Evidence for an alkene metathesis reaction during the unimolecular dissociation of (CH3)(3)COCHCH2 center dot+

The unimolecular reaction of the metastable tert-butyl vinyl ether radical cation leads mainly to elimination of ethylene, From metastable ion studies as well as Fourier transform ion cyclotron resonance measurements and by u sing isotopic labeling, it is proposed that the fragmentation begins with t he cleavage of the O-C(butyl) bond. This behavior, which contrasts with tha t of ionized alkylethers, is the consequence of the presence of a vinyl gro up that allows the formation of a neutral allylic radical. The key intermed iate of the fragmentation is likely to be a [CH2CHOH, (CH3)(2)CCH2](.+) com plex in which a cycloaddition-cycloreversion reaction occurs, This intermed iate is the same as that produced by a McLafferty rearrangement in appropri ate aldehyde radical cations.