Electroreductive Syntheses

Work by Ono et al. [66] has been specifically directed at ultrasonic control of product-selectivity in electroreductions. Using a lead cathode, in dilute methanolic sulphuric acid, at a constant current of 20 mA cm , Ono electroreduced benzaldehyde under stirred, unstirred and ultrasonic conditions (Fig. 6.17). In an unstirred system, benzyl alcohol (two-electron process) was the major product, while mechanical stirring reversed the position in favour of the hydrodimer (one-electron product). Ultrasonic irradiation from a cleaning bath (100 W, 36 kHz) so strongly favoured the hydrodimer that the alcohol was barely evident (Tab. 6.16). 

These authors, using a horn system, also noted less striking but still significant switches towards the one-electron products in other sonoelectrochemical reductions [66] including dimethylmaleate at a lead cathode in an aqueous mixed-phosphate buffer, and benzyl bromide at a lead cathode in methanolic tetraethylammonium bromide solution (Tab. 6.16). 

System Conditions Product ratio one-electron/two-electron 

The reduction of benzoic acid at a lead cathode in aqueous sulphuric/citric acids yields the two-electron products benzaldehyde and the four-electron product benzyl alcohol rather than one-electron hydrodimer. In all cases studied by the authors they found that ultrasound favoured the process involving the smaller number of electrons per molecule. This is the opposite of the sonoelectrochemical effect seen in carboxylate electrooxidation [57,59,60] where the process involving the greater number of electrons was favoured by ultrasound. 

Atobe and Nonaka [67] have used a 20 kHz (titanium-alloy) sonic horn as the electrode (called sonoelectrode) for electroreductions of various benzaldehyde derivatives. This they did after insulating the submerged metal part of the horn-barrel with heat-shrink plastic. They found an improvement in current efficiency with insonation, but in addition noted some change in product selectivity towards one-electron-per-mole-cule products. Although the authors quote enhanced mass transfer across the electrode interface as the origin of the sonoelectrochemical trend towards products from the lesser amount of electrons per substrate molecule, the involvement of surface species on the reactive electrode provides a complication. 

A useful preparative-scale electroreduction reaction involves the conversion of nitroarenes to amines. In practice, this is a complicated system with competing reactions, e.g., hydrodimerization to hydrazo compounds. One of the earliest sonoelectroreductions concerns a direct reduction of nitrophenol to 4-amino-phenol on a 50-g scale at 50 mA cm 2 in a catholyte of 30% aqueous H2SO4 containing some 0.5% of SnC and 0.01% of polyethyleneglycol nonylphenyl ether. 6 Ultrasoimd is applied at 2.375 KW power (cell dimensions are not given, precluding assessment of power density), electrolysis is continued for 16-30 h, and the cell temperature rises to 80-90°C. The product forms in some 70% yield. 

Recent work has been specifically directed at ultrasonic control of product-selectivity in electroreductions. Thus, benzaldehyde can be reduced to either the hydrodimer in a 1-e process or to the benzyl alcohol in a 2-e process (Fig. 15).  

66 Gigi, S. Paucescu, V. Jurth, S. Romanian Patent 1980 RO 72382 Chem. Abstr. 1982,96,151408x. 

Using a lead cathode in dilute methanolic sulfuric acid at a constant current of 20 mA cm 2, the benzyl alcohol was the major product from an unstirred solution. Mechanical stirring favors the hydrodimer, but sonication (cleaning bath, 100 W, 36 kHz) so strongly favors this product that the alcohol was barely evident. The magnitude of the effect depends on the ultrasonic power, however without appreciable stereochemical change. For benzaldehyde there are always imidentified products ca. 30%), whether the reaction is sonicated or not. 

4-Tolualdehyde, which also shows the sonoelectrochemical switch to the pinacol, has a blocked para position and is less affected by side reactions (Table 5). Similar effects were obtained by irradiation from a probe (20 kHz) under a range of different silent and sonicated conditions (powers, sources, cell geometries). 

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Apphcations of ultrasound to electrochemistry have also seen substantial recent progress. Beneficial effects of ultrasound on electroplating and on organic synthetic apphcations of organic electrochemistry (71) have been known for quite some time. More recent studies have focused on the underlying physical theory of enhanced mass transport near electrode surfaces (72,73). Another important appHcation for sonoelectrochemistry has been developed by J. Reisse and co-workers for the electroreductive synthesis of submicrometer powders of transition metals (74). 

TABLE 9. Electroreductive synthesis of dienes from benzenoid compounds... 

More recently Kashimura et al. [96] has investigated the effect of ultrasound on the electroreductive synthesis of polysilanes, polymer germanes and related polymers using magnesium electrodes. They found that the presence of ultrasound greatly facilitated the reactions. 

An alternative which is attractive for large scale work is the electrochemical reduction of aldonolac-tones.42 43 Particular attention has been paid to the electroreductive synthesis of ribose from ribonolac-tone because of the importance of the former in the synthesis of riboflavin (vitamin 62). Processes generally involve a mercury cathode and maintenance of an acidic pH, often with the assistance of a phosphate or borate buffer. It has been reported that alkali metal ions are also necessary, suggesting that the reduction occurs via metal amalgam formation. However, other accounts make no mention of metal... 

Electroreductive synthesis of Se22 and Se - dianions was enhanced by irradiation from a cleaning bath. Instead of employing a sacrificial cathode of elemental selenium, this procedure allowed the direct use of selenium powder with carbon cloth as cathode. This method was also used in the production of the corresponding tellurium anions. These species could be reacted in situ in aprotic solvents such as DMF, THF, and acetonitrile for the synthesis of selenides and tellurides by nucleophilic displacement from haloalkanes (Fig. 17). 

Kashimura S, Ishifune M, Yamashita N, Bu HB, Takebayashi M, Kitajrma S, Yoshihara D, Kataoka Y, Nishida R, Kawasaki S, Murase H, Shono T (1999) Electroreductive synthesis of poiysiianes, polygermanes, and related polymers with magnesium electrodes. J Org Chem 64(18) 6615-6621... 

Ishifune M, Kashimura S, Kogai Y, Fukuhara Y, Kato T, Bu HB, Yamashita N, Murai Y, Murase H, Nishida R (2000) Electroreductive synthesis of oligosilanes and poiysiianes with ordered sequences. J Oiganranet Chem 611(l-2) 26-31... 

The adaptation of these electrochemical reactions to the synthesis of PPP has mainly used Ni complexes. First attempts were reported by Schiavon et al. [158] but their material was not pure and contained a high proportion of nickel centres. Fauvarque et al. [159], however, reported the electroreductive synthesis of an almost nickel-ffee poIy(/ -phenylene). A typical procedure requires completely dry conditions, uses NiX2L2 [X = C1 or Br, L = P(Ph)3 or L2 = (Ph)2PCH2-CH2P(Ph)2] as the starting complex and consists of reducing it electrochemically to a zero-valent species in the presence of dibromobenzene or dibromobiphenyl according to Scheme 6,37 [160] ... 

Ruthenium is a known active catalyst for the hydrogenation of carbon monoxide to hydrocarbons (the Fischer-Tropsch synthesis). It was shown that on rathenized electrodes, methane can form in the electroreduction of carbon dioxide as weU. At temperatures of 45 to 80°C in acidihed solutions of Na2S04 (pH 3 to 4), faradaic yields for methane formation up to 40% were reported. On a molybdenium electrode in a similar solution, a yield of 50% for methanol formation was observed, but the yield dropped sharply during electrolysis, due to progressive poisoning of the electrode. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

This electroreductive alkylation is successfully applied to the synthesis of /1-thujaplicin. 

An unexpected production of 2,4,6-triphenyl-l, 3,5-triazine in the electroreduction of 3,4-diphenyI-l,2,5-thiadiazole 1-oxide has been reported . Synthesis of 1,3-diyne derivatives of 2,4-diamino-l,3,5-triazine, 9a and 9b, has been accomplished by reaction of biguanidine with mono- and di-esters 8a and 8b, respectively <00T1233>. 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Electrocarboxylation is carried out when C02 is used as electrophile offering an interesting alternative to organometallic synthesis. A prerequisite of this type of electroreduction in industrial scale is electrolytic cells especially adapted to use aprotic solvents. These cells must fullfil the following requirements [148,149] ... 

There is a brief reference to electroreductive silicon polymer formation in COMC II (1995) (chapter Organopolysilanes, p 96), but the very limited extent of the field at that time precluded further comment. Since then, the field has seen considerable progress, and the mild conditions have permitted the synthesis of functionalized polymers of moderate molecular weight (104), an example of which is the co-polymer poly(rncthyM-rncthoxymethoxyphcnylsilylene)-r -poly(methylphenylsilylene), 26, with a protected phenolic function, which was prepared with a molecular weight Mw= 19,000.96 Deprotection afforded the phenolic polymer. Several reviews on the area have been published.113-115... 

Electroreduction of aliphatic amides in the presence of chlorotrimethylsilane gives coupling products and this reaction is useful for the synthesis of a-amino ketones (Scheme 22) [41]. In this reaction, the formation of an Mg salt promotes the coupling of two anion radical centers. 

Electroreductive coupling of ketones with silyl-substituted olefins promotes interesting reactions that are useful for organic synthesis. For example, coupKng of ketones with trimethylvinylsilanes affords /I-trimethylsilyl alcohols, which are easily transformed to the corresponding olefins (Scheme 40). This reaction is interesting from the synthetic point of view since vinylsilane behaves as the equivalent to a /I-trimethylsilyl group-substituted anion [77, 83]. 

A demonstration of the utility of the electroreductive cyclization reaction is provided by the formal total synthesis of the antitumor agent quadrone (16, Scheme 4) [17]. The first stage of the synthesis involved a controlled potential reduction of (9) in the presence of dimethyl malonate as the proton donor. An efficient cyclization ensued, leading to the formation of the y-hydroxy ester (10)... 

H. Tanaka, O. Ren, S. Torii, Pd(0)/Pd(II)-mediated electroreductive cyclization of n-alkenyl-2-bromoanilines in Novel Trends in Electroorganic Synthesis (Ed. S. Torii), Kodansha, Tokyo, 1995, pp. 195,196. 

The electroreduction of iminium salts in the presence of alkyl halides has been applied to the synthesis of alkaloid-type compounds as depicted in Scheme 87 [127,128]. 

Electroreduction of y- and 5-cyano ketones in isoPrOH with a Sn cathode gave a-hydroxyketones with good diastereo-selectivities as cyclization products. The reaction has been used as a key step for the synthesis of, for example, guaiazulene, triquinanes, and dihydrojasmone. Similarly, the corresponding intermolecular couplings were realized [315]. 

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