Cyclic (Aryl)(Amido)Carbenes: NHCs with Triplet-like Reactivity

The synthesis and study of a library of cyclic (aryl)(amido)carbenes (CArAmCs), which represent a class of electrophilic NHCs that feature low calculated singlet-triplet gaps (ΔE_ST =19.9 kcal mol^-1 ; B3LYP/def2-TZVP) and exhibit reactivity profiles expected from triplet carbenes, are described. The electrophilic properties of the CArAmCs were quantified by analyzing their respective selenium adducts, which exhibited the largest downfield ⁷⁷ Se NMR chemical shifts (up to 1645 ppm) measured for any NHC derivative known to date, as well as their Ir carbonyl complexes, from which large Tolman electronic parameter (TEP) values (up to 2064 cm^-1 ) were ascertained. The CArAmCs were found to engage in reactions that are typically observed with triplet carbenes, including C-H insertions, [2+1] cycloadditions with alkenes as well as alkynes, and spontaneous oxidation upon exposure to oxygen.