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, a nd fluorocubane, in the presence of aluminum chloride or boron trifluo ride, 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 e nergetic, reactive carbocations in which the empty orbital is of sp3 n ature since the strained bridgehead centers can not flatten out. No lo ng-lived 1-norbornyl, 3-noradamantyl, or cubyl cation can be observed in antimony pentafluoride containing superacid systems under stable io n conditions. 1-Halonorbornanes give the rearranged, sigma-delocalized 2-norbornyl cation whereas fluorocubane decomposes in the system. The difference between ''stable'' and ''reactive'' carbocation intermedia tes is discussed.