Hindered inversion of chiral ion-dipole pairs.
O-Protonated S-(-)-1-phenyl-1-methoxyethane (IS) has been generated in the gas phase by CH3(2)Cl+ methylation of S-(-)-1-phenylethanol (1S). Detailed information on the reorganization dynamics of the intimate ion-dipole pair (IIS), arising from IS by C-O bond dissociation, is inferred from the kinetic study of the intramolecular inversion of configuration of IS vs its dissociation to alpha-methylbenzyl cation (III) and CH(3)OH. The behavior of IIS in the gas phase is compared to that observed in aqueous solutions, where the loss of optical activity of IS is prevented by exchange of the leaving CH3OH with the solvent shell. Hindered inversion of IS in solution is attributed to the operation of attractive interactions between the moving CH3OH moiety and the solvent cage which inhibit internal return in the intimate ion-dipole pair IIS. Similar interactions do not operate in the solvolysis of 18O-labeled 1S in aqueous acids, whose loss of optical activity efficiently competes with exchange of the leaving H2(18)O with the solvent shell.
Filippi
A
A
Contribution from the Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Università degli Studi di Roma "La Sapienza", P.le A. Moro 5, I-00185 Roma, Italy.
Gasparrini
F
F
Speranza
M
M