Carbocations cation-pool generation

Microsystem-controlled cationic polymerization technology requires extremely reactive initiators and cation pools serve as effective initiators for this technology. Usually carbocations are generated by a reversible process from their precursor. Yoshida et al. developed the cation pool method [35], in which carbocations are generated irreversibly by low-temperature electrolysis and are accumulated in relatively high concentration in the absence of nucleophiles. N-Acyliminium ions, alkoxycarbenium ions [36-40] and diarylcarbenium ions [41] have been generated by this method. Such cation pools are expected to serve as extremely reactive initiators for cationic polymerization. 

A divided cell equipped with a sintered glass separator, a carbon fiber anode, and a platinum cathode is used in order to avoid the electrochemical reduction of anodically generated carbocations. Tetrabutylammonium tetrafluoroborate is usually used as supporting electrolyte, and dichloromethane is in most cases suitable as solvent because of less nucleo-philicity and low viscosity at low temperature. Two equivalent of TfOH (trifluoromethane-sulfonic acid) to a cation precursor is added in the cathodic chamber to facilitate the reduction of protons in the cathodic process. The constant current electrolysis (20 mA) was then carried out at —78 °C with magnetic stirring until 2.0-2.5 F/mol of electricity was consumed to give a cation pool. Carbamates (oc-silyl carbamate) (Scheme 2a) [3], a-silyl ethers (Scheme 2b) [4], diarylmethanes (silylated diary Imethanes) (Scheme 2c) [5] can be... 

The cation pool method opens a new aspect of the chemistry based on carbocations, which have been considered to be difficult to manipulate in normal reaction media. These methods involve the generation of carbocations in the absence of nucleophiles, spectroscopic characterization, and reactions with a variety of carbon nucleophiles to achieve direct carbon-carbon bond formation. 

Although the applications to A-acyliminium ions, alkoxycarbeniumions, and benzylic cations were successful, it seems to be difficult to apply the method to less stabilized cations. The applicability of the cation pool method inebitably depends upon the stability of the cation that is accumulated. The cation flow method [20, 21] which involves generation of carbocations in a microflow electrochemical system should be much more favorable because of short residence times and efficient temperature control. 

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. 

A report has described a route to multiply alkylated thiophene derivatives from electrochemically derived diarylcarbenium ions (Scheme 46). Using the cation-pool technique - in which the cationic intermediate is generated in high concentration and pooled - the carbocation (228) alkylates thiophene, providing up to 58% of the trialkylated product (229) and 24% of the dialkylated product (230). 

In 1999, the cation pool method debuted for the first time in a CDC reaction (Scheme 8.55). Yoshida carried out the electrolysis of carbamate (109) at low temperature, and the generated cation pool of carbocations (or iminium ions) (110) was further reacted with various nucleophiles, affording pipelidine derivatives 111 in moderate yields. This is the conventional and direct method for oxidative C-C bond formation. 

The reaction of an N-acyliminium ion pool with an alkene or alkyne followed by trapping of the resulting carbocation by water leads to the formation of the corresponding carbohydroxylation product. Cationic sequential one-pot, three-component coupling reactions of an N-acyliminium ion can also be accomplished using an electron-rich olefin and a suitable nucleophile that traps the thus-generated cationic intermediate as shown in Scheme 5.17. ... 

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