The use of a charge-tagged acrylate derivative bearing an imidazolium tag to study the Morita-Baylis-Hillman reaction via ESI-MS(/MS) monitoring and the effect of such tag (imidazolium cations and ion pairs) over TSs is described. The ionic nature of the substrate was meant to facilitate ESI transfer to the gas phase for direct mass spectrometric analysis. The detection and characterization of charged intermediates has suggested major reaction pathways. DFT calculations considering the effect of a polar and protic solvent (methanol), of a polar and aprotic solvent (acetonitrile), and of no solvent (gas phase) were used to predict possible TSs through a common accepted intermediate. The controversial proton transfer step, which may proceed via Aggarwal's or McQuade's proposals, was evaluated. Calculations predicted the formation of electrostatic intermediate complexes with both the cation and anion when charge-tagged reagents are used. These complexes contribute to the positive ionic liquid effect, and based on the formation of these unique complexes, a rationale for the ionic liquid effect is proposed. These complexes also pointed to a plausible explanation for the positive ionic liquid effect observed in several reactions that are difficult to be carried out in organic solvents but have shown a beneficial effect when performed in ionic liquids.

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Herein we report on the sampling and characterization via electrospray ionization (tandem) mass spectrometry of free, longlived N-heterocyclic carbenes (NHC) bearing negative-charge tags. To facilitate electrospray ''ion fishing'' via electrostatic ejection directly to the gas phase, negative-charge tagged NHC were formed in imidazolium-based ionic liquid solutions via double deprotonation of imidazolium cations bearing acid side groups, viz. CH 2 CO 2 H or (CH 2) 3 SO 3 H. Via ESI-MS/MS experiments, the gaseous N-heterocyclic carbenes were found to display structurally diagnostic dissociations and bimolecular reactions. In perfect parallel to solution chemistry, the gaseous negative-charge tagged NHC were found to react promptly with CO 2 by carboxylation to form negative-charge tagged imidazolium carboxylates. Neutral carbenes were inaccessible for mass spectrometry, but the charge tag strategy opens many new possibilities to explore the intrinsic chemistry of these key but elusive species.

Negatively charge-tagged N-heterocyclic carbenes have been formed in solution via deprotonation of imidazolium ions bearing acid side groups and transferred to the gas phase via ESI(-)-MS. The structure of the putative and apparently stable gaseous carbenes formed in such conditions were then probed via reactions with carbon dioxide using a triple quadrupole mass spectrometer particularly optimized for ion/molecule reactions of ESI-generated ions. Complete conversion to imidazolium carboxylates was achieved, which seems to demonstrate the efficiency of the transfer, the gas-phase stability, and the long-lived nature of these unprecedented charge-tagged carbenes and their predominance in the ionic population. Comprehensive studies on the intrinsic reactivity of N-heterocyclic carbenes with silent charge tags are therefore possible. Graphical Abstract ᅟ.

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