Comprehensive Basicity Scales for N-Heterocyclic Carbenes in DMSO: Implications on the Stabilities of N-Heterocyclic Carbene and CO2 Adducts.

A very broad acidity scale (≈40 pK units) for about 400 N-heterocyclic carbene precursors (NHCPs) with various backbones and electronic features, including imidazolylidenes, 1,2,4-triazolylidenes, cyclic diaminocarbenes (CDACs), diamidocarbenes (DACs), thiazolylidenes, cyclic (alkyl)(amino)carbenes (CAACs) and mesoionic carbenes (MICs), was established in DMSO by a well examined computational method. Varying the backbone structure or flanking N-substituents can have different extent of acidifying effects, depending on both the nature and number of substituent(s). The Gibbs energies (ΔGr s) for the reactions between the corresponding NHCs and CO2 were also calculated. There is a good linear correlation between the pKa s of most NHCPs and ΔGr s, suggesting that a greater basicity of NHC leads to a more stable NHC-CO2 adduct. Interestingly, the nearby asymmetric environment has virtually no differential effect on the acidities of the chiral NHCP enantiomers, but has a pronounced effect on the ΔGr values.