2.4.5- Tris -imidazolium salts

A large number of differently bridged diazolium salts of type 24 (Figure 1.4) have been described. Compound 25, the precursor for an interesting bis-carbene containing carbene and alcoholato donor functions, was prepared by Arnold et al. by the reaction of two equivalents of an A-alkylimidazole with a functionalized epichlorhydrin (Figure 1.4). Dibenzimidazolium salts are also known. Trisimidazolium salt 26 was first described by Dias and Jin, " shortly thereafter followed by the description of trisimidazolium salts of type 27. Tripodal tris-imidazolium salts of types 26, 28 and 29 were prepared by Hu and Meyer. ... 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

A tripodal imidazolium salt 13 was obtained by Howarth from enantiopure imidazoles 14 (Scheme 10) by reaction with l,3,5-tris(bromomethyl)-2,4,6-trimethyl benzene [19]. 

To introduce the Rh-centre in the supported ionic liquid, a solution of [Rh(CO)2(acac)] in acetonitrile was treated with either the ligand tri(m-sulfonyl)triphenyl phosphine trisodium salt (TPPTS) or the ligand tri(m-sulfonyl)triphenyl phosphine tris(l- butyl-3-methyl-imidazolium) salt (TPPTI) (Rh/P ratio of 1 10). The ligand TPPTI was found to dissolve in [BMIM][BF4] and... 

Homoenolates generated catalytically with NHCs can also be employed for C-C and C-N bond formation. Bode and Glorias have independently accomplished the diastereoselective synthesis of y-butyrolactones by annulation of enals and aldehydes [121, 122]. Bode and co-workers envisioned that increasing the steric bulk of the acyl anion equivalent would allow reactivity at the homoenolate position. While trying to suppress the competing benzoin and enal dimerization the authors comment on the steric importance of the catalyst. Thiazolium pre-catalyst 173 proved unsuccessful at inducing annulation. A-mesityl substituted imidazolium salt 200 was found to provide up to 87% yield and moderate diastereoselectivities (Scheme 34). 

An ionic liquid was fully immobilized, rather than merely supported, on the surface of silica through a multiple-step synthesis as shown in Fig. 15 (97). A ligand tri(m-sulfonyl)triphenyl phosphine tris(l-butyl-3-methyl-imidazolium) salt (tppti) was prepared so that the catalyst, formed from dicarbonylacetylacetonate rhodium and the ligand (P/Rh = 10), could be soluble in both [BMIMJBFq and [BMIM]PF6. The supported ionic liquid-catalyst systems showed nearly three times higher rate of reaction (rate constant = 65 min ) that a biphasic system for the hydroformylation of 1-hexene at 100°C and 1500 psi in a batch reactor, but the n/i selectivity was nearly constant the same for the two ( 2.4). Unfortunately, both the supported and the biphasic ionic liquid systems exhibited similar metal leaching behavior. 

Direct arylation of methyl imidazole with 2,7-dichloronaphthyridine leads to a potentially tetradentate and practically tridentate bis-carbene ligand on a naphthyridine scaffold [363]. Reaction of the bis-imidazolium salt with silver(l) oxide in the usual way yields a linear trinuclear silver carbene complex with this tris-bridging hgand (see Figure 3.115). 

Fig. 13 X-ray crystal structures of two 2D assemblies with components that radiate from a central core (a) tris(imidazolium) triflate salt and (b) phloroglucinol and 2,4-dimethyl-pyridine solid.

Whereas most 2D finite crystalline assemblies exhibit a cyclic structure, 2D assemblies with components that radiate, or branch, from a central core have also been reported (Fig. 13).30,31 In particular, Kraft and Frohlich have reported a tris(imidazolium) triflate salt with anions that assemble along the exterior of a 1,3,5-trisubstituted benzene (Fig. 13a).30 Three imidazoline groups directed the assembly... 

Two-dimensional ion conduction was also achieved for the smectic liquid crystals consisting of the mixture of ionic liquids and hydroxyl-functionalized rod molecules [49-51] and smectic LC imidazolium salts having a long alkyl chain [52-55]. One-dimensional ion conduction was performed for aligned columnar LC phases forming ion-channels of fan-shaped imidazolium salts having a tris(alkoxy)phenyl moiety [56]. 

TPPTI Triphenylpbospbine trisulpbonate tris(l-butyl-3-methyl-imidazolium) salt... 

Recently, the present authors have achieved a facile recycling method for both catalyst and reachon medium using F-626 in a Mizoroki-Heck arylation reaction of acrylic acids [11]. The procedure employed a fluorous carbene complex, prepared in situ from a fluorous imidazolium salt, palladium acetate as the catalyst and F-626 as a single reaction medium. When acrylic acid was used as a substrate, separation of the product from the reaction mixture was performed simply by filtration with a small amount of FC-72. The FC-72 solution containing the fluorous Pd-catalyst and F-626 was evaporated and the residue containing the catalyst and F-626 (96% recovery) can be recycled for the next run (Scheme 3.5-6). They tried to reuse the catalyst, and observed no loss of catalytic activity in five re-use cycles. 

Figure 7.18 Supported ionic liquid catalysis applied in the hydroformylation of 1-hexene (tppti = tri(m-sulfonyl)triphenyl phosphine tris(1-butyl-3-methyl-imidazolium) salt. (Taken and modified from Ref. [108].)...

All other reported syntheses of pincer group 9 complexes bearing two NHC donor moieties involve the oxidation of the metal center. The first Rh pincer complex with two NHC donor moieties was reported by Peris and coworkers [30]. They reacted the imidazolium salt l 2HBr [9] with [Rh(COD)(p-Cl)]2 (COD, cyclooc-tadiene) and KBr in presence of triethylamine at 80 °C to obtain the Rh(III) tri-bromido complex 32. Lower temperatures led to the dimetallic species 31, which could be converted to the pincer complex 32 by heating and addition of KBr [30] (Scheme 9.3). 

Oxidative addition reactions of imidazolium salts to electron-rich, coordinatively unsaturated Ni (NHC)2 complexes have been used to synthesize Ni(ii) tris(carbene)-nickel-hydrido complexes (Equation (15))." ... 

Cavell also reported the oxidative addition of imidazolium salts to coordi-natively unsaturated bis-IMes complexes of Pd and Ni (Equation (3.1)). Reactions occurred cleanly, generating tris-NHC metal hydrides, which were remarkably stable to reductive elimination, likely due to the steric constraints of NHCs, which prevent the orbital overlap between the hydride and the car-bene carbon required for reductive elimination. 

In contrast to unsaturated alkoxy-functionalized NHCs, a deprotonation of corresponding imidazolium salts of saturated analogues did not result in alkali metal adducts but in bicyclic products HL (R = Dipp, Mes, i-Pr) 19. Arnold et al reported a series of lanthanide NHC complexes (Scheme 6.2) based on these ligand precursors by using a protonolysis reaction with internal bases such as rare earth tris[bis(trimethylsilyl)amido] or alkyl compounds. ... 

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