Counter-cation effect

The most interesting results of the computational data, however, concern the addition of the second electron which is strongly endothermic in all cases. In other words, these dianions are predicted to dissociate an electron in the gas phase spontaneously. In solution or the solid state, it is possible that the ions could be stabilized by the effects of counter cations. This stabilization has been accomplished for the boron dianions which crystallized as contact ion pairs [47-50]. However, as indicated above, no success has been achieved so far in... 

Several variables were studied such as the counter cation(s) in the zeolite, the activation temperature, the solvent and the effect of additives. 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

A particularly interesting aspect of the reaction is its apparent counter-cation dependence. Our initial screen of bases included inorganic bases with a variety of counterions, including Li+, Na+, K+, and MgX+, but only potassium bases were effective. The precise role of the counterion is not completely understood, but it is critical for success. To date, the use of KOf-Bu or KH have consistently provided the best results, with KHMDS used as a lower-yielding alternative if absolutely necessary (in cases where the substrate was unstable to alkoxide bases). 

There are many papers which purport to record the effect of counter-ion on such factors as transfer constants, co-polymerisation ratios, etc. It is significant that in most of these studies relatively high initiator concentrations have been used, so that counter-ion effects are more likely but before accepting that the observed effects are indeed due to change of counter-ion (derived from different catalysts or co-catalysts) it must be ascertained that these polymerisations are in fact cationic and not pseudo-cationic - in which case the effects would stem from the different reactivities of different esters (see Section 5). 

Figure 7 shows a plot of x as a function of cp. A rough correlation can be seen if cp is positive then x is positive and if cp is negative then x is negative. The crystal structure of XAGWUX contains two independent molecules with opposite conformations. One of the molecules is syn-syn the other is anti-anti. This occurrence frustrates any attempts to correlate the conformation of the cation with the acidic strength of the anion or other counter ion effects. 

To elucidate the role of one of the two carboxyl groups of TA in the adsorbed state on the catalyst, a study was conducted on the effect of the cation which was used for the pH adjustment of the modifying solution on the EDA of MRNi (29). As shown in Fig. 23, the EDA of TA-MRNi was strongly affected by the kind of cation used, and sodium was found to be the most favorable one, although the EDA of (S)-2-hydroxyisovaleric acid-MRNi was not affected, as shown in Table XXII. From this finding it can be deduced that one of the carboxyl groups of TA participates in the adsorption, while the other must exist as a carboxyl anion and that the counter cation must be present near the carboxyl ion. 

The effect of the counter cation (Table XI) may be in a manner similar to those proposed for alkyllithium and allyllithium reagents. 

Both charged and neutral anion binding hosts are common. Charged hosts must compete with the counter cation while neutral hosts are effectively ion-pair binding hosts. 

---