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Loss of benzene to generate an enolate anion by a site‐specific double‐hydrogen transfer during CID fragmentation of o ‐alkyl ethers of ortho ‐hydroxybenzoic acids
Author(s) -
Attygalle Athula B.,
Bialecki Jason B.,
Nishshanka Upul,
Weisbecker Carl S.,
Ruzicka Josef
Publication year - 2008
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/jms.1399
Subject(s) - chemistry , kinetic isotope effect , hydrogen atom , deuterium , fragmentation (computing) , molecule , ion , alkyl , dissociation (chemistry) , benzene , photochemistry , hydrogen , medicinal chemistry , stereochemistry , organic chemistry , quantum mechanics , computer science , operating system , physics
Collision‐induced dissociation of anions derived from ortho ‐alkyloxybenzoic acids provides a facile way of producing gaseous enolate anions. The alkyloxyphenyl anion produced after an initial loss of CO 2 undergoes elimination of a benzene molecule by a double‐hydrogen transfer mechanism, unique to the ortho isomer, to form an enolate anion. Deuterium labeling studies confirmed that the two hydrogen atoms transferred in the benzene loss originate from positions 1 and 2 of the alkyl chain. An initial transfer of a hydrogen atom from the C‐1 position forms a phenyl anion and a carbonyl compound, both of which remain closely associated as an ion/neutral complex. The complex breaks either directly to give the phenyl anion by eliminating the neutral carbonyl compound, or to form an enolate anion by transferring a hydrogen atom from the C‐2 position and eliminating a benzene molecule in the process. The pronounced primary kinetic isotope effect observed when a deuterium atom is transferred from the C‐1 position, compared to the weak effect seen for the transfer from the C‐2 position, indicates that the first transfer is the rate determining step. Quantum mechanical calculations showed that the neutral loss of benzene is a thermodynamically favorable process. Under the conditions used, only the spectra from ortho isomers showed peaks at m / z 77 for the phenyl anion and m / z 93 for the phenoxyl anion, in addition to that for the ortho‐specific enolate anion. Under high collision energy, the ortho isomers also produce a peak at m / z 137 for an alkene loss. The spectra of meta and para compounds show a peak at m / z 92 for the distonic anion produced by the homolysis of the OC bond. Moreover, a small peak at m / z 136 for a distonic anion originating from an alkyl radical loss allows the differentiation of para compounds from meta isomers. Copyright © 2008 John Wiley & Sons, Ltd.