Bidentate imidazolium

Improvement in the catalyst activities and enantioselectivities was realised by the development of the chiral, bidentate alkoxy-functionalised imidazolium and imidazolidinium pro-ligands (134 and 136). 134, after deprotonation, was used to prepare the well-defined complex 135. Both 136 in the presence of BuLi and Cu(OTf)2 or 135 without any additional co-reagents were efficient catalysts in the asymmetric 1,4 addition of dialky Izincs and Grignards to cyclohexen-2-one giving higher ee (83% at rt and 51% at -30°C, respectively) [107, 108]. 

Other species tested in situ are chelating bis-imidazolium structures with phe-nylene spacers [48], and bidentate C-P ligands [49] that achieved, in the case of triarylphosphine-imidazolium salts, very good yields for a wide variety of substituted aryl bromides (Scheme 6.11) [50]. 

Some other enantioselective approaches have been attempted, still with moderate enantioselectivities, by making use of in situ systems containing a chiral NHC precursor. Luo and co-workers reported on the use of the bidentate chiral imidazo-lium salt 16, derived from L-proUne, in combination with [RhCia-COCcod)], leading to an enantiometic excess of around 20% [30]. The use of chiral imidazolium salt 17 in combination with [RhCl(CH2=CHj)j]j by Aoyama afforded slightly better ee (Fig. 7.3) [31 ]. So far, Bohn and co-workers have obtained the best enantioselectivities (up to 38% ee) for the catalytic addition of phenylboronic acid to aromatic aldehydes by using planar chiral imidazolium salts 18, derived from paracyclophane, in combination with [Rh(OAc)2]2 [32]. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

In the imidazolium salts 63, obtained by nucleophihc substitution of the iodo-derivative 61 by an imidazole 62, the oxazoline is bnked by the carbon atom in the 4-position. Coordination of the bidentate bgand to the Ir(COD) + complex fragment is then achieved by in situ deprotonation (Scheme 43). This modular design allows facile and rapid access to a large ligand library by variation of the substituents in the 2-position of the oxazoline and at the terminal N-atom of the heterocyclic carbene. 

Re(CO)3(/t-OCH3)3Re(CO)3] (64) whereas decreasing pressure favoured production of rhenium clusters of higher nuclearity with mixed (/t-oxo)- and (/r-methanolato) bridges. The presence of 1.33 equivalent of base per metal atom in the clusters 44 and 44b facilitates subsequent reactions, that demand base. The reaction of 44 with (HCsMes), for example, resulted in the quantitative formation of 45 [79], In the case of the trinuclear rhenium cluster 44b, reaction with the caibene precursor 1,1 -methylene-bis(3-methyl-1 //-imidazolium)-diiodide yielded the corresponding mononuclear Re(I) complex (65) with the bidentate caibene ligand (see Scheme 14) [80]. 

Akin to group 4 metals, various NHC bidentate or tridentate ligands have been used to stabilize group 5 metals as shown in Scheme 14.24 [49,67]. Vanadium complex 44 could be cleanly oxidized (by 4-methylmorpholine Al-oxide) to the corresponding paramagnetic V (IV) oxo complex LV(=0)Cl2 [66,67]. Like the vanadium (V) complex 39, species 45 was found to be air-stable whether in the solid state or in benzene solution. The vanadium (III) complex 46 resulted from the reaction of V(NMe2)4 with the corresponding imidazolium salt and thus proceeded with a reduction to V(III). 

Chelating Carbene Ligands and Metal Complexes Potentially, bidentate coordinating dicarbenes are accessible by alkylation of 1-methylimidazole with 1,3-dibromopropane to produce bis(imidazolium bromides) (Scheme 2.162) [52]. The benefits of its high water solubility were utiUzed when an aqueous solution of the bromide was treated with 2 equiv of NH4PF5 to produce the corresponding hexafluorophosphate. Addition of [Rh(COD)Cl]2 to the latter in THF and subsequent deprotonation with an excess of KOtBu afforded a cationic Rh(COD) complex [47]. Replacement of COD by CO could be realized in... 

In 1998, Herrmann et al. reported the synthesis of the first chiral carbene containing an oxazoline unit. In this bidentate ligand, the oxazoline ring is linked in its 2-position to the imidazole ring via a methylene bridge [48]. The key step in the synthesis of the imidazolium precursor is the acid-catalyzed cyclization of the oxazoline by reaction of an iminoester, formed in situ from a nitrile function, and the amino alcohol (Scheme 15.12). 

Early forays into this reaction were described by Cavell and Yates. Experimental studies demonstrated that oxidative addition of both C2 H and C2-I imidazolium ions to platinum was feasible. The C2-I substituted imidazolium ion 5 also underwent oxidative addition to [Pd(PPh3)4] however, an attempted reaction with the C2-H imidazolium met with failure (Scheme 3.4). Palladium has been shown to oxidatively add into C2 H imidazolium ions in bidentate systems resulting in palladium complexes 6 and 7 (Scheme 3.5). The oxidative addition of imidazolium ions to iridium, generating (NHC)Ir -hydrides was also reported. ... 

The solvolysis of benzhydryl bromide did not proceed without addition of activator under the chosen reaction conditions. Traditional monodentate XB donors such as 1,4-diiodoperfluorobenzene and 1,3,5-triiodoperfluorobenzene did not increase yields of 22. Bidentate cationic XB donors /M,p-ll-26 70Tf based on the imidazolium motif, however, lead to a yield of more than 80% of 22 under otherwise unaltered reaction conditions. 

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