Electrochemical catalyze reactions

Monitoring enzyme catalyzed reactions by voltammetry and amperometry is an extremely active area of bioelectrochemical interest. Whereas liquid chromatography provides selectivity, the use of enzymes to generate electroactive products provides specificity to electroanalytical techniques. In essence, enzymes are used as a derivatiz-ing agent to convert a nonelectroactive species into an electroactive species. Alternatively, electrochemistry has been used as a sensitive method to follow enzymatic reactions and to determine enzyme activity. Enzyme-linked immunoassays with electrochemical detection have been reported to provide even greater specificity and sensitivity than other enzyme linked electrochemical techniques. 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

Bromonaphthalene does not react with benzenethiol (thiophenol) salts. However, if electric current is passed through a solution containing 1-bromonaphthalene, the tetrabutylammonium salt of thiophenol, and DMSO, then l-(phenylthio)naphthalene is produced in 60% yield. When the reaction is conducted in acetonitrile, it leads to naphthalene above all (Pinson and Saveant 1978, Saveant 1980, Amatore et al. 1982). In the electrochemically provoked reaction, it is sufficient to set up the potential difference corresponding to the initial current of the reduction wave to transform 1-bromonaphtahalene into 1-naphthyl radical. The difference in the consumption of electricity is rather remarkable In the absence of thiophenolate, bromonaphthalene is reduced, accepting two electrons per molecule in the presence of thiophenolate, 1-bromonaphthalene is reduced accepting two electrons for every ten molecules. The reaction with the thiophenolate ion is catalyzed by electric current and takes a reaction path shown in Scheme 5.2. 

Regarding the cobalt-catalyzed reactions, the electrochemical analyses of the involved processes allowed the discovery of a chemical way for the synthesis of these organozinc compounds. This chemical way will be evoked in this chapter. 

The slow addition of the arylzinc compound to a solution containing both an aryl hahde, Ar X, and the palladium complex furnished the biaryl in good to excellent yields. The very reactive Pd(0) complex was likely formed in situ by reduction of the starting palladium(II) complex by ArZnX. The reactions were very rapid (ca 1 or 2 h) compared to most usual Pd-catalyzed reactions involving ArZnX (ca 24 h). The reduction of Pd(II) could account for the formation of a small amount of Ar-Ar (2-5%) in the last non-electrochemical step while no homocoupling of Ar X occurred. 

The electrochemical method described here is a useful alternative to the formation of anhydrous HCIO by chemical methods and can be employed for a variety of acid-catalyzed reactions. Anhydrous HCIO is not commercially available and must be prepared by the reaction of AgClO with dry HCl This method, however, is not feasible for preparation of a small amount of dry and pure HCIO without contamination by HCl. 

Diels-Alder reactions are thermal reactions requiring no catalysts (120). However, over the years both acid- and metal-based homogeneous or heterogeneous catalysts have been developed (121—127). Some catalysts used in Diels-Alder catalyzed reactions of butadiene are Fe(NO)2Cl—(CH3CH2)2A1C1, Pd[P(C H5)3]4, Cu(I) exchanged silica—alumina (128,129), large pore zeolites (130), and carbon molecular sieves. An electrochemical process has also been used to catalyze the self-condensation to vinylcyclohexene (131). When the asymmetric Ni catalyst (4) was used, specificity to the enantomeric (5)-4-vinylcyclohexene (132,133) was observed (26% enantiomeric excess). 

Polyquinolines have been used as polymer supports for transition-metal catalyzed reactions. The coordinating ability of polyquinoline ligands for specific transition metals has allowed their use as catalysts in hydroformylation reactions (99) and for the electrochemical oxidation of primary alcohols... 

While ionic strengths as low as 1 mM have been used with the cell illustrated in Figure 1, most LCEC experiments are carried out with a minimum of 0.05 M buffer salts in the mobile phase. Postcolumn mobile phase changes (pH, ionic strength, solvent content) and post-column reactions (redox cross reactions, derivatiza-tions, enzyme catalyzed reactions) can expand the utility of electrochemical as well as other detectors. These subjects have recently been treated in some detail (9). Suffice it to say that direct detection, without post-column chemistry, is always preferable (more reliable, more sensitive, less expensive). 

Several methods are available for estimating acidity in solution for the homogeneous superacids spectroscopy (UV, NMR), electrochemical methods, chemical kinetics, and heats of protonation of weak bases (9). Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. The most simple and useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions we usually compare the activity with that of Si02-Al20v... 

Another efficient method is the electrochemical oxidation of NADH at 0.585 V vs Ag/AgCl by means of ABTS2- (2,2,-azinobis(3-ethylbenzothiazoline-6-sulfonate)) as an electron transfer mediator [96]. Due to the unusual stability of the radical cation ABTS, the pair ABTS2 /ABTS is a useful mediator for application in large-scale synthesis even under basic conditions. Basic conditions are favorable for dehydrogenase catalyzed reactions. This electrochemical system for the oxidation of NADH using ABTS2 as mediator was successfully coupled with HLADH to catalyze the oxidation of a meso-diol (ws >-3,4-dihydroxymethylcyclohex-l-ene) to a chiral lactone ((3aA, 7aS )-3a,4,7,7a-tetrahydro-3//-isobenzofurane- l-one) with a yield of 93.5% and ee >99.5% (Fig. 18). 

The direct electrochemical oxidation of NAD(P)H is possible but for this purpose relatively high oxidation potentials are necessary and potential passivation of the electrodes can occur. Because of the low speed of NADH oxidation and to avoid fouling of the electrodes, mediators like phenanthroline derivates are often used for advanced electron transfer. Those systems can be coupled efficiently with enzyme-catalyzed reactions which require NAD(P)H oxidation. 

The electrochemical dealkylation of aliphatic amines is a useful way of mimicking enzymatic dealkylation. This has been effectively used for the synthesis of Af-dealkylated metabolites of drugs with much better efficiencies than the enzyme catalyzed reactions (Scheme 38) [2]. 

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