Onium cation

Carbenium ions and onium cations from hetero-PAHs 160... 

A number of clay-like 2D layered and zeolite-like 3D framework host structures have been materialized using isopolycyanocadmate as the linkage unit. The 2D layered and the 3D framed hosts accommodate a variety of organic guest molecules and onium cations of different geometrical characters. 

As listed in Table 3, a number of inclusion compounds of the CdjCCN) host have been obtained with the general formula [CdjCCN) ] [onium] [guest] where the onium cation is always accommodated as the guest in the zeolite-like 30 framework structure [5]. 

Noyori et al.1 have reviewed uses of this triflate (1) for silylation and as a catalyst in various nucleophilic reactions. The review includes several unpublished results obtained by the Nagoya group. The Si atom in the triflate can interact with various heteroatoms, particularly with oxygen to form onium cations, which can act as supercations in aprotic solvents. For example, the reaction of 1 with the keto epoxide 2 involves ring fusion followed by cyclization to 3. 

Crown ether is the other important class of phase-transfer catalysts which critically differs from the onium salt, in that the whole inorganic salt is transferred into the organic phase. The reaction modes described above can generally be accommodated in such crown ether-catalyzed reactions, simply by replacing the onium cation by a metal cation complex of crown ether. 

Oniu m Cations. Major families of ILs are composed of quaternary onium cations such as imidazolium, pyridinium, ammonium, phosphonium, sulfonium cations and so on. As described above, the fact that most ILs are composed of organic cations is attributed to weaker electrostatic interaction among component ions. There have been several reports on the effect of cation structure on the Tm of ILs. The relationship between Tm and the basic structure of onium cations is important in developing a protocol to prepare low melting ILs. 

Effect of Side Chain Length. The side chain bound to the cation also affects the Tm due to flexibility and excluded volume effects. To discuss the relation between side chain structure and Tm, the onium cation structure was fixed. The relation between the side chain structure and the thermal properties of imidazolium salts has already been reported by Seddon et al. [16, 17]. The effect of alkyl chain length of 1-alkyl-3-methylimidazolium tetrafluoroborate on the phase transition temperatures is... 

MacFarlane et al. [129] and Watanabe et al. [24a, 114] discussed the difference in diffusivity of component ions. Reported diffusion coefficients of ILs are shown in Table 3.19 together with viscosity and ionic conductivity. From that table, it is easy to see that lower viscosity ILs show larger diffusion coefficients and higher ionic conductivity. Cations generally have larger diffusion coefficient values than do anions in ILs. This means that the cation diffuses more easily than the anion. However, the transference numbers of onium cation (t+) in ILs calculated from the results of PFG-NMR is in the range 0.5 to 0.6 and their contribution to the ionic conductivity is mostly the same, irrespective of the ion species. In the case of [bpy][BF4], the BF4- shows a larger diffusion coefficient than that of bpy+, and therefore t+ is below 0.5 [24a], Thus, as well as thermal and electrochemical properties, the diffusion behavior of component ions is dependent on their structure. 

Five different types of halogen exchange reactions are known (R and R are alkyl, aralkyl or an aryl groups, M is usually a metal cation but can also be an onium cation). 

Although the tellurocyanate ion was suspected for a long time to exist in solution, attempts to isolate tellurocyanates had failed3 until 1968, when tetraethylammonium tellurocyanate was isolated as a moisture- and air-sensitive, pale-yellow solid. Several other tellurocyanates with large onium cations were subsequently isolated. 

Dimethyl and methyl phenyl 4,4-dimethyl-2,6-dioxocyclohexylidene tellurium compounds transferred a methyl group to triethylamine, triphenylphosphane, tris[di-methylamino]phosphane, and triphenylarsane. The methyl onium cations were isolated as lelraphenylborates. The alkylidene organo telluronium intermediate was identified by 31P-NMR spectroscopy in the reaction of the ylide with tris[dimethylamino]phosphanc . 

Excited electronic states play a crucial role in the chemistry of hypervalent radicals formed by collisional reduction of organic onium cations, where... 

A suspension of nutshell particles in DMF was quatemized with tributyl-phosphine. This converted the encapsulated poly(VBC) into poly[(vinylben-zyl)-tributylphosphonium chloride]. The EDS experiments indicated a low surface concentration of phosphorus in the quaternized samples. Since the elemental analysis indicated a high content of phosphorus in all the samples, one can infer that the quaternary onium cations are hidden inside the polystyrene cages . 

Chloroaluminate systems have been intensively studied over the past few decades mainly in electrochemical research fields. Many of the electrochemical properties of RTILs have been sorted out using the chloroaluminate systems [50]. Chloroaluminate anions form RTILs with various organics, including not only EMI and BP but also triazolium [51] and aliphatic onium cations [7,52-54]. Other unique RTILs similar to the chloroaluminate systems such as the chlorogallate (GaCLj ) [55], chloroborate (BCL ) [56], and bromoaluminate (AIBr4 ) systems [57] have been reported. 

Salt synthesis is a helpful development for solving this problem. Acid esters are foremost among the salts used for this purpose. Tertiary amines are reacted with acid esters to introduce an alkyl group onto the nitrogen atom and prepare the onium cation. The reacted acid then remains as the counter anion. This method is well covered in several other books on ionic liquids [1]. Several acid esters are known to be effective for direct preparation. Recently even TFSI methyl ester was reported as an effective reagent for the preparation of ionic liquid containing TFSI... 

It is well known that amines can be neutralized with acids to generate salts. This neutralization process is useful in preparing organic salts with very low melting points. In 1914 Walden reported [3] neutralized ethylamine with nitric acid to prepare a salt that had a melting point of 12°C. This appears to have been the first ionic liquid prepared by a neutralization method [4]. As seen in equation (19.1), there is a slight difference between onium cations prepared by quaternization (a) and neutralization (b) when tertiary amines are used as a starting material ... 

Some of these ligands are so highly nucleophilic, i.e. basic (see Section II.2), that in an equilibrium like Eq. (1) the carbocation counterpart of the onium cation is not regenerated. Thus, the latter is the only species in solution. 

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