Caesium ion pairs

Due to the high interest in metalation reactions with lithium amide or alkyllithiums, an indicator scale of lithium ion pairs in THF has been developed119. Aggregation studies have indicated that organolithium species exist predominantly, if not exclusively, as monomers in the 10-3-10-4 M concentration range. Particular attention has been devoted to the lithium and caesium ion-pair acidities of diphenylamine in THF120 that, at 25 °C, have been found to be 19.05 and 24.20, respectively. 

Kinetic studies with caesium cyclohexylamide have also been performed593. For reaction of tritiated benzene, the kinetic order was one in both hydrocarbon and caesium cyclohexylamide ion-pair (Table 180) and rate coefficients were... 

If the proposed structure was true, then a solvent of higher solvating power, such as dme, should decrease the stability of the intramolecular complex. Using sodium as the counterion part of the active centres should exist in the form of externally solvated contact ion pairs or even solvent-separated ion pairs. With caesium as the counterion there should be little change, because this cation is only poorly solvated by both solvents and consequently the possibility of solvent-separated ion pairs to be found should be extremely small. 

Equilibrium contact ion-pair acidities have been reported for monomeric lithium and caesium salts of several sulfones and a sulfoxide in TE1F.19... 

The ion pair status is not predictable only on the basis of the cation used. For highly delocalizable carbanions like 9-fluorenyl in THF, the CIP fraction was found to increase rapidly in the order Li < Na < K < Cs [22] and the caesium salt was found to be the most bathochromic in the series. The degree of association of carbanion ion pairs in non polar solvents is highly dependent upon the extent of charge delocalization [23]. With highly delocalizable anions SSIP will be formed with Li cation. A completely different sequence of ion pairs was shown in the case of naphtholate anions the CIP fraction increases in the order K < Na < Li [24], These anions are thus to be considered as more localized anions in order to agree with the preceding observations. 

In a recent study [175] of the methoxide ion catalysed tritium exchange in methanol of weakly acidic fluorinated bicycloalkanes (pIT ca. 20), comparison of the rates of exchange for six compounds, with ion-pair pK values determined in cyclohexylamine containing caesium ions, gave a Bronsted plot with slope (a) within experimental error of unity. The isotope effects for proton compared with deuteron transfer (feH/feD = 1.2) are similar to those observed with chloroform. The charge on these polyfluorocarbanions may also be localized on carbon [175]. 

Perhaps more important, however, are the initial studies of Van Beylen and his collaborators on the dynamics of dissociation of carboanionic species studied by the technique of electric field relaxation. With fluorenyl lithium in pure diethyl ether and added traces of tetrahydrofuran the overall rate constant for dissociation displays a negative activation energy, strongly suggesting that dissociation does not occur directly from contact on tight ion pairs, which are present in vast excess, but rather via a small number of solvent-separated species. This is important because under identical conditions loose ion pairs cannot be detected spectroscopically. Similarly, with polystyryl caesium in tetrahydrofuran the results point to the presence of a small concentration of loose ion pairs and seems to support the hypothesis of Lohr and Schulz. 

With fluorenyl caesium (FICs) in dimethoxymethane, addition of tetrahydro-furan appears to generate triple ions, Fl2Cs and FICs2, in equilibrium with ion pairs which are only weakly dissociated into free ions. The derived thermodynamic data indicate that dimethoxymethane, although having a lower dielectric constant than tetrahydrofuran, specifically solvates Cs". ... 

Lohr and Schulzhave re-determined kji ) in THF for polystyryl caesium as 150 M s at 25 °C, with ATj = 5 x 10 M. These data now correlate well with similar data for DMA and tetrahydropyran as solvents, and this removes a previous anomaly. The frequency factor for the reaction, however, is still low and arguments have been presented against the previous submission that loose ion pairs do not make a contribution with this counterion. 

Copolymerization studies involving the Cs salts of living polymers of p-substituted styrenes and 1,1-diphenylethylene in THF have enabled the reactivities of the corresponding free ions and ion pairs to be assessed. The method used for the calculation of rate constants appears to be satisfactory, and the values obtained in the case of poly-(p-methylstyryl) caesium and poly-(/ -methoxystyryl) caesium correlate well with the accepted values for the unsubstituted styrene system. For example, kp —) values in THF at 0 °C are 95 000, 220 000, and 1 000 000 M" s for the series styrene, p-methylstyrene,... 

Most recently the polymerization of methyl methacrylate with Ba counterion has been reported. In THF at —70 °C the rate constant for propagation is independent of the active centre concentration and growth seems to occur via ion pair species only. The kinetic data obtained compare favourably with those for polymethylmethacryl sodium and caesium.As before, active centres are terminated by side-reactions involving the ester group. 

The kinetics of the polymerization of styrene initiated by cumyl caesium in dimethoxyethane have been re-investigated and it was concluded that loose ion pairs are kinetically significant. The initiation of the polymerization of ethylene oxide initiated by iluorenyl sodium is not a straigditforward process. When a molecule of ethylene oxide is added to fluorenyl carbanion, the hydrogen atom on the 9-position of the adduct is susceptible to removal by a second fluorenyl... 

Moyer et al. demonstrated that anion complexes of 57 could include large charge-diffuse cations, such as caesium and imidazolium, in the anion-induced calix [4]pyrrole cup [114]. Shielding of the imidazolium CH atoms in CD2CI2 in the presence of an anion-binding calixpyrrole was observed and considered as evidence for ion-pair complexation in solution. Similar behavior was also observed for N-ethylpyridinium cation [115]. 

Figure 3.39 Formation of anomeric pairs of glycosyl trichloroacetimidates and 2,4-dinitrophenolates. Anhydrous caesium carbonate is also used for trichloroacetimidate formation. In the 2,4-dinitrophenolate epimerisat-ion, nucleophilic attack by oxygen rather than sulfur is proposed since sp -hybridised carbon is a hard electrophile.

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