Anionic N-Heterocyclic Carbenes from Mesoionic Imidazolium-4-pyrrolides: The Influence of Substituents, Solvents, and Charge on their ⁷⁷Se NMR Chemical Shifts

Abstract

Mesoionic compounds are the starting material for the synthesis of unique anionic N-heterocyclic carbenes. Herein, mesoionic imidazolium pyrrolides synthesized from pyrrole-2carbaldehyde via various N-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 77 Se NMR shifts investigated under systematic variation of conditions are dependent on the substitution pattern (Δδ Se = 133 ppm) and the steric demand of the substituents. Solvent dependencies of the 77 Se NMR shifts were investigated applying toluene-d 8 , THF-d 8 , CDCl 3 , CD 2 Cl 2 , pyridine-d 5 , acetone-d 6 , DMSO-d 6 , CD 3 CN, AcOD, and MeOD. The influences of the referencing method on the 77 Se shifts using external or internal Me 2 Se or Ph 2 Se 2 and solvent can add up to Δδ Se = ca. 80 ppm. In addition, we observed a temperature dependence of both the selenoureas and the reference reagent Ph 2 Se 2 as well as a 77 Se shift difference of the analyte caused by interaction with internally added Ph 2 Se 2 . The negative charge of deprotonated selenoureas shifts the values by an additional −20 ppm.