The gas-phase reactions of localized and delocalized carbanions with aryl halides: Competition between attack on the π-system and the periphery

• Gas-phase examples of unusual SN2 processes on bromine centers.
• Unusual leaving group patterns in gas-phase nucleophilic aromatic substitution reactions.
• Evidence of non-statistical behavior in reactions of relatively large ions and substrates.

The gas-phase reactions of a series of polyfluorobromobenzenes with stabilized carbanions (substituted benzyl and phenyl anions) were studied in an ion trap mass spectrometer. The carbanions were formed via collision-induced decarboxylation of appropriate carboxylate precursors that were generated in an electrospray ionization (ESI) source. The systems show two types of reactivity. Attack on the π-system leads to nucleophilic aromatic substitution reactions (SNAr) with the loss of bromide or fluoride anions. Alternatively, attack on the periphery leads to proton transfer or nucleophilic attack on bromine (SN2@Br), each of which leads to a new phenyl anion. The two types of nucleophiles present different reactivity. The phenyl anion nucleophiles mainly undergo proton transfer reactions, presumably due to their more localized charge as well as destabilizing steric interactions in the SNAr transition states. The benzyl anions offer more diverse reactivity and greater competition among the entire set of possible reaction pathways. Thermodynamic factors have some control over the measured rate constants and branching ratios, but the preference for bromide vs. fluoride loss in the SNAr processes does not show a discernable correlation with structural features in the substrates. In addition, there is evidence suggesting non-statistical behavior in the reactions.

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