Sequential Methyl Elimination from Ionized Dimethyl[2. n ] Para cyclophane-Enes ( n = 3,4,5,6): Interaction of Ionized Aromatic Rings in Strained Cyclophane Conformations?

Abstract

The fragmentations of the molecular ions of a series of dimethyl[2.n]paracyclophane-enes (n = 3, 4, 5, 6) have been studied by mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labeling. The characteristic fragmentation sequence of the molecular ions of all dimethyl[2.n]paracyclophane-enes is the sequential elimination of three methyl radicals. The first methyl radical originates predominantly from the methyl substituent of the ethylene bridge and this process is accompanied by a large kinetic energy release (KER) indicative of a preceding rearrangement. The kinetic energy release distribution (KERD) observed is independent of the size of the polymethylene bridge of the dimethyl[2.n]paracyclophane-enes and even of the source of the methyl radical. Abundant loss of a second methyl radical is observed from metastable [M – CH₃]⁺ ions in spite of a violation of the “even-electron-rule”. Again, the methyl radical lost is chiefly a methyl substituent at the ethylene bridge, although methyl loss from the inner methylene groups of the polymethylene bridge occurs also. A small KER is observed for this fragmentation, which is again independent of the source of the methyl radical. These observations are rationalized by a rearrangement of the molecular ions by bond formation between the aromatic rings and subsequent hydrogen migrations either before or after a benzylic cleavage of the saturated bridge.