Highly specific fragmentation processes of isomeric mixed esters of phenylsuccinic acid under electron impact

Isomeric mixed dialkyl phenylsuccinates, PhCH(COOR)CH₂COOR′, undergo a highly specific elimination of ROH under electron impact. A deuterium-labelling study showed that the hydrogen atom from the benzylic position 2 is abstracted in this process. These results suggest the occurrence of a ‘hidden’ hydrogen migration of the benzylic hydrogen atom to the carbonylic oxygen of the remote ester group, followed by the elimination of ROH from the adjacent ester group with the involvement of that hydrogen. Alkoxyl group migrations resulting in the formation of [PhCH?OR]⁺ and [PhCH?OR′]⁺ ions are less specific, although the migration of the remote R′O˙ is significantly preferred in all the pairs of isomers examined. Mechanisms are suggested for the formation of the two ions.