Ion-pair exchange

Similarly to classical PTC reaction conditions, under solid-liquid PTC conditions with use of microwaves the role of catalyst is very important. On several occasions it has been found that in the absence of a catalyst the reaction proceeds very slowly or not at all. The need to use a phase-transfer catalyst implies also the application of at least one liquid component (i.e. the electrophilic reagent or solvent). It has been shown [9] that ion-pair exchange between the catalyst and nucleophilic anions proceeds efficiently only in the presence of a liquid phase. During investigation of the formation of tetrabutylammonium benzoate from potassium benzoate and tetrabu-tylammonium bromide, and the thermal effects related to it under the action of microwave irradiation, it was shown that potassium benzoate did not absorb micro-waves significantly (Fig. 5.1, curves a and b). Even in the presence of tetrabutylammonium bromide (TBAB) the temperature increase for solid potassium benzoate... 

The rate of this ion-pair exchange could be measured in various solvents using picosecond laser absorption spectroscopy. The rate decreases as the ability of the solvent to solvate the metal cation increases. It even falls to zero rate in the presence of appropriate crown... 

An ion-pair effect on carbonylation reaction occurs e.g., the exchange of NaCo(CO)4 with CO is favored by ion pairing . Exchange can also be carried out on supported metal carbonyls e.g., CO can be bonded to Cu(I) complexes supported on polystyrene . 

The excellent fit provided for the data is strong evidence for the validity of the scheme. While more than one chemical interpretation is possible, it would appear that, in addition to a diazonium ion and carbonium ion, at least one other intermediate, most plausibly the diazoester isomeric with the pyrazolenine reactant, is required. A final decision between this and a mechanism involving ion pair exchange will need further investigation. 

Huisgen also found that in acetic acid as solvent the three reactions lead to much the same proportion of n- and iso-propyl acetates. It seems probable that the carbonium ions are here formed predominantly within ion pairs. Most likely, however, ion pair exchange occurs with the acidic solvent leading almost exclusively to propyl-acetate pairs as the carbonium ion intermediates. 

To occur, these halide displacements require anionic activation which can be achieved by PTC coupled with the addition of a transfer agent (NRJ, X ) to provoke ion-pair exchange. 

In the best situation, lattice energies for LiBr and LiCl are 788 and 834 kj/mol, respectively. Consequently, ion-pair exchange occurs preferentially with LiBr of a lower lattice energy. LiBr can thus be selectively activated in the presence of LiCl [Eq. (52)]. 

Due to ion-pair exchange, there is formation of loose ion pairs Nu", NR which are very reactive, lipophilic and polar species. They are consequently highly sen-... 

The E° value for FeCpCC H ) /" is about - 2 V vs SCE. Thus with L = PMe3, the ET responsible for the disproportionation found in the presence of NaPF is about isoergonic but with P(OMe)3, it is endergonic by about 0.25 V (24 kJ Mole ) and nevertheless driven by the follow-up double ion-pair exchange and ensuing irreversible reactions. ... 

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