Cation-pool method steps

In the cation pool method organic cations are generated by electrochemical oxidation and are accumulated in a solution. In the next step, a suitable nucleophile is added to the thus-generated solution of the cation. In the cation flow method organic cations are generated by electrochemical oxidation using a microflow cell. The cation thus generated is allowed to react with a nucleophile in the flow system. 

The cation pool method is based on the irreversible oxidative generation of organic cations. In the first step, the cation precursor is oxidized via an electrochemical method. An organic cation thus generated is accumulated in the solution in the absence of a nucleophile that we want to introduce onto the cationic carbon. Counter anions which are normally considered to be very weak nucleophiles are used to avoid the nucleophilic attack on the cationic center. In order to avoid thermal decomposition of the cation, electrolysis should be carried out at low temperatures such as -78 °C. After electrolysis is complete, the nucleophile is then added to obtain the desired product. The use of a carbon nucleophile results the direct carbon-carbon bond formation. 

Chemical generation of cation pools is also effective. As an active reagent used for the cation generation is prepared electrochemically, this method is called the indirect cation-pool method. The method involves three steps. In the first step, an active reagent is generated and accumulated electrochemically. In the second step, the active reagent thus generated... 

The following example demonstrates the utility of the indirect cation-pool method. The first step is the electrochemical generation of ArS(ArSSAr) which was characterized by NMR and CSI-MS. ArS(ArSSAr) serves as a quite effective chemical reagent for the generation of alkoxycarbenium ions from a-ArS-substituted ethers, presumably because of its high thiophilicity (Scheme 5.25). The conversion is complete within 5 min at —78 °C. The alkoxycarbenium ion pool thus obtained exhibits similar stability and reactivity to that obtained with the direct electrochemical method. Therefore, alkoxycarbenium ion pools generated by the indirect method also serve as powerful reagents for flash chemistry. 

The electrochemical method is also effective for the oxidation of heteroatom compounds. For example, oxidation of carbamates using a microflow electrochemical cell leads to the formation of N-acyliminium ion, which is allowed to react with various carbon nucleophiles such as allylsilanes in the flow system (Figure 7.5). This is a microflow version of the cation pool method, in which highly reactive organic cations are generated and accumulated in the absence of nucleophile and are allowed to react with nucleophiles in the next step [36-47]. The microflow version is called the cation flow method [48, 49]. The cation flow method can be applied, in principle, to more reactive and unstable organic cations, which are difficult to accumulate in a macro-scale batch system. 

Each ion-radical reaction involves steps of electron transfer and further conversion of ion-radicals. Ion-radicals may either be consnmed within the solvent cage or pass into the solvent pool. If they pass into the solvent pool, the method of inhibitors will determine whether the ion-radicals are prodnced on the main pathway of the reaction, that is, whether these ion-radicals are necessary to obtain the hnal prodnct. Depending on its nature, the inhibitor may oxidize the anion-radical or reduce the cation-radical. Thns, quinones are such oxidizers whereas hydroquinones are reducers. Because both anion and cation-radicals are often formed at the first steps of many ion-radical reactions, qninohydrones— mixtures of quinones and hydroquinones—turn out to be very effective inhibitors. Linares and Nudehnan (2003) successfully used these inhibitors in studies on the mechanism of reactions between carbon monoxide and lithiated aromatic heterocycles. 

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