Evidence for an Alkene Metathesis Reaction during the Unimolecular Dissociation of (CH₃)₃COCHCH₂^•+

Abstract

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 using isotopic labeling, it is proposed that the fragmentation begins with the cleavage of the O–C(butyl) bond. This behavior, which contrasts with that of ionized alkylethers, is the consequence of the presence of a vinyl group that allows the formation of a neutral allylic radical. The key intermediate of the fragmentation is likely to be a [CH2CHOH, (CH3)2CCH2]•+ complex in which a cycloaddition–cycloreversion reaction occurs. This intermediate is the same as that produced by a McLafferty rearrangement in appropriate aldehyde radical cations.