An experimental and computational investigation on the fragmentation behavior of enaminones in electrospray ionization mass spectrometry.

The dissociation pathways of protonated enaminones with different substituents were investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS) in positive ion mode. In mass spectrometry of the enaminones, ArCOCHCHN(CH(3) )(2) , the proton transfers from the thermodynamically favored site at the carbonyl oxygen to the dissociative protonation site at ipso-position of the phenyl ring or the double bond carbon atom adjacent to the carbonyl leading to the loss of a benzene or elimination of C(4) H(9) N, respectively. And the hydrogendeuterium (H/D) exchange between the added proton and the proton of the phenyl ring via a 1,4-H shift followed by hydrogen ring-walk was witnessed by the D-labeling experiments. The elemental compositions of all the ions were confirmed by ultrahigh resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR-MS/MS). The enaminones studied here were para-monosubstituted on the phenyl ring and the electron-donating groups were in favor of losing the benzene, whereas the electron-attracting groups strongly favored the competing proton transfer reaction leading to the loss of C(4) H(9) N to form a benzoyl cation, Ar-CO(+) . The abundance ratios of the two competitive product ions were relatively well-correlated with the σ(p) (+) substituent constants. The mechanisms of these reactions were further investigated by density functional theory (DFT) calculations.