Rearrangement by Intermediate Ion/Neutral Complexes during the McLafferty Fragmentation of Unsaturated Ketones

The fragmentation of 1-phenylhex-5-ene-1-one 1 was studied by mass analyzed ion kinetic energy spectrometry of its deuterium and ¹³carbon labelled derivatives as a further example of interactions within ion/neutral complexes. The analysis of the MIKE spectra demonstrate two important reaction channels of metastable molecular ions. The first one is a McLafferty rearrangement by elimination of a 1,3-butadiene molecule. The second energetically even more favoured fragmentation corresponds to the loss of an ethyl radical. The MIKE spectra of labelled molecular ions, especially that of the ¹³carbon labelled compound, indicate a skeletal rearrangement specifically of the butenyl moiety of the alkyl side chain by interchanging the terminal carbon atoms C-6 and C-5 with the inner C atoms C-3 and C-4 prior to the loss of the ethyl radical. This specific skeletal rearrangement is explained by an internal return within an intermediate ion/neutral complex f, consisting of the acetophenone enol radical cation and the neutral butadiene, by addition of the enol radical cation to the remote double bond of the diene. The second reaction channel is the irreversible cyclization of an intermediate distonic ion c leading to the formation of an ionized ethyl-α-dihydronaphthol d. This ion is stabilized by loss of an ethyl radical resulting in protonated α-naphthol e.