Cation-pool method electrochemical oxidation

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. 

The cation pool method and the cation flow method, which are based on low temperature electrochemical oxidation, have opened up a new chapter in the chemistry of organic cations, which have been considered to be difficult to manipulate in normal reaction media. The indirect cation pool method, which... 

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. 

By using the low temperature electrochemical oxidation, other reactive cationic species such as iodonium ions (I" ) [18] and sulfonium ion equivalents (ArS(ArSSAr) ) [19] can also be generated and accumulated in a similar fashion to the cation pool method. 

As illustrated in Fig. 1, one prominent path followed by electrogenerated cation radicals involves loss of a proton to form a neutral radical (R). Such radicals are easier to oxidize than the initial alkene. This second one-electron oxidation produces a carbocation (R - e R" ). If the electrochemical oxidation is carried out in an inert solvent such as dichloromethane, the carbocation is highly reactive. Many useful reactions have been carried out using carbocations generated in this manner (Fig. 4) [1, 6, 7]. Sometimes the carbocation may be unstable at temperamres near ambient, decomposing before it can react with an added reactant. Concern over this problem has led to the very useful concept known as the cation pool method in which the electrochemical reaction is carried out at a low temperature (typically 78 °C) at which the carbocation is stable. The resulting solution is known as the cation pool. 

We will focus on reagent-mediated oxidations and will therefore not cover electrochemical oxidations, though significant work has been done on the relevant reactions.This means we will not be covering Yoshida s cation pool method for the electrochemical generation of cations to which nucleophiles may be added,though some of the mechanistic details will overlap. Since a major benefit of CDC reactions is their one-pot nature we will deprioritize stepwise processes, for example Af-halogenations that can be used in Hofmann-Loffler-type cyclizations. ... 

However, the use of low-temperature electrochemical oxidation allows generation and storage of organic cations such as Af-acyliminium ions in a normal reaction medium such as dichloromethane in the absence of a nucleophile. This method is called the cation pool method (Fig. 7.4) [5-7]. 

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