Generation of regiospecific carbanions under electrospray ionisation conditions and their selectivity in ion-molecule reactions with CO₂

Abstract

Regiospecific formation of carbanions from a set of geometrical (cis and trans isomers) and five different sets of positional isomers (ortho, meta and para isomers) of aromatic carboxylic acids is reported under negative electrospray ionisation conditions by decarboxylation of the carboxylate anions. The structures of decarboxylated anions, [(M-H)-CO₂]^-, are studied by ion-molecule reactions with carbon dioxide in the collision cell of a triple quadrupole mass spectrometer. The [(M-H)-CO₂]^- ions generated from the trans and meta/para isomers react with CO₂ to produce product ions corresponding to the addition of one CO₂, which confirms the survival of the [(M-H)-CO₂]^- ions as carbanions. On the other hand, the [(M-H)-CO₂]^- ions generated from cis and ortho isomers failed to react with CO₂ due to rapid isomerisation of the initially generated carbanion to a aromatic carboxylate/oxide anion, which is unreactive with CO₂, through a facile intramolecular proton transfer from the proton-containing substituent to the carbanion site. When the experiments were performed at high desolvation temperatures (300°C), instead of 100°C, the relative abundance of [(M-H)-CO₂]^- ions and the corresponding CO₂ adduct in ion-molecule reaction experiments increased significantly due to minimisation of proton exchange. Quantum chemical calculations on some of the generated isomeric carbanions and their isomerised products due to proton transfer support the selective stability of carbanions.