Collision-induced dissociation study of stereospecific elimination processes of stereoisomeric phenylcyclohexanols and their derivatives upon electron ionization

Abstract

The electron ionization (EI)-induced elimination of water, methanol and acetic acid from the M+• ions of stereoisomeric 2-, 3- and 4-phenylcyclohexanols and their methyl ethers and acetates, respectively, was studied by deuteration and collision-induced dissociation (CID) techniques. The highly stereospecific elimination processes in trans-3- and -4-phenylcyclohexanols and in the corresponding methyl ethers and acetates take place with the involvement of the benzylic hydrogen atom, suggesting syn-1,3- and syn-1,4-elimination via cyclic transition states. The elimination processes from the cis-alcohols and their methyl ethers occur mainly after ring opening, and result in the formation of mixtures of product ions. The elimination processes are non-stereospecific in the stereoisomeric 2-phenyl-substituted systems, and are preceded by ring cleavage in both cis- and trans-isomers, resulting in mixtures of cyclic and acyclic product ions. All cis-2-, -3- and -4-phenylcyclohexyl acetates undergo elimination by a McLafferty-type mechanism with the abstraction of a hydrogen atom from positions 2 and/or 6. An interesting outcome of this work is that the majority of the gas-phase isomeric hydrocarbon phenyl-C6H9 radical cations, formed either by EI-induced fragmentation of stereoisomeric phenylcyclohexyl derivatives or by EI of a variety of phenyl-C6H9 isomers, retain their structural integrities. On the other hand, the CID spectra of the even-electron phenyl-C6H10+ ions produced by chemical ionization (CI)-induced fragmentation from isomeric phenylcyclohexanols and their methyl ethers and acetates are similar, indicating loss of structural information under CI in this system. © 1998 John Wiley & Sons, Ltd.