Unusual fragmentation of 1,1,2,2,3,3-hexamethylindan. Methyl group equilibration and multi-step skeletal rearrangements in the [M - CH3]+ ions prior to the formation of t-C4H9+ and other fragment ions

Kuck D, Mehdizadeh A. Unusual fragmentation of 1,1,2,2,3,3-hexamethylindan. Methyl group equilibration and multi-step skeletal rearrangements in the [M - CH3]+ ions prior to the formation of t-C4H9+ and other fragment ions. Organic Mass Spectrometry. 1992;27(4):443-452.Based on the surprising observation of an intense C4H9+ (m/z 57) peak in the electron impact mass spectrum, the fragmentation of 1,1,2,2,3,3-hexamethylindan (2) was studied by mass-analysed ion kinetic energy Spectrometry of its deuterium-labelled analogues. While methyl loss from ions [2]+ occurs with high selectivity (92%) from the positions 1 and 3 without any rearrangement, ions [2 CH3]+ undergo complete equilibration of the five methyl groups as intact entities. Subsequent multi-step skeletal rearrangement of the [2 - CH3]+ ions leads to formation of tert-butyl ions and to the loss of isobutene and propene, again without concomitant hydrogen exchange. Several kinetic isotope effects and also probably a thermodynamic isotope effect associated with each of these fragmentation processes have been found and their origin is discussed. The possibility of the formation of ion-neutral complexes [t-C4H9+ C10H10] and [s-C3H7+ C11H12] is considered on the basis of the labelling and reactivity pattern