Ionic Cycloreversion Reactions in Tetralin Derivatives. Structure of C₈H₈O Ions from 1-Tetralol

By a combination of deuterium labeling and metastable ion measurements, it has been demonstrated that 1-tetralol ion (1,2,3,4-tetrahydro-1-naphthalenol) undergoes cycloreversion by two pathways to eliminate ethene bearing either carbons 2 and 3 or carbons 3 and 4. Although the two mechanisms are competitive for rapidly decomposing molecular ions, the elimination of ethene bearing carbons 3 and 4 is preponderant in the metastable time region. Each process produces a structurally distinct C₈H₈O radical cation and this is verified by isotopic labeling, comparisons of metastable ion intensities, and studies of kinetic energy release. The actual structure of [C₈H₈O]·⁺ produced by elimination of carbons 3 and 4 is the enol form of acetophenone, which undergoes methyl loss by abstraction of one of the randomized ring hydrogens. Expulsion of carbons 2 and 3 gives a structure similar to that produced by water loss from o-hydroxymethylbenzyl alcohol. These results sound a cautionary note for literal interpretations of metastable intensities and kinetic energy release data in ion structure determinations.