Ion/Neutral Complex-mediated Reactions Generated from Some Ionized Tetrahydroimidazole-substituted Methylene β-Diketones

Abstract

The dissociation of gaseous metastable ions of m/z 153 and the formation of ions of m/z 139 from the unimolecular fragmentations of ionized tetrahydroimidazole-substituted methylene -diketones were examined by tandem mass spectrometry. In addition, some other fragments accompanying the elimination of either an H 2 O molecule or an CHO ᭹ radical were also observed in the collision-induced dissociation spectra of molecular ions of the compounds bearing an aromatic ring. Collision-induced dissociation and isotopic labeling showed that these processes may involve reactions of intermediate ion/neutral complexes and multistep rearrangements. The corresponding mechanisms are discussed. © 1997 by John Wiley & Sons, Ltd. Received 8 January 1997; Revised 8 March 1997; Accepted 10 March 1997 Rapid. Commun. Mass Spectrom. 11, 723-726 (1997 9,10 However, related compounds in which the two hydrogens of the methylene group of -diketones are substituted, as for example in our system, have not been studied. Tandem mass spectrometric techniques, involving collision-induced dissociation (CID) at low energy have now been used for this purpose. Some interesting common characteristic fragmentations in ionized tetrahydroimidazole-substituted methylene -diketones mediated by ion/neutral complexes are reported in this paper. EXPERIMENTALMass spectrometric measurements were performed on a multiple quadrupole instrument (VG Quattro, Fisons Instruments, Manchester, UK) with a source temperature of 200°C. The instrument was operated in the electron ionization mode using an ionization energy of 70 eV and an emission current of 200 µA. The samples were dissolved in acetone and inserted via a desorption probe; samples were loaded onto a rapidly heated wire filament and evaporated with a desorption current of 400 µA. CID spectra and parent-ion spectra were obtained using argon as the collision gas at a collision energy of 50 eV. The pressure of argon was adjusted to reduce the main beam intensity by 50% of its original value.The tetrahydroimidazole-substituted methylene -diketones were synthesized in our laboratory using a method that has been reported previously, 11,12 in which a suitable -diketone was used as starting material, reacting in succession with carbon disulfide, ethyl bromide and ethylenediamine. The products were then purified by recrystallization. The deuterated samples were prepared by the classical method, mixing the sample with an excess of CD 3 OD. RESULTS AND DISCUSSIONSome major ions and corresponding relative abundances observed in the electron ionization (EI) mass spectra of the compounds investigated (1-5) are given in Table 1 (Fig. 2) indicated that the specific fragmentations were derived directly from dissociation of their molecular ions. It is clear that in these cases, the observations cannot be explained by simple bond cleavage and rearrangement; therefore, the experimental results implied that an unexpected and specific skeletal multi-step rearrangement reaction may be occurring before dissociation. In order to o...