ION-MOLECULE COMPLEXES IN 1,2-ALKYL SHIFTS

The internal return of neutral leaving groups was studied in rearrange ments of polycyclic systems (2-norpinyl --> 2-norbornyl, endo- --> exo -tricyclo[5.2.1.0(2,6)]dec-8-yl, bicyclo[3.2.0]hept-2-yl --> 7-norborn yl, and 4-protoadamantyl --> 2-adamantyl). Acid catalysis was applied to O-18-labeled alcohols in aqueous organic solvents, to alcohols in m ethanol, and to ethers R-O-R' in alcohols R''-OH. The leaving group wa s found to attack the migration origin in competition with solvent mol ecules, Return:exchange ratios were obtained from product distribution s, either directly or by kinetic simulation (in cases of partial excha nge prior to rearrangement). If departure and return of the leaving gr oup occur on the same side of the carbon framework, return:exchange ra tios ranging from 1 to 11.5 were observed. Less internal return was fo und for bridged than for open carbocations, Migration of the departing molecule to the opposite face (exo reversible arrow endo) or to a bet a carbon is a minor process (return:exchange 0.1), in accordance with previous reports on inverting displacements and allylic 1,3 shifts. Th ese data are rationalized in terms of short-lived ion-molecule (ion-di pole) complexes whose collapse competes with ligand exchange.