Preparative scale electrolysis

With reference compounds, from which the number of electrons n that are transferred is known, e.g. ferrocene with n = 1, for an unknown compound can be approximately calculated from the peak current in the cyclovoltammogram. New and p a peaks in the cyclovoltammogram can be compared with the values of possible electroactive products. From cyclovoltammetry of 10-20 mg of substrate, valuable information can be obtained prior to preparative scale electrolysis on the best suited electrolyte and electrode, the oxidation or reduction potential of the substrate, the reactivity of the intermediate, the number of electrons n transferred and possible products. More information on the mechanism and kinetics of the reactive intermediates can be gained from a variety of sophisticated electroanalytical techniques [12]. 

In preparative laboratory scale electrolysis, in general between 1 and 100 mmol of substrate is converted. For that purpose the undivided or divided cell shown in Figs. 2 and 3, respectively, are suitable. 

For larger scale eonversions, the circulation cell [2b], the capillary gap cell [14a] and the Swiss roll eell [14b] are available. All three eells work at low cell voltage 

For performing a preparative electrolysis the following procedure is recommended  

At first a current/voltage curve from the electrolyte is taken. High currents below the decomposition potential point to electroactive impurities in the electrolyte. These can often be removed by purging with nitrogen to remove oxygen or by preelectrolysis close to the decomposition potential. 

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Preparative-scale electrolysis is a second type of electrochemical experiment, where, conversely, consumption of the maximal amount of substrate in... 

Although separate determination of the kinetic and thermodynamic parameters of electron transfer to transient radicals is certainly important from a fundamental point of view, the cyclic voltammetric determination of the reduction potentials and dimerization parameters may be useful to devise preparative-scale strategies. In preparative-scale electrolysis (Section 2.3) these parameters are the same as in cyclic voltammetry after replacement in equations (2.39) and (2.40) of Fv/IZT by D/52. For example, a diffusion layer thickness S = 5 x 10-2 cm is equivalent to v = 0.01 V/s. The parameters thus adapted, with no necessity of separating the kinetic and thermodynamic parameters of electron transfer, may thus be used to defined optimized preparative-scale strategies according to the principles defined and illustrated in Section 2.4. 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

The considerations, prior to beginning, must include special characteristics of electrochemical reactions and their practical consequences in a preparative scale electrolysis ... 

The C=C double bond in 4-methylcou-marin has been hydrogenated at a Hg cathode in the presence of alkaloids with 17% ee [388]. By systematic variation of pH, supporting electrolyte, working potential, substrate, and alkaloid (yohimbine) concentration, the enantioselectivity has been increased to 67% ee. A mechanism supported by electroanalytical data and preparative scale electrolysis is proposed. 

An analogous mechanism, such as for 4-methylcoumarin, is supported by preparative scale electrolysis and cyclovoltammetry. The mechanistic model enabled the synthesis of simple nonracemic catalysts, from which (S)-A-methylprolinooctadecylester leads to... 

Refs. [i] Steckhan E (1996) Electroorganic synthesis. In Kissinger PT, Heineman WR (eds) Laboratory techniques in analytical chemistry, 2nd edn. Marcel Dekker, New York, pp. 641-682 [ii] Jorissen J (2004) Practical aspects of preparative scale electrolysis. In Bard AJ, Stratmann M, Schafer HJ (eds) Organic electrochemistry. Encyclopedia of electrochemistry, vol. 8. Wiley-VCH, Weinheim, pp 29... 

The absence of substitution was confirmed by preparative-scale electrolysis. 

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