RESUMO
We describe the synthesis of two tetrachloroindate ionic liquids used as probes to study the involvement of NHCs (N-heterocyclic carbenes) in the distillation of imidazolium derivatives. Atmospheric-pressure chemical ionization mass spectrometry (APCI-MS), electrospray ionization mass spectrometry (ESI-MS), atmospheric-pressure thermal desorption ion mass spectrometry (APTDI-MS) and laser-induced acoustic desorption (LIAD) were used to depict the possibility of the involvement of NHCs during the distillation process. Each type of imidazolium derivative showed a particular mechanism of distillation, pointing firmly to the dependence of both the cation and the anion natures to distil as ion pairs or NHCs. Ionic liquid 1-n-butyl-3-methylimidazolium tetrachloroindate (1a) exhibited a preference to distil as ion pairs, whereas 3,3'-(ethane-1,2-diyl)bis(1-methyimidazolium)bis-tetrachloroindate (1b) may react with the Lewis acid anion, affording a bidentate NHC complex to distil. Thermodynamics, quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses of the ionic liquid 1a were also conducted and helped understand the preference for ion pairs instead of NHCs. The performed theoretical calculations did not forwent the possibility of NHC formation; however, they clearly indicated the high stability of the anions (Lewis acids in nature) and also indicated that the possible reaction between NHC and the anion is not favoured. The calculated thermodynamic values were in accordance with the features observed by MS and indicated ion pairs as the feasible species for the distillation of imidazolium-based ionic liquids.
RESUMO
Recent studies have reported surprising results related to the rearrangement of carbenes under ultracold conditions, making use of sophisticated models of quantum tunnelling to explain the observed phenomena. Here, we demonstrate that a methylhydroxycarbene (H3C-C-OH) rearrangement is possible by making changes in molecularity (i.e., through cooperative effects), owing to intermolecular hydrogen bond/H-transfer. The model used for accomplishing these changes in molecularity suggests the occurrence of two chemical species during the rearrangement and preferential formation of acetaldehyde. We propose an alternative interpretation for the methylhydroxycarbene rearrangement, as well as for a bimolecular isomerization mechanism for acetaldehyde formation with an activation barrier, Ea, of +0.25 kcal mol(-1), relative to 1a' (−8.06 kcal mol(-1) relative to 1a); this barrier is lower than that required by H-tunnelling as proposed by Schreiner et al. We also note that the mechanism for obtaining vinyl alcohol leads to the simultaneous formation of acetaldehyde through an Ea of +13.53 kcal mol(-1), relative to 1a (+0.93 kcal mol(-1) relative to 1b), again confirming the predominant presence of acetaldehyde.
RESUMO
The mechanism of the Ugi four-component reaction has been investigated by electrospray ionization (tandem) mass spectrometry using charge-tagged reagents (a carboxylic acid or an amine) to favour detection. Key intermediates were transferred directly via ESI(+) from the reaction solution to the gas phase and characterized by MS measurements and MS/MS collision induced dissociation. The Mumm rearrangement (final step) was also investigated by both travelling wave ion mobility mass spectrometry and DFT calculations. The data seem to consolidate the amazingly selective mechanism of this intricate four-component reaction.