DIRECT EVIDENCE FOR ANCHIMERIC ASSISTANCE IN ALCOHOL ELIMINATION FROMGAS-PHASE MH-DIALKOXYCYCLOHEXANES UNDER CHEMICAL-IONIZATION - EXPERIMENT AND THEORY( IONS OF 1,4)

trans-1,4-Dialkoxycyclohexanes afford very abundant [MH - ROH](+) ions upon chemical ionisation (CI), in contrast to the cis-isomers, sugges ting anchimeric assistance in the alcohol elimination from the MH+ ion s of the trans-diethers. Collision induced dissociation (CID) measurem ents of the [MH - ROH]+ ions, obtained from various suitably deuterium labelled stereoisomeric 1-ethoxy-4-methoxycyclohexanes, indicate form ation of symmetrical bicyclic ethyl and methyl oxonium ions by an anch imerically assisted alcohol elimination from the trans-diethers. On th e other hand these measurements suggest that the cis-isomers afford is omeric monocyclic 0-protonated 4-alkoxycyclohexene cations, in which t he 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 symme trical 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 i n the cis-isomers, are suggested to be direct evidence for anchimeric assistance in a gas-phase ion dissociation process, Ab initio calculat ions at the MP3/6-31G//6-31G* level support the anchimerically assist ed elimination mechanism observed in trans-1-ethoxy-4-methoxycyclohexa ne, but also show that the energy difference between the anchimericall y assisted and non-assisted elimination mechanisms is small (ca. 2-3 k cal mol(-1))(1 cal = 4.184 J).