Stereomutation of Methoxycarbenium Ions. 2. Experimental Evidence for an Inversion Process

Article abstract of DOI:10.1021/ja00294a040

The stereomutation about the C-O partial double bond of a series of 1-methoxy- (2/3) and 1-hydroxy-3-arylallyl cations (5) has been examined experimentally in strong acid solutions and theoretically by using HF/STO-3G calculations.The interconversions of cations 2/3 were shown to be intramolecular processes in FSO3H, CF3SO3H, and FSO3H/SbF5.The rate constants of the stereomutation of these cations, which were measured by using dinamic NMR methods, were found to depend on the substituents on the aryl ring.Apart from the case where the aryl ring bears two m-CF3 groups, 2g/3g, the rate constants for stereomutation are linearly related to ?⁺ (ρ = -2.37) with ? donors accelerating the reaction.The calculations performed for 5 show that the lowest energy pathway for isomerization involves rotation rather than inversion.They also reveal that ?-donor substituents on the aryl ring lower the barrier to rotation while raising the barrier to inversion.In both cases, the calculated barriers for substituted systems show a linear correlation with ?⁺.It is concluded that the preferred pathway for stereomutation of most of these 1-hydroxy and 1-methoxyallyl cations in both strong acid solution and the gas phase involves rotation.When the aryl ring bears very strongly electron-withdrawing groups, e.g., two CF3 groups in 2g/3g, the inversion pathway would seem to be preferred.This is the first example of methoxycarbenium ion which undergoes stereomutation by inversion rather than rotation