Imidazol-2-ylidene type NHC

Several studies reported the application of established five-membered ring (imidazol-2-ylidene type) NHC ligands in Heck chemistry. Whilst most authors generally focus on catalytic activity, Jutand and co-workers examined the mechanism of oxidative addition of aryl halide to NHC-Pd complexes. Complexes [Pd L2] (L = saturated NHC) were generated electrochemically and tested in situ for oxidative addition. They proposed that either an associative or dissociative mechanism could operate, depending on steric and electronic properties of the NHC ligand. 

Rh(III)(NHC) hydrides have been studied as catalysts for this type of hydrogenation. The products from the reaction of Rh(I) complexes with are dependent on the natnre of the NHC. The reaction of [RhCl(IPr)2(N2)] 1 (IPr = Af,A -bis-[2,6-(di-tTo-propyl)phenyl]imidazol-2-ylidene) with gave the monomeric complex 3 [1], which was also obtained from the reaction of [RhCl(COE)(IPr)]2 2 with and excess IPr, while the reaction of [RhCl(COE)(lMes)]2 with gave the chloride bridged species 4 (Scheme 2.1) [2],... 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

DFT studies showed that annulation of imidazol-2-ylidenes generally destabilizes the NHC. This becomes also apparent when analogies between benzannulated N-heterocyclic carbenes and saturated imidazolin-2-ylidenes 86 (see Figure 1.8) are considered. Both types of NHC dimerize rapidly if the nitrogen atoms of the heterocycle are substituted with sterically less demanding substituents. No such dimerization is observed with imidazol-2-ylidenes 78, which possess an unsaturated heterocycle like the benzimidazol-2-ylidenes. 

Lewis adducts of the type RA1---NHC (with the formation of an Al-C bond with a double bond character) do not readily form upon reaction of AIR and NHCs derivatives, which further supports that both classes of compounds exhibit related properties. However, prolonged heating of HC C(Me)NDipp 2Al with imidazol-2-ylidenes produced a carbene adduct of a rearranged aluminum(III) hydride... 

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