A copper-masked monosubstituted carbene as a general transmetalating agent toward stable carbene complexes

Abstract

Metal complexes bearing carbene ligands LnM=CR1R2 are relevant intermediates in catalytic transformations. The stability of isolated carbene complexes decreases for the case of monosubstituted carbenes (R1 = H). We have discovered that the copper complex [TpMsCu(C(H)NEt2)] (1) readily transfers the aminocarbene :C(H)NEt2 to other complexes of metals, such as rhodium, iridium, palladium, platinum, copper, or gold, leading to complexes bearing the M–C(H)NEt2 moiety. Experimental data allow us to propose that this monosubstituted aminocarbene (MAC) ligand displays a nucleophilicity similar to that of NHC (N-Heterocyclic Carbene) ligands bearing bulky substituents but it has a considerably lower volume. These features allow unprecedented reactivity patterns, such as the generation of the first stable monosubstituted carbene adduct of Rh2(OAc)4, a well-known catalyst for carbene transfer reactions. Density functional theory (DFT) studies carried out on the transfer of the MAC ligand from [TpMsCu(C(H)NEt2)] to trans-[PdCl2(NCMe)(PPh3)] led us to ascertain that the transmetalation process involves several intermediates in which the TpMs ligand shifts between the κ2 and κ3 coordination modes.