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 degrees 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 [v(CO)] scale from [RhCl(3)(CO)(2)] (6) The average value of v(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.