Electrochemical generation highly reactive

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

The highly reactive nature of (P)Rh is perhaps best demonstrated by the reaction of electrochemically generated (TPP)Rh with terminal alkenes and alkynes. The overall reaction with alkynes is given by Equation 7 and the suggested mechanism... 

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. 

Yoshida et al. reported that generation and online detection of highly reactive carbocations from carbamates were accomplished by integrating an electrochemical microreactor with an FTIR spectrometer [57]. They also demonstrated that both the carbocations and nucleophiles could be generated using the paired electrochemical flow system to give the coupling products in reasonable yields (Scheme 4.42) [58]. 

The high yield reduction of 1,2-dihalides to produce olefins has been employed to advantage to prepare reactive olefins. Electron transfer in electrochemistry is proportional to the diffusion coefficient, which is related in a much less sensitive way to temperature changes than is chemical reactivity. Thus it may become possible to synthesize and study electro-chemically species whose chemical reactivity is high by working at low temperatures. Electroreduction of 1,2-dibromobenzocyclobutene (144) in acetonitrile or butyroni-trile/TEAP or chemical reduction using the biphenyl radical anion resulted in the formation of benzocyclobutadiene (145)128. Efforts to observe the electrochemically generated anion radical or dianion of benzocyclobutadiene indicated that dimerization to 146 was faster than further reduction (equation 84). 

From InCl3, highly reactive low-valent indium(i) species are electrochemically generated and regenerated. These are used for allylation and propargylation of carbonyl compounds (Scheme 3). Of special interest are bisallylations of aromatic and aliphatic esters, since such conversions cannot be achieved by using conventional stoichiometric allylations of esters by means of indium metal or Ini.66... 

The synthetic value of double bonds arises from their high reactivity towards simple electrophiles, enophiles and radicals. The latter species can easily be generated by oxidation on BDD electrodes. Thus, advantage can be taken of electrogenerated oxyradicals. Although aliphatic olefins proved to be stable towards electrochemical oxidation on BDD anodes in aqueous media (Baumer and Schafer 2005), activated olefins were successfully converted using acidic methanolic electrolytes (Griesbach et al. 2005). 

Benzyne, generated by the diazotization of anthranilic acid with isoamyl nitrite, is added to Gd Cs2 forming two isolable isomers of mono-adducts. Electrochemical measurements disclosed that the electronic structure of pristine Gd Cs2 has changed dramatically. Because of the high reactivity of benzyne, multiple adducts are not avoidable, even at lower temperatures [146]. 

PC is of potential use in organic electrochemistry [404-406]. It has a high dielectric constant (e = 69) and a wide liquid range (—49 to 242°C), but it is more reactive than acetonitrile, and the potential limit for anodic reactions is lower than for acetonitrile. It has so far been used mostly for electrochemical generation of cation radicals [406]. 

In general, the thermodynamics of free radicals has been, up to now, almost unknown due to their extremely high reactivity and electroactivity. For clarification, several methods were proposed [225,226] from anodic half-wave potentials of carbanions and organo-lithium compounds. Redox catalysis may strongly help to estimate redox potential of a certain number of radicals generated electrochemically. Thus, as seen previously in Scheme 4 and in Scheme 6, in the absence of other side reactions, there are two main different ways... 

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