Carboxylate exchange

Carboxylate exchangers contain —COOH groups which have weak acidic properties and will only function as cation exchangers when the pH is sufficiently high (pH > 6) to permit complete dissociation of the —COOH site. Outside this range the ion exchanger can be used only at the cost of reduced capacity. 

The complexes are 1 1 electrolytes in solution. Other such complexes can be made by a similar route or by halide (or carboxylate) exchange. The first monomeric system Ru2C1(02C.C4H4N)4 (thf), where the ruthenium at one end of the lantern is bound to a thf and the other to a chloride, has recently been made [97], [Ru2Cl(02CBut)4(H20)] and [R Cl CPr thf)] are also monomeric [98],... 

Reaction of OsCl6 with acetic acid/acetic anhydride mixtures containing concentrated HC1 gives the diosmium compound Os2(OAc)4C12 (rather than mixed-valence species, see section 1.8.3) other carboxylates can be made by carboxylate exchange ... 

Scheme 20 Synthesis of heteroleptic tetramethylaluminate complexes via [carboxylate] [alkyl] exchange (top) or [alkyl] - [carboxylate] exchange (bottom, tetramethylaluminate route ) [147,148]...

Certain carboxylates R3Sn02CR (e.g. R = H, C=CR, CH2=CH, or CeFs) can be decarboxylated to RsSuR (equation 101). Other reactions include halide/carboxylate exchanges (equation 102). 

Hara and Cady (105) prepared U02(02CCF3)2 by carboxylate exchange from the corresponding acetate and showed that the compound is very moisture-sensitive and virtually insoluble in trifiuoro-acetic acid. 

It is equally interesting to compare the Ni ion distribution in ion exchanger VPC which is highly selective to Ni ions and in weak-acid carboxylic exchanger CB-4 with no selectivity for Ni ion (Fig. 11). It is evident that Ni concentration distribution depends on the type of exchange isotherm, rectangular for CB-4 resin and unfavorable for VPC resin. 

The inhibition of pyruvate formate-lyase by hypophosphite was first observed by Novelli in work on the CoA-independent carboxylate exchange reaction between pyruvate and formate (186). In a more detailed study by Knappe et al., time-dependent inactivation is observed to occur with concomitant loss of the enzyme free radical EPR signal (180). The inactivation kinetics are first order and the rate of inactivation is accelerated when the enzyme is in the acetylated form. Furthermore, inactivation by [ HJhypophosphite leads to the stoichiometric release of tritium to H2O, and treatment of PFL with [ P]hypophosphite produces an alkali-labile radiolabel that is covalently bound to the inactive enzyme (180). 

One route to weak cation exchangers based on styrenic resins also starts from the chloromethylated resin. The chloro group is converted to a cyano group by reaction with KCN, and the cyano group is then hydrolyzed to a carboxylate. Exchangers based on this chemistry are rarely used in HPLC. A more convenient route to weak cation exchangers is through acrylic add derivatives. 

Although compounds 3a and 4a yield very similar products following oxidation, the kinetics of the two reactions highlight important differences. Compound 4a is coordinatively saturated, but undergoes rapid carboxylate exchange at room temperature. The intermediate in the exchange process is the carboxylate-shifted form, which is probably stabilized by methanol coordination. Carboxylate shifts of this kind have been previously delineated, and the present data require such a shift for meaningful interpretation. Monomer/dimer equilibria have also been observed in some diiron(II) compounds and the consequences of such an equilibrium are manifest in the kinetics. 

In addition to the commonly employed sulfonic acid cation exchanger, resins based on the carboxylic acid groups are sometimes employed under special conditions. These resins are effective for exchange reactions in neutral and alkaline solution. For example, the effluent from a saltsplitting reaction. Equation 5.23, can be treated with a carboxylic exchanger according to the reaction ... 

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