Imidazolium Carboxylates as Versatile and Selective N-Heterocyclic Carbene Transfer Agents:  Synthesis, Mechanism, and Applications

Abstract

N,N'-Disubstituted imidazolium carboxylates, readily synthetically available, isolable, air- and water-stable reagents, efficiently transfer N-heterocyclic carbene (NHC) groups to Rh, Ir, Ru, Pt, and Pd, to give novel NHC complexes, e.g., [Pd(NHC)3OAc]OAc and [Pt(NHC)3Cl]Cl (NHC = 1,3-dimethyl imidazol-2-ylidene). The NHC esters are also effective. Tuning the reaction conditions for NHC transfer can give either mono- or bis-NHCs, or bis- and tris-NHCs. A net N to C rearrangement of the N-alkyl imidazole complex to the corresponding NHC complex was seen with (MeO)2CO (DMC). DFT calculations identify the steps needed to form the carboxylate from imidazole and DMC: SN2 methyl transfer from DMC to imidazole, followed by proton transfer from the imidazolium CH to the carboxylate counterion, produces the free NHC H-bonded to MeOH with a weakly associated CO2. The nucleophilic NHC attacks CO2 to form NHC-CO2. NHC transfer to the metal with loss of CO2 has been calculated for Rh(cod)Cl. A proposed two-cis-site reactivity model rationalizes the experimental data: two such vacant sites at the metal are needed to allow coordination of the NHC-CO2 carboxylate and subsequent CC cleavage with NHC transfer. Partial cod decoordination or chloride loss is thus required for Rh(cod)Cl. Chloride dissociation, calculated to be easier in polar solvent, is confirmed experimentally from the retarding effect of excess chloride.

N-Heterocyclic carbene bonding to cobalt porphyrin complexes

“…The air-stable NHC cobalt complex 2 was successfully prepared by reacting the imidazolium carboxylate 1 [18] in the presence of equimolar amounts of meso-tetraphenylporphyrin cobalt(III) chloride (TPP-CoCl; Scheme 1). Complex formation was indicated by the intense purple color, which is reflected in the UV-vis spectrum by the Soret band (λ max 434 nm) and two absorption maxima for the Q band (λ max 560 and 595 nm).…”

C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands

“…Carbon-carbon bond cleavage is silver(I)-mediated and proceeds via a stepwise oxidation of the methyl substituent at the C2 position to a formyl group (C) with concomitant silver reduction. The carbonyl group is cleaved readily 27 in the presence of water, formed during the oxidation of 1, and another equivalent of Ag + ions to give the normal C2-bound silver-bis(carbene) intermediate D. While the proposed intermediates C and D have not been detected directly, closely related species have been isolated and support this mechanism. For example, a similar reactivity has been observed with 2-benzylated imidazolium salts, leading to the formation of benzoic acid as easily detectable side product.…”

Organic reactions for the electrochemical and photochemical production of chemical fuels from CO2 – The reduction chemistry of carboxylic acids and derivatives as bent CO2 surrogates

“…Most excitingly, the N-heterocyclic carbene fragments have been shown to react with CO 2 to form zwitterionic imidazolium carboxylates (Eq. (1), 11) [32].…”

“…In Scheme 7, such binding is demonstrated to occur with a phosphine Lewis Base and a borane Lewis Acid. Catalytic reductions of CO 2 to methanolic species have been shown with the help of phosphine-borane (33) binding and catalysis [60,61] with a borane terminal reducing agent (32).…”