Competitive isomerization processes of 5‐methyl‐2‐hexanone radical cations

Abstract

A great number of isomerization reactions occur before dissociation of metastable [C,H2,+,COR]+' radical cations (R = OH, CH3, C6H,, n = 4,5). These processes are often identical for carboxylic acids and ketones. For instance, it -has been shown that pentanoic acid radical cations isomerize: (i) by 1-2',2, 1-3 and 1-43-9 CH202 migration, respectively, into the 0, y and 6 distonic ions (Scheme 1, mechanisms (a), (b) and (c)), and (ii) by rearrangement of the y distonic ion either through a cyclopropanoic transition state3 or through an [enol-alkene]+' ion/molecule ~o m p l e x~,~ leading to reversible isomerization of n-pentyl ion into isopentyl ion (mechanism (d)).The same processes have been observed for the metastable 2-hexanone radical cation". ' I . Rearrange- A comprehensive study of labelled n-hexanoic acid radical shows that these ions undergo fewer skeletal rearrangement processes than pentanoic acid radical cations.Two transposition reactions have been proposed for [CH,COC,H,,]+' ions: (i) l e 2 4 313 (Scheme 2) involving the mechanism (c) (Scheme l ) , and (ii) 5c34-,614.15 (Scheme 2) involving, in the first step, mechanism (d) and in the second step mechanism (b).However, these hypotheses have not been confirmed by 13C labelling and the fragmentation pathways leading to the fragment ions have not been established except for [4 -C2H,]+.. '5 In this study we report a comparison of the massanalysed ion kinetic energy (MIKE) spectra of ions 4, 5, 7 and 8 which allows the elucidation of the origin of the main fragment [M -C2Hs]+, the confirmation of the reversible isomerization of 4 into 5 and the evidencing of other rearrangements. Scheme 2