Anionic N-Heterocyclic Carbenes from Mesoionic Imidazolium-4-pyrrolides: The Influence of Substituents, Solvents, and Charge on their Se NMR Chemical Shifts.

Mesoionic compounds are the starting material for the synthesis of unique anionic N-heterocyclic carbenes. Herein, mesoionic imidazolium pyrrolides synthesized from pyrrole-2-carbaldehyde via various -alkyl-4-pyrroyl-imidazoles are described. These were converted into nine new 4-(pyrrol-2-yl)-substituted imidazolium salts and transformed into the mesoionic title compounds using an anion exchange resin. The DFT-calculated (B3LYP/6-311++G**) CREF values indicate a great potential for the formation of anionic N-heterocyclic carbenes by deprotonation, which were generated and reacted with selenium to obtain selenoureas. The Se NMR shifts investigated under systematic variation of conditions are dependent on the substitution pattern (Δδ = 133 ppm) and the steric demand of the substituents. Solvent dependencies of the Se NMR shifts were investigated applying toluene-, THF-, CDCl, CDCl, pyridine-, acetone-, DMSO-, CDCN, AcOD, and MeOD. The influences of the referencing method on the Se shifts using external or internal MeSe or PhSe and solvent can add up to Δδ = ca. 80 ppm. In addition, we observed a temperature dependence of both the selenoureas and the reference reagent PhSe as well as a Se shift difference of the analyte caused by interaction with internally added PhSe. The negative charge of deprotonated selenoureas shifts the values by an additional -20 ppm.