Facile Derivatization of a “Chemo-active” NHC Incorporating an Enolate Backbone and Relevant Tuning of Its Electronic Properties
Résumé
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” carbene ligand noted “5H” adopts the keto form. By contrast, treatment of 4 with electrophilic reagents E-X (E1 = tBuCO; E2 = Me; E3 = Tf) results in the O-functionalized imidazolium derivatives [4E1−3]+X−, which can be subsequently complexed under the form of the carbene 5E1−3 to give [(5E1−3)RhCl(COD)] (7E1−3). Alternatively, the carbene ligand 5 can be post-functionalized from its complex [(5H)RhCl(COD)] (7) by a simple sequence involving (i) base-induced deprotonation and (ii) addition of E-X, exemplified by the generation of [(5E4−5)RhCl(COD)] (7E4−5) (with E4 = Ph2PO; E5 = tBuSiMe2 (TBDMS)). In parallel, C-functionalization of the metal-bound ligand can be achieved from 7 by deprotonation with LiHMDS, leading to [(5−)RhCl(COD)]Li(thf)n+ followed by a classical aldolization/elimination sequence leading to [(5CH2)RhCl(COD)] (8), in which the C5 carbon of the NHC bears a methylene group. Parallel direct transformations can be achieved after ligand complexation to copper, as exemplified by the generation of [(5H)CuCl] (9) or [(5E5)CuCl] (9E5) (E5 = TBDMS). The donor properties of all the above NHC ligands were evaluated from the standard complexes (NHC)RhCl(CO)2 against the classical IR ν(CO) scale. The resulting order of nucleophilicities for the modulated carbenes is 5CH2 < 5H ≪ 5E4 < 5E1 < 5E2 < 5E5 ≪ 5− . The X-ray structures analyses of 7E2 and 7E4 are reported.