisguanidines to benzodiimidazole-diylidenes

•Catalytic cyclization of guanidines was studied by on line monitoring of the reaction by ESI-MS.•All-important intermediates organic species were intercepted during the experiment.•The key copper-containing intermediate was intercepted and characterized.•The plausible key mechanistic steps are presented.