Decarboxylation of carboxylate ions in the gas phase
provides a
useful window into the chemistry displayed by these reactive carbanion
intermediates. Here, we explore the species generated by decarboxylation
of two benzoate derivatives: 2-formylbenzoate (2FBA) and 2-benzoylbenzoate
(2BBA). The nascent product anions are transferred to a cryogenic
ion trap where they are cooled to ∼15 K and analyzed by their
pattern of vibrational bands obtained with IR photodissociation spectroscopy
of weakly bound H2 molecules. The structures of the quenched
species are then determined by comparison of these spectra with those
predicted by electronic structure calculations for local minima on
the potential energy surface. The 2-phenide carbanion generated by
decarboxylation of 2FBA occurs in two isomeric forms that differ in
the orientation of the formyl group, both of which yield a very large
(∼110 cm–1) redshift in the stretching frequency
of the H2 molecule attached to the anionic carbon center.
Although calculated to be a local minimum, the analogous 2-phenide
species could not be isolated upon decarboxylation of 2BBA. Rather,
the anionic product adopts a ring-closed structure, indicating efficient
nucleophilic attack on the pendant phenyl group by the nascent phenide.
The barrier for ring closing is evaluated with electronic structure
calculations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.