Electrochemical reduction esters

One development involves the use of vitamin B 2 to cataly2e chemical, in addition to biochemical processes. Vitamin B 2 derivatives and B 2 model compounds (41,42) cataly2e the electrochemical reduction of alkyl haUdes and formation of C—C bonds (43,44), as well as the 2inc—acetic acid-promoted reduction of nitriles (45), alpha, beta-unsaturated nitriles (46), alpha, beta-unsaturated carbonyl derivatives and esters (47,48), and olefins (49). It is assumed that these reactions proceed through intermediates containing a Co—C bond which is then reductively cleaved. 

Erickson and Fischer have not been able to reproduce this result, however 23f They found that the products isolated from reduction of either 13 or its ethyl ester (14) are essentially completely racemic, thus making the stereochemistry of electrochemical reduction of 13 consistent with that of 11. 

Feoktistov and coworkers 57) found a third mode of reactivity in a closely related system. The electrochemical reduction of the diethyl esters of dichloro-maleic (56) and dichlorofumaric (57) acid was studied. Neither afforded the corresponding acetylene. 

While electrochemical cleavage of several other kinds of single bonds has been reported, stereochemical information is sparse. Erickson and Fischer examined the electrochemical reduction of O-benzoylatrolactic acid (76, R = H) and its methyl ester (76, R = CH3) 69>. The products (77, R = H and CH3) of the... 

On the other hand, it has been found that the electrochemical reduction is a very unique and useful tool in synthetic organic chemistry when magnesium is used as the material of the electrode. The cathodic reduction of 1,3-dienes with magnesium electrode gives very unique products, i.e. 3-cyclopentenol derivatives when it is carried out in the presence of a carboxylic acid ester (equation 23)17. 

In the cathodic reduction of activated olefins, chlorosilanes also act as trapping agents of anionic intermediates. Nishiguchi and coworkers described the electrochemical reduction of a,/ -unsaturated esters, nitriles, and ketones in the presence of Me3SiCl using a reactive metal anode (Mg, Zn, Al) in an undivided cell to afford the silylated compounds [78]. This reaction provides a valuable method for the introduction of a silyl group into activated olefins. 

Cobalamin catalysed electrochemical reduction of the 2-chloroethanol ester 68 at negative potentials, without photochemical assistance, leads to a 1,2-elimination process (see p. 115) [228]. This contrasts with the lack of 1,2-elimination during reaction of 66 and 67, Thus in the purely electrochemical carbon-cobalt bond... 

The deoxygenation of phenyl esters and ethers can be achieved by electrochemical reduction provided the fimetion is converted into a good leaving group. 

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Alkenes may be activated toward electrochemical reduction by electron-withdrawing sucstituents. Thus acrylonitrile and acrylate esters are easily reduced and, depending among other factors on the proton availability of the medium, they undergo either hydrogenation or hydrodimerisation. The basic character of the radical-anions of such substrates has been put to use in EGB promoted Michael additions of the type outlined in Scheme 15 the case where the probase is azobenzene has already been discussed. 

A famous use of electrochemical reduction of porphyrin systems is that used to convert a chlorophyll a derivative, chlorin-e6 trimethyl ester (64), into a chlorophyll b derivative, rhodin-g7 trimethyl ester (65) (71LA(749)109). Electrolysis of chlorin-e6 trimethyl ester gave the chlorin-phlorin which was photolyzed in dioxane/water to give the trans-d o (Scheme 7) this was simply transformed into rhodin-g7 trimethyl ester (65) to accomplish the first ever interconversion of a chlorophyll a series pigment into one of the companion b series. 

The radical anions of phosphafulvene (80) and dibenzophosphafillvene (81) were generated by electrochemical reduction and their EPR spectra recorded between 110 K and room temperature.213 Comparisons of calculated spin densities and charge distributions are presented along with experimental data. The radical anions of o-, m-, or p-thiobenzoate esters were generated by electro-reduction and found by their ESR spectra to be persistent.214... 

The electrochemical properties of all these bis-adducts have been studied in reasonable detail, using either cyclic voltammetry (CV) or controlled potential electrolysis (CPE) [8], Reductive electrolysis of ester-containing methanofullerenes results in the removal of the adducts in a versatile and useful reaction, initially called the retro-Bingel reaction (Figure 1). Additional work resulted in the discovery of an intramolecular electrochemically induced isomerization of C6o-bis-adducts. Exhaustive reduction with one electron per molecule resulted in seven regio-isomers regardless of which pure bis-adduct regioisomer was electrolyzed. Recently, it has been observed that, in addition to the malonates, electrochemical reduction of other methano-adducts can also lead to removal of the addends,... 

Scheffold and colleagues subsequently reported that alkyl iodides 249 add to xfl-unsaturated esters 248 catalyzed by 23 mol% hydroxycobalamine 247 (X=OH) and mediated by light to homolyze 253B [302]. The generation of the active Co(I) species 253A proceeded by electrochemical reduction at a potential of —1.2 V vs SCE. The transformation required a large excess of 248 to promote the addition of the initially generated alkyl radical. The reaction was applied to the total synthesis of the California red scale pheromone. The product... 

Katsumata and colleagues reported the first electrochemically mediated Ni(I)-catalyzed tandem radical cyclization reactions [139]. A bromoacetaldehyde dienyl acetal or the corresponding enynyl acetal underwent tandem 5-exo/5-exo cyclization reactions in the presence of Ni(cyclam) complex 98a under electrochemical reductive conditions (see Part 2, Sect. 5.3.3, Fig. 68). Bicyclic esters were isolated in 35% and 72% yields as a mixture of two and four diastereomers, respectively. 

In general, the dissolving metal or electrochemical reductions of acyl halides or acyclic anhydrides are not useful for the preparation of primary alcohols. Such reductions invariably provide acyloin esters, ene-diolate diesters or related species. Cyclic anhydrides may be reduced to give lactones. For example, the reduction of phthalic anhydride at a mercury cathode has been used in the synthesis of phthalide (90%). In general, however, such reduction are not widely employed in synthesis. 

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