Alkali-metal-mediated zincation (AMMZn) meets N-heterocyclic carbene (NHC) chemistry : Zn–H exchange reactions and structural authentication of a dinuclear Au(I) complex with a NHC anion

Merging two evolving areas in synthesis, namely cooperative bimetallics and N-heterocyclic carbenes (NHCs), this study reports the isolation of the first intermediates of alkali-metal-mediated zincation (AMMZn) of a free NHC and a Zn–NHC complex using sodium zincate [(TMEDA)NaZn(TMP)(tBu)2] (1) as a metallating reagent. The structural authentication of (THF)3Na[:C{[N(2,6-iPr2C6H3)]2CHCZn(tBu2)}] (2) and [Na(THF)6]+[tBu2Zn:C{[N(2,6-iPr2C6H3)]2CHCZn(tBu2)}]− (4), resulting from the reactions of 1 with unsaturated free NHC IPr (IPr = 1,3-bis(2,6-di-isopropylphenylimidazole-2-ylidene) and NHC complex ZntBu2IPr (3) respectively demonstrates that in both cases, this mixed-metal approach can easily facilitate the selective C4 zincation of the unsaturated backbone of the NHC ligand. Furthermore, the generation of anionic NHC fragments enables dual coordination through their normal (C2) and abnormal (C4) positions to the bimetallic system, stabilising the kinetic AMMZn intermediates which normally go undetected and provides new mechanistic insights in to how these mixed-metal reagents operate. In stark contrast to this bimetallic approach when NHC-complex 3 is reacted with a more conventional single-metal base such as tBuLi, the deprotonation of the coordinated carbene is inhibited, favouring instead, co-complexation to give NHC-stabilised [IPr·LiZntBu3] (5). Showing the potential of 2 to act as a transfer agent of its anionic NHC unit to transition metal complexes, this intermediate reacts with two molar equivalents of [ClAu(PPh3)] to afford the novel digold species [ClAu:C{[N(2,6-iPr2C6H3)]2CHCAu(PPh3)}] (6) resulting from an unprecedented double transmetallation reaction which involves the simultaneous exchange of both cationic (Na+) and neutral (ZntBu2) entities on the NHC framework

Synthesis, characterization, dynamics and reactivity toward amination of η3-allyl palladium complexes bearing mixed ancillary ligands. Evaluation of the electronic characteristics of the ligands from kinetic data.

On the basis of an original protocol, we have synthesized several complexes of the type [Pd(eta(3)-C(3)H(3)R(2))(LL')]ClO(4) (R = H, Me; L, L' = PPh(3), P(OEt)(3), 2,6-dimethylphenylisocyanide, t-butylisocyanide, 1,3-dimesitylimidazolidine, 1,3-dimesitylimidazol-2-ylidene). The complexes, some of which are completely new species, were fully characterized and their behaviour in solution was studied by means of (1)H NMR. The reactions of the complexes bearing the symmetric allyl moiety [Pd(eta(3)-C(3)H(5))(LL')]ClO(4) with piperidine in the presence of the olefin dimethylfumarate were followed under kinetically controlled conditions. Formation of allyl-amine and of the palladium(0) derivatives [Pd(eta(2)-dmfu)(LL'] was observed. The reaction rates k(2) proved to be strongly dependent on the ancillary ligand nature and allowed a direct comparison among the electronic characteristics of the ligands. The reactivity trend determined appears to be mainly influenced by the capability of the ancillary ligands in transferring electron density to the metal centre and consequently on the allyl fragment