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

✍ Scribed by Dietmar Kuck; Ahmad Mehdizadeh

• Publisher: John Wiley and Sons
• Year: 1992
• Tongue: English
• Weight: 908 KB
• Volume: 27
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/oms.1210270416

✦ Synopsis

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 [ 21 +' occurs with high selectivity (92%) from the positions 1 and 3 without any rearrangement, ions [ 2 -CH,] + undergo complete equilibration of the five methyl groups as intact entities. Subsequent multi-step skeletal rearrangement of the [ 2 -CH, 1 + 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+ C,,H,,] and [s-C,H,+ CllH,,] is considered on the basis of the labelling and reactivity pattern.