Electrochemical Microflow Cells

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. 

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. 

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... 

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. 

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. 

Using a similar type of the microflow cell, Haswell s group reported the electroreductive coupling of activated olefins and benzyl bromide derivatives (Equation 12.8) [40]. The microflow electrochemical reactor can be easily multiplexed to generate a number of parallel flow cells, which offer the performance of a single cell while increasing the volumetric throughput of the system. 

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. 

The electrochemical reductive coupling reaction of various aikenes with benzyl bromides can also been achieved in the absence of supporting electrolyte using the microflow cell (Figure 7.20) [lOlj. When the inter-electrode gap is 160 pm, the desired cross coupling product is obtained effectively, whereas a significant amount of homocoupling product is obtained when the gap is 320 pm. 

As mentioned in Section 7.1.2, the electrochemical reduction of allylic halides in the presence of chlorotrimethylsilane can be achieved using a microflow cell and the desired allylic silanes are obtained (Table 7.2). 

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

MWNTs favored the detection of insecticide from 1.5 to 80 nM with a detection limit of InM at an inhibition of 10% (Fig. 2.7). Bucur et al. [58] employed two kinds of AChE, wild type Drosophila melanogaster and a mutant E69W, for the pesticide detection using flow injection analysis. Mutant AChE showed lower detection limit (1 X 10-7 M) than the wild type (1 X 10 6 M) for omethoate. An amperometric FIA biosensor was reported by immobilizing OPH on aminopropyl control pore glass beads [27], The amperometric response of the biosensor was linear up to 120 and 140 pM for paraoxon and methyl-parathion, respectively, with a detection limit of 20 nM (for both the pesticides). Neufeld et al. [59] reported a sensitive, rapid, small, and inexpensive amperometric microflow injection electrochemical biosensor for the identification and quantification of dimethyl 2,2 -dichlorovinyl phosphate (DDVP) on the spot. The electrochemical cell was made up of a screen-printed electrode covered with an enzymatic membrane and combined with a flow cell and computer-controlled potentiostat. Potassium hexacyanoferrate (III) was used as mediator to generate very sharp, rapid, and reproducible electric signals. Other reports on pesticide biosensors could be found in review [17],... 

Marken and co-workers accomplished electrolysis without an intentionally added electrolyte by using a simple microflow electrochemical cell having a parallel electrode configuration [37]. Two electrodes are placed facing each other at a distance of the order of micrometers, and a substrate solution flows through the chamber. In this system, the liquid flow and the current flow are perpendicular. 

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. 

Electrochemical oxidation of fiirans can also been carried out without intentionally added electrolyte using a microflow system. I n this case, an electrochemical thin-layer flow cell, which has a simple geometry with a glassy carbon anode and a platinum cathode directly facing each other at a distance of 80 pm apart is used (Figure 7.9) [65, 66]. 2,5-Dimethoxy-2,5-dihydrofuran is obtained in 98% yield by the oxidation of furan in methanol solvent. Similar electrochemical methoxylation and acetoxylation of various organic molecules can also be carried out using this system. 

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