Imidazolium salts phosphino

From the three components, the 2-phosphino substituted imidazolium salt cannot serve as a normal carbene ligand, but both the amino functionalised imidazoUum salt (given the role as the base) and the nonfunctionalised imidazolium salt can act as carbene ligands and would enhance the catalytic activity of the system, if they did [149]. Furthermore, the amino functionalised imidazolium salt would stabilise the Pd(0) species due to its hemila-bile behaviour. One should use these ionic liquid systans with caution. 

Having introduced the golden rule of phosphine chemistry to its carbene analogues we will proceed to break it several times in the following brief sununary of routes to synthesise phosphino functionalised carbenes. The best way to synthesise an imidazolium salt is to react an N-substituted imidazole with an alkyl or aryl halide [235], Thus, it is a good idea to utilise a functionalised alkyl halide. The functional group can then be used to introduce the phosphino group. 

Figure 3.78 Introduction of a phosphino group onto the sidechain of an imidazolium salt.

Figure 3.80 Stepwise introduction of a phosphino group and an imidazolium salt on a chiral paracyclophane.

A similar protocol was developed by Wang et al. starting from benzaldehyde, N,N-dimethylaminomethyl benzene was formed, Uthiated in the ortho position and reacted with ClPPhj to introduce the phosphino group. Then the amino group is substituted with chloride and the molecule reacted with the respective imidazole to generate the mono- or bisphosphino imidazolium salt (see Figure 3.81). 

Phosphino functionalised carbenes do not need to be used in situ or the carbene complexes generated without the formation of the free carbene. Danopoulos et al. have isolated and structurally characterised a phosphino functionahsed carbene after deprotonation of the corresponding imidazoUum salt with KNfSiMej) [280], a feat repeated by Hodgson and Douthwaite using a chiral imidazolium salt [243]. 

The more popular routes towards transition metal phosphino functionalised carbene complexes avoid the formation of the free carbene and apply one of several in situ methods. Lee et al. reacted the phosphino functionalised imidazolium salt with palladium(II) chloride in the presence of sodium acetate as base [241,281] resulting in the expected paUadium(II) chelate complex. 

Figure 3.89 Syntheses of a phosphino functionalised imidazolium salt using the silane reduction route.

Our now familiar phosphino functionalised imidazolium salt - PhjPCHjCHjlmMes - has attracted further attention to warrant a third route for its synthesis. Nolan and cowoikers used arylimidazole, dibromoethane and KPPh and obtained a rather low yield (21%) [238], whilst Tsoureas et al. chose the reduction of the corresponding phosphane oxide with SiHClj under harsh conditions and had no control over the anion [282]. This prompted Wolf et al. to develop a third route [290], another modification of Nolan s protocol [238]. 

Wolf et al. used the phosphino functionalised imidazolium salt to react it with nickel(II) bromide forming the P-coordination product (see Figure 3.95). Attempts to deprotonate... 

Figure 3.94 Another route towards phosphino functionalised imidazolium salts.

We are in for a few surprises with the chemistry of rhodium and iridium. Let us stay with the phosphino functionalised imidazolium salt PhjPCHjCHJmMes and react it with the precursor complexes [M(cod)OEt]j (M = Rh, Ir) [296]. As expected, the corresponding cationic phosphino functionalised carbene complexes are formed (see Figure 3.96). 

The corresponding reactions with iridium(I) precursors again behave differently. When the phosphino functionalised imidazolium salt is reacted with [IrlcodlCl], the phosphane adduct with a pendant imidazolium moiety is formed. A similar reaction with the more reactive [Ir(cod)( Li-H)(p,-Cl)2]2 yields a five coordinate iridium(I) complex that might be described as having square pyramidal geometry with the bromide in apical position and the carbene in abnormal coordination mode [47-49] (see Figure 3.97). 

Figure 3.98 Synthesis of phosphino functionalised imidazolium salts carrying two phosphino wingtip groups.

Switching from palladium to rhodium, we encounter some very interesting chemistry. Zeng et al. [302] reacted the tiidentate PCP phosphino functionalised imidazolium salt with silver(I) oxide and subsequently transferred the carbene to rhodium(I) (see Figure 3.100). Careful selection of the rhodium precursor complex and reaction conditions enables tetrahedral, square bipyramidal and octahedral rhodium(I) and rhodium(III) complexes to be formed. As the authors explained, the activation of the C-Cl bond in methylene chloride in an oxidative addition reaction on rhodium(I) resulting in a rhodium(in) complex requires an electron rich rhodium(I) complex. The presence of a NHC ligand is advantageous in this respect. 

Ketz et al. synthesised a keto functionalised imidazolium salt using the standard protocol by reacting the appropriate N-aryl substituted imidazole with the respective keto halide [95], Their intention was to use the corresponding nickel(II) carbene enolate complex in olefin polymerisation reactions similar to the phosphino enolates in the SHOP process [1 ]. It proved difficult to prepare the intended precatalyst with only one carbene enolate ligand attached to the nickel centre. Initially, the homoleptic complex with two carbene enolate ligands was formed. The authors pointed out that the high proportion of n-olefins is unusual. 

While Bertrand and co-workers described in 1988 the stable k -phosphino-carbene 8, which did not act as a ligand, Arduengo et al. prepared in 1991 the first free and stable bottleable N-heterocyclic carbene 9 by deprotonation of the corresponding imidazolium salt (Scheme 1.4). This deprotonation method was later supplemented by Kuhn, who introduced the reductive desulfurization of thiones for the preparation of stable imidazol-2-ylidenes. ... 

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