Reprogramming of a Malonic N‐Heterocyclic Carbene: A Simple Backbone Modification with Dramatic Consequences on the Ligand's Donor Properties

Reaction of N , N′ ‐dimesitylformamidine with dimethylmalonyl dichloride in dichloromethane in the presence of an excess of triethylamine gives the 2‐chloro‐4,5‐dioxohexahydropyrimidine 1 . The corresponding diamidocarbene 3 is generated in situ by further deprotonation with KHMDS at –40 °C and identified by trapping with S 8 to give the fully characterized (including X‐ray structure) sulfur adduct 4 . It also reacts with [RhCl(cod)] 2 to yield the NHC complex [RhCl( 3 )(cod)] ( 5 ) (characterized also by X‐ray structure). The donor properties of 3 were evaluated against the established IR [ ν (CO)] scale from [RhCl( 3 )(CO) 2 ] ( 6 ). The average value of ν (CO) = 2045 cm –1 indicates that the diamidocarbene 3 is much less nucleophilic than structurally relevant six‐membered NHCs including the anionic diaminocarbenes previously reported in our group.