INTRACOMPLEX CH3OH WALKING AROUND OPTICALLY ACTIVE 1-METHYL-3-ETHYLALLYL CATIONS. A GAS PHASE KINETIC STUDY

The kinetics and the mechanism of the racemization and regioisomerization of O-methylated (S)-trans-4-hexen-3-ol (IS') or (R)-trans-3-hexen-2-ol (IIR') have been investigated in the gas phase at 720 Torr and in the 40-120 C temp. range. The starting oxonium intermediates were generated in the gas phase by the reaction of (CH3)2Cl+ ions, formed by stationary .gamma.-radiolysis of bulk CH3Cl, on the corresponding optically active alcs. The rate const. of the gas-phase regioisomerization of IS' [(3.4-16.0) 106 s-1] was found to exceed that of its racemization [(1.9-9.8) 106 s-1] over the entire temp. range. Similar differences were obsd. for the regioisomerization [(2.9-15.0) 106 s-1] and the racemization of IIR' [(1.8-9.6) 106 s-1]. By analogy with previous exptl. and theor. evidence, these results are consistent with intramol. racemization and regioisomerization processes involving the intermediacy of two distinct hydrogen-bonded complexes, wherein the CH3OH mol. is coplanarly coordinated to the in-plane hydrogens of the 1-methyl-3-ethylallyl moiety. The activation parameters for their formation from the IS' and IIR' were evaluated and compared with those concerning the racemization and regioisomerization of O-protonated (S)-trans-4-hexen-3-ol, previously measured in the gas phase under similar exptl. conditions. The comparison reveals that gas-phase racemization and regioisomerization of O-protonated (S)-trans-4-hexen-3-ol involve transition structures located early along the reaction coordinate, whereas the transition structures involved in the rearrangement of O-methylated (S)-trans-4-hexen-3-ol and (R)-trans-3-hexen-2-ol are placed later along the reaction coordinate and are characterized by a strong coordination of the moving CH3OH mol. with the hydrogens of the allylic moiety.