Experimental Assessment of the π-Acidity of Anionic and Neutral Malonate-Derived N-Heterocyclic Carbenes

The pi-acidity of anionic malonate-derived N-heterocyclic carbenes (maloNHC)(-), [1](-), has been determined experimentally by recording the Se-77 NMR chemical shifts of the corresponding maloNHC center dot Se adducts. Though anionic in nature, [1](-) displays a significant pi-accepting character, which can additionally be fine-tuned depending on the nature of the countercation. The pi-acidity of neutral maloNHC-derived carbenes was also assessed by neutralization of the malonate backbone, and it was shown to increase upon O-methylation, giving rise to the (amino)(acrylamido)carbene 1(Me) and to increase even more dramatically upon protonation of the central carbon atom, which formally generates the diamidocarbene 1(H).