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J. Chem. Soc., Perkin Trans. 2, 1997 1221 Direct evidence for anchimeric assistance in alcohol elimination from
 

Summary: J. Chem. Soc., Perkin Trans. 2, 1997 1221
Direct evidence for anchimeric assistance in alcohol elimination from
gas-phase MH ions of 1,4-dialkoxycyclohexanes under chemical
ionisation. Experiment and theory
Ronit Shvily, Thomas Müller, Yitzhak Apeloig* and Asher Mandelbaum*
Department of Chemistry and the Minerva Center for Computational Quantum Chemistry,
Technion ­ Israel Institute of Technology, 32000 Haifa, Israel
trans-1,4-Dialkoxycyclohexanes afford very abundant [MH ROH] ions upon chemical ionisation (CI),
in contrast to the cis-isomers, suggesting anchimeric assistance in the alcohol elimination from the MH
ions of the trans-diethers. Collision induced dissociation (CID) measurements of the [MH ROH] ions,
obtained from various suitably deuterium labelled stereoisomeric 1-ethoxy-4-methoxycyclohexanes,
indicate formation of symmetrical bicyclic ethyl and methyl oxonium ions by an anchimerically assisted
alcohol elimination from the trans-diethers. On the other hand these measurements suggest that the cis-
isomers afford isomeric monocyclic O-protonated 4-alkoxycyclohexene cations, in which the hydrogens at
positions 2 and 3 (as well as those at positions 5 and 6, and 1 and 4) are not equivalent. The two results,
namely the symmetrical bicyclic structure and the high abundance of the [MH ROH] ions in the CI
mass spectra of the trans-diethers, in contrast to the non-symmetrical monocyclic structure and low
abundance of these ions in the cis-isomers, are suggested to be direct evidence for anchimeric assistance in
a gas-phase ion dissociation process. Ab initio calculations at the MP3/6-31G*//6-31G* level support the
anchimerically assisted elimination mechanism observed in trans-1-ethoxy-4-methoxycyclohexane, but

  

Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology

 

Collections: Chemistry