Electrospray Ionization Mass Spectrometry Reveals an Unexpected Coupling Product in the Copper-Promoted Synthesis of Pyrazoles

The reaction mechanism of the intermolecular oxidative formation of pyrazole 2 via a C-C/N-N bond-formation cascade of the enaminone 1 is investigated by means of ESI-MS. No direct reaction intermediates are observed; however, the formation of an unexpected imidazolid-3-one derivative X is observed that involves an oxidative dimerization of 1 in the presence of dioxygen. The derivative X is isolated and characterized by means of multidimensional H-1 and C-13 NMR spectroscopy.