Carboxylate anions cations, reaction with

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

According to this mechanism, initiation includes reaction between the tertiary amine and epoxide, and the primary active centre is represented by a zwitterion with an alkoxide anion and an irreversibly bound amine in the form of an ammonium cation (Eq. (69)). This zwitterion reacts in the next step with the anhydride (Eq. (70)) yielding a carboxylate anion. The growth reactions (Eqs. (71) and (72)) include interactions of the carboxylate anion with epoxide, and of the alkoxide anion with the anhydride. 

A further dependence of the selectivity between different nucleophiles on the stability and reactivity of carbocations was found by Richard and Amyes in a study of reactions of alcohols and carboxylate anions with -substituted a-trifluoromethyl benzyl cations (75, X = Me, OMe, SMe, N(Me)CH2CF3, and NMe2) monitored using the azide clock.305 Apart from the methyl-substituted substrate, for which the reactions approached diffusion control,... 

Mono- or poly-carboxylated anions always gives rise to complex salts with the metal cations, which are insoluble in the reaction medium, or can be made insoluble by addition of appropriate cosolvents, making the isolation procedure more easy. 

Diazomethane methylates carboxylic acids because carboxylic acids readily protonate it, giving an extremely unstable diazonium cation. This compound is desperate to lose N2, the world s best leaving group, and so it does, with the N2 being substituted by the carboxylate anion. The carboxylate anion is in exactly the right position to carry out an S 2 reaction and that is what we have drawn. 

As already developed in the start of this section, the reaction of carboxylate anions with 1-chloroethyl carbonates is widely used for the preparation of commercial prodrugs. The hard nucleophile R COO" attacks selectively the soft center B, that is apparently contrary to the HSAB theory. However, the required use of added Nal may favour a cation-like transition state, the cationic intermediate having therefore two hard electrophilic centers and B attack would not be in violation of the rule [Scheme 80],... 

The second-order kylation rates of carboxylate salts are influenced by the nature of the cation, solvent and the alkyl halides. In general, carboxylate anions with soft (large) counterions are more reactive than those with hard counterions because of higher charge separation in the former. Thus, the anticipated order of increasing reactivity Li (l) < Na (l.l) < K Cl.S) < Cs+(2) was observed in the reaction of 2-methyl-2-propylpentanoate with 1-iodopentane at (Xi C in HMPA-EtOH (1 1 v/v). ... 

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