Direct evidence for anchimeric assistance in alcohol elimination from gas-phase MH+ ions of 1,4-dialkoxycyclohexanes under chemical ionisation. Experiment and theory

Abstract

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 also show that the energy difference between the anchimerically assisted and non-assisted elimination mechanisms is small (ca. 2–3 kcal mol^-1) (1 cal = 4.184 J).