Showing papers by "Asher Mandelbaum published in 1991"
TL;DR: In this paper, a deuterium-labelling study showed that the hydrogen atom from the benzylic position 2 is abstracted in this process, suggesting the occurrence of a hidden hydrogen migration of the benzelic hydrogen atom to the carbonylic oxygen of the remote ester group, followed by the elimination of ROH from the adjacent ester groups with the involvement of that hydrogen.
Abstract: 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 [PhCHOR]+ and [PhCHOR′]+ 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.
6 citations
TL;DR: The electron impact mass spectra of isomeric methyl ethym and ethyl methyl halosuccinates (X=Cl and Br) are surprisingly different as mentioned in this paper, and a low-energy collision-induced dissociation study of deuterium-labeled analogues of the former isomers indicates that the [M-X] + ions are mixtures of protonated methyl ethyl maleate (major component, >85%) and fumarate, and the loss of the halogen atom is a multi-step process including at least two specific hydrogen transfers.
Abstract: The electron impact mass spectra of isomeric methyl ethym and ethyl methyl halosuccinates (X=Cl and Br) are surprisingly different. Only the isomers with the ethyl group remote from the halogen give rise to [M―X] + ions. A low-energy collision-induced dissociation study of deuterium-labelled analogues of the former isomers indicates that the [M―X] + ions are mixtures of protonated methyl ethyl maleate (major component, >85%) and fumarate, and the loss of the halogen atom is a multi-step process including at least two specific hydrogen transfers. Migration of a β-hydrogen atom to the carbonyl oxygen within the ethoxycarbonyl group produces a primary radical site in a distonic intermediate which, by subsequent abstraction of a hydrogen atom from C(3), triggers the ejection of X from C(2) with concomitant double fond formation. Whereas in the other isomer an [M-X] + ion is absent or negligible, a characteristic double loss of C 2 H 4 and CO 2 is observed
5 citations