Cations electrochemical generation

Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA) differs from the other promoters in that its cation is a radical, and as such produces radical cationic sulfonium ions as glycosylating species from thioglycosides.85 The use of this promoter arose from earlier work on the electrochemical generation of 5-glycosyl radical cations as glycosylating species. 

ESR spectra of the radical anions of 3-phenoxy-, 3,4-diphenoxy-, and 3,4-dichloro-l,2,5-thiadiazole and of the radical cations of various 3-aryloxy-4-morpholino-l,2,5-thiadiazoles have been obtained by the electrochemical generation of the ions in 3 x 10-3M solutions of the thiadiazoles in the system MeCN/Et4NC104 (ca. 0.1 M) on a platinum helix electrode directly in an ESR resonator at first wave potentials <2003RJC806>. 

Energetic electron transfer reactions between electrochemically generated, shortlived, radical cations and anions of polyaromatic hydrocarbons are often accompanied by the emission of light, due to the formation of excited species. Such ECL reactions are carried out in organic solvents such as dimethylformamide or acetonitrile, with typically a tetrabutylammonium salt as a supporting electrolyte. The general mechanism proposed for these reactions is as follows. 

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. 

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. 

We chose to study the generation of alkoxycarbenium ion 26 from thioacetal 28. The electrochemically generated ArS(ArSSAr)+, 37 which was well characterized by CSI-MS, was found to be quite effective for the generation of alkoxycarbenium ions, presumably because of its high thiophilicity (Scheme 17). The conversion of 28 to 26 requires 5 min at -78 °C. The alkoxycarbenium ion pool 26 thus obtained exhibited similar stability and reactivity to that obtained with the direct electrochemical method. The indirect cation pool method serves a powerful tool not only for mechanistic studies on highly reactive cations but also for rapid parallel synthesis. 

It has been found that the electrochemically generated NO radical addes to the substituted olefins 81, and the radical species 81a formed is further oxidized to the cationic intermediate 81b which reacts with acetonitrile and yields 82 (Scheme 41). The anodic oxidation was carried out in a mixed solvent CH3CN-Et20 with NaNOa as a supporting electrolyte. The oxazoline derivatives 82 were isolated in 69-77% yield [103],... 

Electrochemically generated solutions of radical-cations will react with nucleophiles in an inert solvent to generate a radical intemiediate. Under these conditions the intermediate is oxidised to the carboniiim ion by a further radical-cation. Generally, an aromatic system is then reformed by loss of a proton. Reactions of 9,10-diphenylanthracene radical-cation nucleophiles in acetonitrile are conveniently followed either by stop flow techniques or by spectroelectrochemistry. Reaction with chloride ion follows the course shown in Scheme 6.2, where the termination... 

An electrochemical and ESR study of 2,7-disubstituted phenazines has appeared <1996CPB1448>. The electrochemically generated radical cation of phenazine A, A -dioxide was investigated by ESR electrolysis and cyclic voltammetry <2002MI4245>. Time-resolved and steady-state ESR spectra were observed for the lowest excited triplet (Ti) states of phenazine and its monoprotonated cation (phenazinium) in sulfuric acid-ethanol mixture at 77 K <2005SAA1147>. 

By media variables we mean the solvent, electrolyte, and electrodes employed in electrochemical generation of excited states. The roles which these play in the emissive process have not been sufficiently investigated. The combination of A vV-dimethylformamide, or acetonitrile, tetra-n-butylammonium perchlorate and platinum have been most commonly reported because they have been found empirically to function well. Despite various inadequacies of these systems, however, relatively little has been done to find and develop improved conditions under which emission could be seen and studied. Electrochemiluminescence emission has also been observed in dimethyl sulfite, propylene carbonate, 1,2-dimethoxyethane, trimethylacetonitrile, and benzonitrile.17 Recently the last of these has proven very useful for stabilizing the rubrene cation radical.65,66 Other electrolytes that have been tried are tetraethylam-monium bromide and perchlorate1 and tetra-n-butylammonium bromide and iodide.5 Emission has also been observed with gold,4 mercury,5 and transparent tin oxide electrodes,9 but few studies have yet been made1 as to the effects of electrode construction and orientation on the emission character. 

For example, alkyl ammonium-stabilized metal nanoparticles were generated by electrochemical process. A target bulk metal sheet is settled as an anode in an electrochemical cell as shown in Figure 9.1.1. Metal cations are generated at the anode and move to the cathode. Metal ions are reduced there by electrons generated from the cathode to form zero-valence metal atoms. In many cases, the zero-valence metal atoms are deposited onto the cathode metal sheet (usually platinum) or precipi-... 

Extremely high ECL efficiencies seem to be a common feature of the homoleptic-IrL3 as well as the heteroleptic-L2Ir(X) iridium(III) cyclometallated complexes. Extremely high ECL efficiencies (up to 0.55) were observed via ion annihilation between the electrochemically generated L2Ir(acac)+ or L2Ir(pico) + cations (where... 

Oxidative cleavage by means of electrochemically generated cation-radicals is also possible thus benzyl ethers may be cleaved and carboxylates decarboxylated using cation-radicals of brominated triphenylamines in acetonitrile containing a weak base.34 35 Such as indirect reaction makes it... 

The anodic oxidation of 2,4,5-triarylimidazole was studied in aprotic solvents.291-295 The 2,4,5-triarylimidazole anions undergo a one-electron oxidation, forming dimeric bis-(2,4,5-triarylimidazolyls).294 The isomeric bis imidazolyls consist of imidazole and isoimidazole systems. The dimerization is a result of a nucleophilic attack of 2,4,5-triarylimidazole anions on the electrochemically generated 2,4,5-triarylimidazolium cations. 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

The previous review in this series1 discussed the variations which can be expected in the structure and reactivity of electrochemically generated anion radicals and cation radicals as the group X in C=X is varied through C, N and O. The reader is referred to the previous review for that discussion, which will not be repeated here. 

Recently, a general synthesis of a-formyloxycarbonyl compounds was reported. Yields ranging from 35-90% were achieved via electrochemical generation of enol carbonate cation radicals in DMF156. The cation radicals are trapped by the solvent, and the resulting formiminium ion is hydrolyzed during workup. The mechanism is shown in Scheme 62. 

In the majority of cases, the electrochemical generation of organic cation radicals takes place in an ampoule lowered into an ESR cavity. Sometimes, however, exhaustive external generation and use of a flow system allow one to obtain an ESR spectrum that is far better resolved (see, for example, Seo et al. 1966). 

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