CHIRAL IONS IN THE GAS-PHASE .1. INTRAMOLECULAR RACEMIZATION AND ISOMERIZATION OF O-PROTONATED (S)-TRANS-4-HEXEN-3-OL

The acid-catalyzed racemization and regioisomerization of (S)-trans-4- hexen-3-ol (1S) has been investigated in gaseous CH4 and C3H8 at 720 T orr and in the 40-120 degrees C temperature range. The contribution to the racemization and isomerization products by free 1-methyl-3-ethyla llyl cations, arising from unimolecular fragmentation of excited O-pro tonated (S)-trans-4-hexen-3-ol (IS), was evaluated by generating them from protonation of isomeric 2,4-hexadienes and by investigating their behavior toward (H2O)-O-18 under the same experimental conditions. Th e rate constant of the gas phase racemization of IS (1.4-21.3 x 10(6) s(-1)) was found to exceed that of its isomerization (1.0-9.9 x 10(6) s(-1)) over the entire temperature range. The experimental results, co mbined with ab initio theoretical calculations on the model [C3H5+/H2O ] system, are consistent with a gas phase intramolecular IS racemizati on and isomerization involving the intermediacy of structured ion-mole cule complexes, wherein the H2O molecule is coplanarly coordinated to the hydrogen atoms of the 1-methyl-3-ethylallyl moiety. The rate of fo rmation of these structured complexes, their relative stability, and t he dynamics of their evolution to the racemized and isomerized product s depend, in the gas phase, on the specific conformation of IS. The re levant activation parameters point to transition structures wherein a substantial fraction of the positive charge is located on the allyl mo iety. The results obtained in the present gas phase investigation conf irm previous indications about the occurrence and the role of intimate ion-molecule pairs in acid-catalyzed racemization and isomerization o f optically active alcohols, including allylic alcohols, in solution.