Imidazolylidene Ligands

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

The main synthetic routes leading to the formation of NHC complexes are depicted in Scheme 1.2. The methodologies given are shown with imidazolidinylidene and imidazolylidene ligands, however, they are applicable to other NHCs [16, 17]. 

Scheme 29.10 Carbenoid imidazolylidene ligands based on the JM-PHOS ligand (32).

Much of the chemistry of vinylidene complexes has been developed with catalytic applications in mind, as detailed later in this volume. Early examples had low activity for alkene metathesis, although complexes containing imidazolylidene ligands showed improved efficiencies [35]. However, in many cases, reactions of the vinylidene ligand have resulted in transformation to other carbon-based ligands which have not been released from the metal fragment. 

Scheme 43 Synthesis of an iridium(I) complex bearing Burgess s chiral oxazoline-imidazolylidene ligand...

Bidentate oxazoline-imidazolylidene ligands, in which both units are linked by a chiral paracyclophane, have been studied in Bolm s group [129]. In this case, the planar chirality of the pseudo-orfho-paracyclophane is combined with the central chirality of an oxazoline (Scheme 48). Compounds 70 were tested in the asymmetric hydrogenation of olefins displaying moderate selectivity (ee s of up to 46% for dimethylitaconate in the presence of 70b). 

Imidazolylidene ligands are enjoying a rapidly increasing use in cata-... 

Similar trends were observed in a complementary study using the palladium complexes 14 and 15 comprising sterically identical normal and abnormal bis(imidazolylidene) ligands (Scheme 5.4). ° In the presence of chlorine, complex 14 was stable and did not react, whereas the abnormal carbene complex decomposed to [PdCU] " and a doubly chlorinated bisimidazolium dication 16. This outcome was explained by oxidative CI2 addition to complex 15, followed by reductive Ccarbene Cl bond formation. Obviously, this process was unfavourable with normally bound imidazolylidenes. It is worth noting that an analogue of complex 15 with no alkyl substituents at the C5 and C5 positions induced reductive Ccarbene-Ccarbene bond formation. The higher propensity of abnormal carbenes to be reductively cleaved was rationalised by the enhanced electron donor properties of the non-classical carbenes, which made them more susceptible towards elimination processes. Evidently, steric factors could be ruled out in these systems. 

Synthesis of iridium complexes with novel planar chiral chelating imidazolylidene ligands. Tetrahedron Asymmetry, 14,1733-1746. 

Among them are planar chiral chelating iridium complexes bearing phosphinyl-imidazolylidene ligands 238 " dihydrooxazole-imidazolylidene ligands 239 " axial chiral Ir(I)-QUINAP-complexes 240 " planar chiral Ir(I)-FcPHOX-complexes 241 and analogs, " and so on. 

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