Wagner-Meerwein Rearrangements in the Gas Phase: Deuterium Isotope Effects on Acid-Induced Dissociation of Optically Active Phenylpropanols.

The D isotope effects on the kinetics of H2O loss from O-protonated (S)-PhCH2CHMeOH (1sH+) and (S)-PhCHMeCH2OH (2sH+) were studied in the gas phase at 750 Torr and 25. The measured primary and secondary .alpha.-D1, .beta.-D1, and .beta.'-D3 kinetic isotope effects provide conclusive evidence on the detailed mechanism of the gas-phase dissocn. of 1sH+ and 2sH+; this involves competing anchimeric assistance from all the groups adjacent to the reaction center (C.alpha.). Their anal., combined with that of the relevant activation parameters reported in the preceding paper, is consistent with: (i) a transition state (TS) structure involving Ph-group participation, which is placed rather early along the reaction coordinate and in which the partial C.alpha.-OH2+ bond cleavage is coupled with a weak Ph-C.alpha. interaction; (ii) a tight TS structure for .beta.-H participation, in which a limited C.alpha.-OH2+ bond cleavage is fully outweighed by the advanced H(D)-C.alpha. bonding;, and (iii) a borderline TS structure for .beta.-Me group participation in 2sH+, in which intense Me-C.alpha. bonding is coupled with pronounced C.alpha.-OH2+ bond elongation. The values of the primary and secondary .alpha.-D1, .beta.-D1, and .beta.'-D3 kinetic isotope effects are discussed and compared with those of related gas-phase and solvolysis reactions.