Stereoisomeric fragment ions arising from decomposition of enol cation radicals of different internal energy content

The least‐squares analysis of collisional activation mass spectra provides evidence that 1,1‐dihydroxy‐3‐methylbutene cation radical (a) and 1‐hydroxy‐1‐methoxy‐3‐methylbutene cation radical (c) (generated in a McLafferty‐type rearrangement from 2‐isopropyl‐3‐methylbutanoic acid and its methyl ester) give rise to product ions b and d , the stereochemistry of which at the double bond is determined by the lifetime (internal energy) of a and c , respectively. Decompositions of a and c in the ion source yield predominantly the less stable cis ‐configurated stereoisomers of b and d , whereas dissociation of a and c at longer lifetimes (lower internal energy) results in the formation of the thermochemically more stable trans isomers of b and d . A qualitative MNDO analysis is provided which takes into account the special features of stereoelectronic control of radical chemistry in the cleavage of a carboncarbon bond.