Facile Derivatization of a “Chemo-active” NHC Incorporating an Enolate Backbone and Relevant Tuning of Its Electronic Properties

Abstract

The present report discloses a modular synthetic route to a new type of anionic N-heterocyclic carbene ligand incorporating an enolate group as a reactive backbone component of its heterocyclic framework. The presence of such a reactive unit facilitates further tailoring of the ligand, even after its complexation to transition metals, with concomitant tuning of the electronic donor properties of the carbene center. Simple acylation of the formamidine Ar-NH-CH═NAr (1a,b) (a: Ar = mesityl (Mes); b: Ar = 2,6-diisopropylphenyl (DIPP)) by chloroacetyl chloride gives an acylated formamidine (2a,b), which undergoes a thermally induced cyclization to afford the 4-hydroxyimidazolium salt 3a,b. Single deprotonation by NEt3 gives the zwitterionic imidazolium-4-olate 4, whereas double deprotonation by 2 equiv of LiHMDS gives the imidazolin-2-ylidene-4-olate ligand 5−, which reacts with 0.5 equiv of [RhCl(1,5-COD)]2 to give, after acidification with HCl, the complex [(5H)RhCl(COD)] (7), in which the “acidified” carben...