Probing the Mechanism of the Baylis–Hillman Reaction by Electrospray Ionization Mass and Tandem Mass Spectrometry

"Ion fishing": Protonated intermediates in the catalytic cycle of the Baylis–Hillman reaction between an activated alkene and an electrophile have been intercepted and characterized by electrospray ionization tandem mass spectrometry. This approach provides direct evidence for the currently accepted mechanism (see scheme).