Fluoride ion induced generation of isobutylidene from isopropyl trimethylsiiyl ketone enol triflate in the presence of 6,6-dimethylfulvene gives the [2.2]-m-cyclophane dimer of alpha,alpha,alpha’alpha’-tetramethyl-m-xylylene. When isobutylidene is added to 6,6-bis(trideuteriomethyl)fulvene, the C-13 NMR spectrum of the dimer reveals three quaternary carbons in a 1:1:1 ratio, progressively shifted upfield to suggest that two dimers are formed. Saturation of the reactant solutions with dioxygen resulted in no dimer formation despite undiminished loss of reactants. A control experiment indicated that dioxygen does not affect the formation of the methylenecyclopropane from cyclohexene and isobutylidene. Reaction of 6-cyclopropyl-6-methylfulvene with isobutylidene gave dimers with intact cyclopropane rings. Addition of isobutylidene to 6-tert-butyl-6-methylfulvene resulted in a non-cyclophane dimer.