Imidazolium salts oxazoline

Gade and Bellemin-Laponnaz have reported the synthesis, in good yields, of chiral oxazoline-imidazoliums salts 10a (Scheme 8) obtained by reaction of 2-bromo-4(S)-t-butyl oxazoline with several mono-N-substituted imidazoles [16]. Similaly an imidazolium salt 10b bearing a paracyclophane substituent was prepared by Bolm [17]. 

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. 

Finally, two chiral monodentate N-heterocyclic carbene ligands that contain an oxazoline unit have been reported. Glorius et al. reported the synthesis of the imidazolium salts 76 by cyclizing the corresponding bisoxazolines 75 (Scheme 50) [133]. 

Glorius F, Altenhoff G, Goddard R, Lehmann C (2002) Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands. Chem Comm 2002 2704-2705... 

Linker-free oxazoline functionalised NHC can be synthesised using a modular approach. In this modular approach, a standard aUcyl or aryl substituted imidazole (module 1) is reacted with a 2-halo substituted oxazoline (module 2) to form the corresponding oxazoline functionalised imidazolium salt (see Figure 3.25). In a second step, the oxazoline functionalised NHC can then be reacted with a suitable metal precursor to form the corresponding transition metal carbene complex [93,98,102] (see Figure 3.26). 

Figure 3.29 Synthesis of an oxazoline functionalised chiral imidazolium salt from serine.

The imidazolium salts depicted in Scheme 15.12 were subsequently coordinated as carbene-oxazoline ligands to rhodium(I) and palladium(II). The rhodium complex was tested in the hydrosilylation of ketones, giving the secondary alcohols with poor enantioselectivity [49]. 

In view of these previous developments, we directly coupled various N-substituted imidazoles, which display nucleophilic reactivity, with 2-bromo-oxazolines to give the imidazolium precursors (Scheme 15.15) [55]. This direct condensation of an oxazoline and an imidazole salt provided a straightforward and modular route to the development of a new family of stereodirecting ligands. NHC - rhodium complexes could be obtained by reaction of the imidazolium salt with [ Rh(p-OtBu)(nbd))2] (nbd, norbornadiene) generated iw sifn [56]. 

The last group of chiral NHC ligand precursors is comprised of oxazoline-substituted imidazolium salts. The first derivative of this type 63 was reported by Herrmann et al. in 1998. A slight modification of the ligand backbone and use of the oxazoline C4 atom for bridging gave the imidazolium salt 64. The direct connection of the two heterocycles led to 65. Imidazolium salts built from oxazoline heterocycles like 66 were prepared by Glorius et alP ... 

Notably, the recent literature has described interesting new complexes (Figure 10.9), such as (i) carbene carbonyl- or bis(carbene)-Pt complexes obtained by a transfer reaction between a W-complex and [(PhCNjzPtClz] (ii) Pt -hydrido/NHC complexes (obtained by an oxidative addition of imidazolium salt to Pt complex) (iii) original mixed Pt/oxazoline-NHC, ... 

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