Salts imidazolium

Neither pyrazoles nor pyrazolium salts react by this mechanism which has been described for imidazoles and imidazolium salts (Section 4.01.1.7.4). As exchange rates show (Section 4.04.2.1.7(iii)), it is considerably more difficult to generate an ylide from a pyrazolium salt than from an imidazolium salt (at C-2). 

Imidazolium halides pyrolysis, 5, 449 Imidazolium ions acylation, 5, 402 H NMR, 5, 352 hydrogen exchange, 5, 417 nucleophilic attack, 5, 375 reactivity, 5, 375 ring opening, S, 375 Imidazolium oxides in pyrrole synthesis, 4, 344 Imidazolium perchlorate, 1,3-diphenyl-acylation, 5, 402 Imidazolium salts 1-acetyl-... 

Chichibabin reaction, 5, 409-410 UV spectra, 5, 356 Naphthimidazoles, 2-amino-tautomerism, 5, 368 Naphth[2,3-h]imidazoles oxidation, 5, 405 Naphth[l,2-d]imidazolium salts nucleophilic substitution, 5, 412 Naphth[l, 2-h]isoquinolines... 

One method to transform imidazolium salts (00AGE3773) into carbene ligands, imidazol-2-ylidenes, is by deprotonation with sodium hydride or other suitable hydride in a mixture of THE and liquid ammonia (73JCS(D)514, 96CEJ1627, 98IC6412). 

A variety of silver(I) carbenes can be prepared by interaction of a series of imidazolium salts with silver(I) oxide or silver(I) carbonate (OOJCS(D) 4499). With 3-tert-butyl-l-(2 -pyridylmethyl)imidazolium bromide hydrate and 3-(2", 6"-di-Ao-propylphenyl)-l-(2 -pyridylmethyl)imidazolium bromide hydrate, complexes 85 (R = t-Bu, 2",6"-/-Pr2CgH3) result. 3-(2",4",6"-Trimethylphenyl)-l-(2 -pyridylmethyl)imidazolium bromide in turn leads to 86 (R= 2",4",6"-MejCgH2). 3-(2",6"-Di-wo-propylphenyl)-l-(2 -pyridyl)... 

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. 

However, ionic liquids containing other classes of organic cations are known. Room-temperature ionic liquids containing organic cations including quaternary ammonium, phosphonium, pyridinium, and - in particular - imidazolium salts are currently available in combination with a variety of anions (Figure 3.1-1 provides some common examples) and have been studied for applications in electrochemistry [7, 8] and in synthesis [9-11]. 

In the binary haloaluminate ionic liquids, an increase in the mole percent of the imidazolium salt decreases the density of the liquid (see Table 3.2-2). The bromo-aluminate ionic liquids are substantially denser than their chloroaluminate counterparts, being between 0.57 g cm and 0.83 g cm denser than the analogous chloroaluminate ionic liquids (see Table 3.2-2). Variation of the substituents on the imidazolium cation in the chloroaluminate ionic liquids has been shown to affect the density on the basis of the cation size [17]. 

Table 3.5-1 lists the E-r values for the allcylammonium thiocyanates and nitrates and the substituted imidazolium salts. It can be seen that the values are dominated by the nature of the cation. For instance, values for monoallcylammonium nitrates and thiocyanates are ca. 0.95-1.01, whereas the two tetraalkylammonium salts have values of ca. 0.42-0.46. The substituted imidazolium salts lie between these two extremes, with those with a proton at the 2-position of the ring having higher values than those with this position methylated. This is entirely consistent with the expected hydrogen bond donor properties of these cations. 

The n values were high for all of the ionic liquids investigated (0.97-1.28) when compared to molecular solvents. The n values result from measuring the ability of the solvent to induce a dipole in a variety of solute species, and they will incorporate the Coulombic interactions from the ions as well as dipole-dipole and polarizability effects. This explains the consistently high values for all of the salts in the studies. The values for quaternary ammonium salts are lower than those for the monoalkylammonium salts. This probably arises from the ability of the charge center on the cation to approach the solute more closely for the monoalkylammonium salts. The values for the imidazolium salts are lower still, probably reflecting the delocalization of the charge in the cation. 

The difference in the hydrogen bond acidities and basicities was far more marked. The a value is largely determined by the availability of hydrogen bond donor sites on the cation. Values range from 0.8-0.9 for the monoalkylammonium salts, and are slightly lower (0.3-0.8) for the imidazolium salts. In the absence of a... 

Hardacre et al. have developed a procedure for the synthesis of deuterated imidazoles and imidazolium salts [65]. The procedure involves the platinum- or palladium-catalyzed deuterium exchange of 1-methyl-d -imidazole with D2O to give 1-methylimidazole-d , followed by treatment with a deuterated alkyl halide. 

An investigation of the ligands for in situ generation of the carbene ligands from their salts revealed that IMes.HCI is our best choice (Table XVI, entry I) all other imidazolium salts investigated resulted in longer reaction times for complete... 

In an attempt to increase the ionic liquid/hexane partition coefficient, a new salen ligand appended with an imidazolium salt was developed (Fig. 5) [18]. Unfortunately, modification of the ligand caused a dramatic reduction in the enantioselectivity - down to 57% ee at most - although the reasons for this behavior remain unclear. 

Symmetrical N, N -disubstituted imidazolium salts are usually obtained by addition of paraformaldehyde on a bis-imine of glyoxal under acidic conditions. A one-pot procedure has been developed. Several enantiomerically pure amines were used to prepare the corresponding symmetrical salts 6 (Scheme 4) [12,13]. 

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]. 

The synthesis of the unsymmetrical imidazolium salt 11 bearing a planar-chiral ferrocene was described by Bolm starting from (Rp)-[2-(trimethysilyl)-ferrocenyl] methanol 12 which afforded the salt in good yield after reaction with Ar,M-carbonyl diimidazole and methylation (Scheme 9) [18]. 

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]. 

RajanBabu reported the first preparation of a bis-imidazolium salt 15 bearing a chiral linker (Scheme 11). The starting material was the enantiomerically pure (S)-l,l -bi-2-naphtol bis(trifluoromethanesulfonate) which was transformed in two steps into the dibromomethyl derivative 16 and then into the bis-imidazole. Quaternarization of this compound afforded 15 [20]. 

A bis-imidazolium salt was also prepared by Burgess, starting from the dichloride 20 derived from optically pure N,N -dimethyl-tmns-l,2-diaminocyclohexane (Scheme 13). The salt 21 was obtained by addition of this compound to several 1-alkylimidazoles [22]. 

An other type of bis-imidazoliums salts 22, resulting of a base-induced 1,3-cycloaddition of tosyl-methylisocyanide (TosMlC) to the enantiopure diimine 23, was obtained by Douthwaite according to Scheme 14 [23]. 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

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