Chemistry and Stereochemistry of Benzyl-Benzyl Interactions in MH+ Ions of Dibenzyl Esters upon Chemical Ionization and Collision-induced Dissociation Conditions

Abstract

Isobutane chemical ionization mass spectra of dibenzyl esters of a wide variety of aliphatic, olefinic, alicyclic and aromatic dicarboxylic acids exhibit abundant m/z 181 C 14 H + 13 ions, indicating a highly general rearrangement process involving the formation of a new bond between the two benzyl groups. An extensive collision-induced dissociation and deuterium labeling study suggested that these ions are an almost equimolar mixture of isomeric α-o-tolylbenzyl, α-p-tolylhenzyJ and p-benzylbenzyl cation structures, and this composition is identical for all the diesters examined. This structural assignment of the C 14 H 13 + ions suggests a mechanistic pathway for their generation, based on the formation of the new bond between the benzyl methylene group of the protonated benzoxycarb-onyl and the phenyl ring of the other ester moiety via π- (and/or ion-neutral) and α-complexe. Stereoisomeric diesters show an unusual steric effect: trans-isomers give rise to much more abundant C 14 H + 13 ions than the cis counterparts. This behavior is explained by stabilized proton-bridged structures of the MH+ ions of the cis-isomers.