Role of hydrogen migration in the mechanism of acetic acid elimination from MH+ ions of acetates on chemical ionization and collision-induced dissociation

The elimination of acetic acid from the MH+ ions of acetates of stereoisomeric 2-methyl-1-cyclohexanols and 1-hydroxy-trans-decalins exhibits a significant degree of stereospecificity under isobutane chemical ionization and collision-induced dissociation (CID) conditions, resulting in more abundant [MH - AcOH](+) ions in the cis-isomers 4c and 5tc than in their trans-counterparts 4t and 5tt, These findings suggest the involvement of a 1,2-hydride shift from the beta- to the alpha-position in the course of the acetic acid elimination from the MH+ ions of the above cis-acetates, resulting in tertiary carbocation structures. The proposed mechanism of the elimination is supported by a considerable deuterium isotope effect detected in beta-deuterium-labeled cis-2-methyl-1-acetoxycyclohexane and by a CID study of the structures of the [MH - AcOH](+) ions obtained from cis- and trans-1,2-diacetoxycyclohexanes. Copyright (C) 1999 John Wiley & Sons, Ltd.