Fragmentation and skeletal rearrangements of 2-arylylamino-5-aryl-1,3,4-oxadiazoles and their noncovalent complexes with cobalt cation and cyclodextrin studied by mass spectrometry.

Mass spectrometric fragmentation pathways of title compounds were studied by electron ionization (EI) and electrospray ionization (ESI) as methods of ion generation. To explain the observed complex skeletal rearrangements, tandem mass spectrometry, accurate mass measurement and isotope labeling (compounds containing one 13C atom in oxadiazole ring) were used. Loss of CO, N2 and H atoms under EI conditions led to the formation of 9,10-dihydroacridine-type ions, loss of NH3 under ESI conditions yielded the 4-phenylphthalazinone-type ions and the loss of HNCO under ESI conditions produced N-arylamino-benzonitrilium ions; however, this process can be affected by the electron-donor/electron-withdrawing properties of groups substituted at the phenyl rings. The ESI was used to study the complexes of the compounds with cobalt as well as with cyclodextrin. It was found that the compounds studied tend to form inclusion complexes with cyclodextrin of stoichiometry 1:1 and complexes of different stoichiometries with cobalt, although those of stoichiometry 6:1 and 4:1 are favored and the attachment of counter ion may stabilize the complexes 3:1 and 2:1.