FRIEDEL-CRAFTS ALKYLATION OF AROMATICS WITH 1-CHLORONORBORNANE, 3-HALONORADAMANTANE, AND FLUOROCUBANE VIA THEIR REACTIVE SP3-HYBRIDIZED BRIDGEHEAD CARBOCATIONS

1-Chloronorbornane, 3-chloronoradamantane, and 3-bromonoradamantane, and fluorocubane, in the presence of aluminum chloride or boron trifluoride, respectively, readily alkylate benzene and substituted benzenes at or below room temperature. As back-side S(N)2-type displacement at bridgehead positions is not possible, the reported new Friedel-Crafts alkylations must involve strongly polarized bridgehead halide-aluminum chloride (or boron trifluoride) complexes in equilibrium with their energetic, reactive carbocations in which the empty orbital is of sp3 nature since the strained bridgehead centers can not flatten out. No long-lived 1-norbornyl, 3-noradamantyl, or cubyl cation can be observed in antimony pentafluoride containing superacid systems under stable ion conditions. 1-Halonorbornanes give the rearranged, sigma-delocalized 2-norbornyl cation whereas fluorocubane decomposes in the system. The difference between ''stable'' and ''reactive'' carbocation intermediates is discussed.