Assessing the Proton Affinities of N,N′-Diamidocarbenes

The gas-phase proton affinities (PAs) of a series of novel diamidocarbenes (DACs) were assessed and compared to various imidazolylidene-based N-heterocyclic carbenes (NHCs) through experimental and computational methods. Apart from a perfluorinated-phenyl derivative (PA = 233 kcal/mol), the calculated and measured PM for a range of DACs (256-261 kcal/mol) were comparable to those of the NHCs (260-266 kcal/mol). Proton transfer from the protonated carbene to various reference bases, as observed by mass spectrometry, was inhibited by steric bulk and precluded the direct measurement of the PA for the known DACs, N,N'-dimesityl-4,6-diketo-5,5-dimethylpyrimidin-2-ylidene and N,N'-diisopropylphenyl-4,6-diketo-5,5-dimethylpyrimidin-2-ylidene. However, DACs featuring less hindered N-aryl substituents facilitated proton transfer, and the measured PA values were found to be consistent with density functional theory calculations (B3LYP/6-31+G(d)). Notably, the PM of the DACs studied were similar to those of the NHCs, indicating that the former retain many of the nucleophilic characteristics intrinsic to their parent diaminocarbenes and that the observed differences in chemical reactivity may be primarily attributed to an enhanced electrophilicity.