Electrochemical microflow

In the cation flow method an organic cation is generated continuously by low temperature electrolysis using an electrochemical microflow reactor. The cation thus generated is immediately allowed to react with a carbon nucleophile in the flow system. This method, in principle, enables the manipulation of highly reactive organic cations. 

A schematic diagram of the cation flow method for generating N-acyliminium ion 2 is shown in Fig. 5. A solution of carbamate 1 is introduced into the anodic compartment of electrochemical microflow cell, where oxidation takes place on the surface of a carbon fiber electrode. A solution of trifluoromethanesulfonic acid (TfOH) was introduced in the cathodic compartment, where protons are reduced to generate dihydrogen on the surface of a platinum electrode. A-Acyliminium ion 2 thus generated can be analyzed by an in-line FT-IR analyzer to evaluate the concentration of the cation. The solution of the cation is then allowed to react with a nucleophile such as allyltrimethylsilane in the flow system to obtain the desired product 3. 

Suga, S. Okajima, M. Fujiwara, K Yoshida, J., Cation flow method a new approach to electrochemical conventional and combinatorial organic syntheses using electrochemical microflow systems,./. Am. Chem. Soc. 2001, 123, 7941-7942... 

Figure 5.8 Electrochemical microflow reactor for the cation-flow method (a) outside (b) inside (anodic part). Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

For example, the anodic oxidation of a silyl-substituted carbamate to generate a solution of N-acyliminium ion and the cathodic reduction of cinnamyl chloride in the presence of chlorotrimethylsilane to generate the corresponding allylsilane can be carried out simultaneously in a single electrochemical microflow cell under continuous flow conditions (Figure 5.10). The N-acyliminium ion, the anodic product, is allowed to react with the allylsilane, the cathodic product, to give the coupling product. 

An electrochemical microflow reactor consisting of a plate-to-plate electrode configuration mounted in a nonconducting housing has been developed. As shown in Figure 7.23, a 75 im thick polyimide foil between the working electrode and the counter electrode defines a unique distance. This polyimide foil contains microstructured slits (250 im wide). 

An electrochemical microflow reactor, which is composed of diflone and stainless steel bodies produced by a mechanical manufacturing technique, is shown in Figure 7.24. The reactor consists of a two-compartment electrochemical cell, which is divided by a PTFE membrane. Carbon felt (7 mm x 7 mm x 5 mm) made of carbon fibers = 10 im) is used as the electrode. 

Figure 7.23 Electrochemical microflow reactor with a plate-to-plate electrode...

Various electrochemical organic reactions have been carried out using electrochemical microflow reactors. Sometimes, electrolysis can be conducted without intentionally added supporting electrolyte by virtue of the extremely short distance between the anode and the cathode. 

Figure 12.9 An electrochemical microflow reactor for the cation flow method.

Electrochemical microflow systems have also attracted significant research interest from the viewpoint of electrolysis without an intentionally added supporting electrolyte, becausethe short distance between the electrodes andthehigh electrode surfaceto reactor volume ratio are advantageous for conductivity and reaction efficiency. 

Using this electrochemical microflow system, the anodic methoxylation of p-meth-oxytoluene was accomplished effectively without an intentionally added electrolyte. The oxidation of p-methoxytoluene was carried out in methanol. The anodic reaction... 

Figure 12.12 Methoxylation of N-methoxycarbonylpyrrolidine using an electrochemical microflow system without an intentionally added electrolyte.

The generation of the cation can be monitored using an FTIR spectrometer (ATR method) equipped with a low-temperature flow cell attached to the outlet of the electrochemical microflow reactor. The absorption at 1814 cm , which is assigned as the C=0 vibration, increases with increase in the electric current. An interesting application of the cation flow method is continuous sequential combinatorial... 

There is another type of microflow cell that is used for electrolyte-free electrolysis [64]. Two carbon fiber electrodes are separated by a spacer (porous PTFE membrane, pore size 3 pm, thickness 75 pm) at a distance of the order of micrometers. A substrate solution is fed into the anodic chamber where the oxidation takes place. The anodic solution flows through the spacer membrane into the cathodic chamber where the reduction takes place. The product solution leaves the cell from the cathodic chamber. In this cell, the electric current flow and the liquid flow are parallel. The effectiveness of the cell is shown by the oxidation of p-methoxytoluene. A solution of p-methoxytoluene in methanol is fed into the electrochemical microflow system and the reaction is carried out under constant current conditions to obtain the desired product in more than 90% yield based on consumed starting material (Figure 7.8). The microflow system can also be used for the oxidative methoxylation of N-methoxycarbonylpyrrolidine and acenaphthylene. 

Figure 7.7 Electrochemical microflow process without adding supporting electrolyte.

Yoshida, Cation Flow Method. A new approach to conventional and combinatorial organic syntheses using electrochemical microflow systems, J. 

Electrochemical Microflow Systems Electrosynthesis in Ionic Liquid... 

Rigo et al. [15] proposed a coulometric biosensor equipping an electrochemical microflow cell. A1 pL of H2O2 sample over 0.3-100 pM was introduced into the cell, in which the horseradish peroxidase (HPR)-modified electrode was installed. In the reaction, the 1, 4-benzoquinone was enzymatically generated and coulometrically electrolyzed at 0.1 V vs. Ag/AgCl. Due to the... 

Electrochemical Microflow Systems, Table 1 Common flow geometries and their corresponding limiting current expressions (in SI units)... 

Electrochemical Microflow Systems, Fig. 3 Viton microcharmel flow electrolysis cell (Redrawn from [42]) for two-electrode configuration electrolysis... 

Electrochemical Microflow Systems, Fig. 4 Galactosidase reaction monitored by fluorescence in a nano-channel reactor (Redrawn from [58])... 

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