A synthetically useful approach of catalytic intramolecular cyclization and oxidation of 2',2'-(1,4-pheny lene)bis(1,3-dialkyl)guanidines (Alkyl = isopropyl 1 or cyclohexyl 2) catalyzed by copper acetate in acetonitrile under air was studied by on line monitoring of the reaction by ESI-MS. All-important intermediates organic species were intercepted during the experiment confirming for the first time the stepwise (1,4-phenylene)bisguanidines cyclization and oxidation mechanism. Moreover, performed collision induced dissociation (CID) experiments were also applied as a structure elucidation tool. Bimetallic copper intermediates Cu1 ([C28H48Cu2N6O10+H](+)) of m/z 755 and Cu2[C22H36Cu2N6O4 + H](+) of m/z 575 were documented. The plausible key mechanistic steps involving the formation of organic and inorganic intermediates detected by in situ monitoring of the reaction are presented. (C) 2019 Elsevier Inc. All rights reserved.