Intracomplex CH3OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

Abstract

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 temperature range. The starting oxonium intermediates were generated in the gas phase by the reaction of (CH3)2Cl+ ions, formed by stationary γ-radiolysis of bulk CH3Cl, on the corresponding optically active alcohols. The rate constant 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 temperature range. Similar differences were observed 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 experimental and theoretical evidence, these results are consistent with intramolecular racemization and regioisomerization processes involving the intermediacy of two distinct hydrogen-bonded complexes, wherein the CH3OH molecule 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 (IS), previously measured in the gas phase under similar experimental conditions. The comparison reveals that gas-phase racemization and regioisomerization of O-protonated (S)-trans-4-hexen-3-ol (1S‘) (AOHH2O) 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 (1S‘) and (R)-trans-3-hexen-2-ol (IIR‘) (AOH = CH3OH) are placed later along the reaction coordinate and are characterized by a strong coordination of the moving CH3OH molecule with the hydrogens of the allylic moiety.