Electrochemical dimerisation

M. Y. Fioshin u. A. P. Tomilov, Electrochemical Dimerisation - a promising Method for the Synthesis of organic Compounds, Khim. Prom. 1964. 649. 

Summaries on dimerisation and oligomerisation reactions in ionic liquids have been published previously.[3,4] Predominantly complexes of either palladium or nickel have been employed in ionic liquids for dimerisation/oligomerisation reactions, but there are also examples where iron,[5] tungsten1 and rhodium17 have been used. Apart from metal catalysed dimerisation/oligomerisation reactions in ionic liquids, examples of electrochemical dimerisation have been reported, which include arylhalidcs17 and 3-(4-fluorophenyl) tluophcnc1 as substrates. 

The final method was an electrochemical reductive dimcrisation working extremely efficiently and capable of dimerising many electron-deficient olefins. 

The natural assumption made by a large number of researchers in the field of electrochemical C02 reduction was that the intermediate was C02, as postulated by Haynes and Sawyer (1967). The observation of oxalate as a major product in addition to, or in competition with, the formation of CO, CO, HCOj and HCOO , increased the attention focused on the reactive intermediate and the mechanisms by which it reacted. However, controversy has arisen over whether the subsequent reaction of the CO 2 was via dimerisation (the EC mechanism) or via attack on another C02 molecule (the ECE mechanism). In addition, the existence of such species as CO 2 (ads) and HCOO (ads) have also been suggested but, as we shall see, these are not now thought to play a major role on simple metals. 

The first class includes non-redox reactions like isomerisation, dimerisation or oligomerisation of unsaturated compounds, in which the role of the catalyst lies in governing the kinetic and the selectivity of thermodynamically feasible processes. Electrochemistry associated to transition metal catalysis has been first used for that purpose, as a convenient alternative to the usual methods to generate in situ low-valent species which are not easily prepared and/or handled [3]. These reactions are not, however, typical electrochemical syntheses since they are not faradaic they will not be discussed in this review. 

A synthesis of great industrial interest is the electrochemical anodic reductive dimerisation of two molecules of acrylonitrile to give adiponitrile, from which adipic acid and 1,6-hexanediamine are prepared by hydrolysis and reduction, respectively, of the two nitrile groups. Polycondensation of the resulting products leads to Nylon 66 (Scheme 5.27). 

In the past few years, however, very efficient new methods of cyclisation proceeding via radical intermediates have been developed and several reviews [19a] and a comprehensive book by Giese [19b] have been published. Rather than reactions involving the dimerisation of two radicals -as in the Kolbe electrochemical synthesis [20] or the radical induced dehydrodimerisation developed by Viehe [21]-more important are the reactions between a radical with a non-radical species. The advantage of this type of reaction is that the radical character is not destroyed during the reaction and a chain-reaction may be induced by working with catalytic amounts of a radical initiator. However, in order to be successful two conditions must be met i) The selectivities of the radicals involved in the chain-reaction must differ from each other, and ii) the reaction between radicals and non-radicals must be faster than radical combination reactions. 

Boujlel et al. <2003SC1675> have prepared oxazolooxazoles such as 90 by the rather more unusual method of electrochemical reduction of 91 which then undergoes dimerisation giving good yields of the symmetrical heterocycles. Various substituents have been included. Their proposed mechanism involves capture of an electron to give radical anion 92 followed by dimerisation and cyclization as outlined in Scheme 9. 

Organic electrochemical reactions are classified in the same way as other organic reactions [1,2]. The most important prototypes include additions (Scheme 6.1) [ 13,14], eliminations (Scheme 6.2) [15, 16], substitutions (Scheme 6.3) [17, 18], couplings and dimerisations (Scheme 6.4) [19-21], cleavages (Scheme 6.5) [22,23], and catalytic reactions (Scheme 6.6) [24,25]. Hundreds of other examples maybe found in the literature [1,2]. 

There is a great variety of such reactions including dimerisation, disproportionation and catalytic reactions, both preceding and following the electrochemical step(s) and it is not useful to attempt to list them all here. The point is merely to stress that they are (with greater or lesser difficulty) digitally tractable, as will be shown in Chaps. 5 and 9. 

Although the electrochemical reduction of halosilanes exhibits a single irreversible wave (a fast dimerisation of the presumed intermediate radical occurs at the electrode), the half-wave reduction potentials are much more negative than the E1/2 values measured... 

Bimolecular reduction to give ditropyl is brought about not only electrochemically but also by many metal powders and metal ions [55]. The use of zinc powder [3,16,55,56] and chromium(II) salts [55,57] for the reduction of tropylium and substituted tropylium salts has been particularly studied and yields of >95% are reported. Evidence suggests that a one-electron reduction is followed by dimerisation of the tropyl radical so formed ... 

The simplest case corresponds to the one-electron transfer between the electrode and species that are chemically stable on the time scale of the experiments (Eq. (1.1)). However, electrochemical systems are frequently more complicated and the electroactive species take part in successive electron transfer reactions at the electrode (multistep processes) and/or in parallel chemical reactions in solution such as protonation, dimerisation, rearrangement, electron exchange, nucleophilic/electrophilic addition, disproportionation, etc., the product(s) of which may or may not be electroactive in the potential region under study. The simulation of these cases is described in Chapters 5 and 6. 

Reductive coupling is a direct method for C-C bond construction between two organic electrophiles. The challenge is the difficulty in achieving selectivity for the crosscoupled product over dimerisation products. Electrochemical methods or stoichiometric reductants such as Zn(0) and Mn(0) are required to perform the transformation in addition to a catalyst. Nickel has been demonstrated to form effective catalysts and organic halides are widely used as coupling partners. 

A classical and long-established electrochemical synthesis is the formation of alkanes (R-R) from the dimerisation of radicals generated via electro-decarboxylation of carboxylate salts known as the Kolbe reaction. The radical species are produced through a single electron process and... 

Mellah M, Gmouh S, Vaultier M, Jouikov V (2003) Electrocatalytic dimerisation of PhBr and PhCH2Br in [BMIM] + NTf2 — iraiic liquid. Electrochem Commim 5 591-593... 

Complex mechanisms have been classified and given a nomenclature. For details, see Bard and Faulkner (1980) and for a reasonably extensive compilation, Hanafey et al (1978) and Nielsen (1985), who provides numerous references. The nomenclature consists, in large part, of chains of the letters C (for chemical) and E (for electrochemical) in the order of occurrence, as first proposed by Testa and Reinmuth (1961). Thus, a CE reaction is a chemical reaction followed by an electrochemical one, EC the reverse, and so on. There are other reactions, hard to fit into this mould DISP (disproportionation), DIM (dimerisation, given as EC2 by Bard and Faulkner 1980), all with various suffixes added, to further specify what is going on. This book is not the place to expand on the subject and this brief introduction will have to do. 

Simulations of ac voltammetry are rare. There is the work of Hayes et al (1974A, 1974B) and Bond et al (1976). These authors examined specific electrochemical situations Hayes et al (1974A) dealt with disproportionation and (1974B) irreversible dimerisation Bond et al (1976) with the interplay of ac and LSV. No analytical solutions for these exist as yet. These workers assumed that the dc and ac components of all quantities are independent. The assumption is reasonable for sufficiently small ac amplitudes and sufficiently high frequencies of the ac modulation. Then, the ac solution can be obtained analytically from the dc solution, and one needs only to simulate the latter. 

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