Proton transfer via strained transition states in the elimination of alcohols from MH+ ions of stereoisomeric diethers and hydroxy esters upon chemical ionization and collision‐induced dissociation

Abstract

AbstractThe isobutane chemical ionization (CI) mass spectra of cis‐ and trans‐1,4‐di(alkoxymethyl)cyclohexanes, with a tertiary alkoxy group ROH and a primary group R′OH, are identical, and they exhibit exclusive elimination of the alcohol ROH originating from the tertiary alkoxyl. The high abundance of the [MH − ROH]+ ion and the absence of [MH − R′OH]+ and MH+ ions is unexpected in the case of trans‐diethers, and it suggests proton transfer from the primary alkoxyl OR′ to the tertiary OR group prior to the elimination of ROH, despite the large distance between them in the trans configuration. Various isomerization pathways were explored in order to account for the similar behavior of the cis‐ and trans‐isomers upon chemical ionization. The results show that the hydrogens at positions 1 and 4 are not involved in the elimination of ROH. Methyl substitution at positions 1 and 4 leads to the competitive elimination of ROH and R′OH, indicating suppression of the proton transfer. Methyl substitution at position 1 adjacent to the primary alkoxy group has a minor effect on the chemical ionization behavior of the diethers. On the other hand, methyl substitution at position 4, adjacent to the tertiary alkoxy group, suppresses the proton transfer, (i.e. both [MH − ROH]+ and [MH − R′OH]+ are abundant in the CI mass spectra of the trans‐isomers), indicating an effect of steric hindrance. The results suggest that direct proton transfer via a strained proton‐bound transition‐state occurs, and methyl substitution at positions 1 and 4 can affect the relative stability of the transition‐state structures involved. Similar behavior has been observed in the CI and collision‐induced dissociation spectra of 1,4‐hydroxy ester cyclohexanes. Copyright © 2001 John Wiley & Sons, Ltd.