Imidazolium ligand

A very convenient method for the selective preparation of aNHC complexes is to block the imidazolium ligand precursors by substitution at C2 with alkyl or aryl groups [29, 30]. Following this procedure, the coordination of 1,2,3-trimethylimid-azolium iodide to [Cp IrCl2]2 afforded Cp Ir(aNHC), as shown in Scheme 3.14 [31]. 

Figure 5.5 Scheme illustrating potential reorientation of bonded imidazolium ligands in response to deprotonation of residual silanols. Anion is not shown for clarity. (Adapted from Wang, Q., Baker, G. A., Baker, S. N., and Colon, L. A., Analyst, 131,1000-1005, 2006.)... 

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

Bolm et al. reported the first planar chiral NHC at the beginning of 2002 [87]. The synthetic strategy is based on an oriented ortho-metallation starting from a chiral sulfoxide, followed by the conversion of the sulfoxy group to a hydroxymethyl unit. The imidazole ring is then linked to this intermediate with the aid of W,AT-carbonyl diimidazole and subsequently quarter nized with methyl iodide to give the imidazolium ligand precursor of the carbene 41 (Scheme 31). 

Note Chelate coordination to lithium is only possible for the amino carbene, but not for the amide imidazolium ligand. The formation of a tetracoordinated lithium atom in a dimer structure is probably the driving force for the formation of a carbene. 

Figure 4.18 Molecular design of iminoalkyl imidazolium ligand.

Wolf, J., Labande, A., Daran, J.C. and Poli, R., Nickel(ll) conplexes with bifunctional phosphine-imidazolium ligands and their catal)4ic activity in the Kumada-Corriu coupling reaction, J. Organomet. Chem. 691 (3), 433 43 (2006). 

The copper-NHC-catalysed addition of organoaluminium reagents to cyclo-hexenones and cycloheptenones reported by Hoveyda [58] is, in some cases, slightly less selective than those performed in the presence of phosphoramidites or phosphinamine catalysts (<90% ee for the imidazolium ligands versus <99% ee that the phosphoramidites and phosphinamine can provide). However, NHC-Cu catalysis provides better results (up to 97% ee and 97% conv) when challenging cyclopentenones and bulky p-substituted cycUc enones (bearing n-butyl, alkynyl, aryl or an ester group as the p substituent) are used as substrates (Scheme 25). 

Alexakis et al. have proposed a catalytic cycle for the ECA with imidazolium ligands and Grignard reagents (Scheme 43) [72]. 

This methodology also allows very good regio- and enantioselectivities when cyclic enynones such as 28 are used as substrates [80, 84]. The use of Cu(OTf)2 and imidazolium ligand L19 as catalysts in DCM leads to the unique formation of the... 

Very recently, Hoveyda et al. have demonstrated that the conjugate addition of organoboron reagents to p-substituted cyclic enones, catalysed by the readily accessible imidazolium ligand L36 and in the absence of any transition metal,... 

Aryloxo-NHC-containing complexes could also be produced from amido precursors where the ligands also acted as internal base as shown by the Shen group. Hence, a hydroxyaryl-imidazolium ligand reacted with [LiY N(z-Pr2) 4] and BuLi at — 78 °C to give the NHC-yttrium complex [(NHQ3Y] 21 (Scheme 6.2). " From [LiYb N(z -Pr2) 4], a bis-substituted ytterbium compound [(NHC)2Yb N(z-Pr2) ] 22 was prepared. Of note, all attempts to prepare the mono-substituted complex were unsueeessful. 

Imidazolium ligands afforded the preparation of dirhodium species such as Rh2( -Bzim-A, C )2(CO)4 and chelate cationic Rh(i) dicarbonyl complexes [Rh((mim)2CH2)(GO)2]BPh4 19 [mim = Af-methylimidazol-2-yl] were also obtained. The related methylbenzimidazol complexes were also prepared. ... 

Nickel-catalyzed Grignard cross-coupling at room temperature using imidazolium ligand... 

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