Electroreduction benzaldehyde

Phenol (508) is found to be produced continuously from benzene (507) by aerial oxidation in aqueous sulfuric acid when a Cu(I)/Cu(II) redox couple is used as a mediator (Scheme 176) [581]. The Cu(I)-mediated electroreduction of oxygen in the presence of chloride is found to be effective for toluene oxidation, leading to benzaldehyde and benzyl chloride [582]. Recently, benzene has also been oxidized... 

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). 

Tab. 6.16. Electroreduction of benzaldehyde, dimethyl maleate and benzyl bromide.

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. 

Work at Coventry has shown the same switch from a two-electron to a one-electron pathway is occuring at higher insonation frequency in that the electroreduction of benzaldehyde at 800 kHz gives almost entirely the pinacol. A major benefit of the use of the higher frequency however is that there is little weight loss of the lead cathode compared to lower frequency (20 kHz) where it is substantially abraded away. 

A copper(O) complex, electro-generated from Cu(acac)2, is able to undergo an oxidative addition with benzyl and allyl bromides. Further reduction leads to the coupling products bibenzyl and 1,5-hexadienes Methyl-3-hexene-l,6-dicarb-oxylate can be prepared from butadiene and CO by electroreduction if di-Fe dicyclopentadienyl tetracarbonyl is used as redox catalyst Electro-generated low-valent tungsten species are able to reductively dimerize benzaldehyde to stilbene according to Eq. 83. The reduction potential was controlled at the third wave of the WClg catalyst (V = -1900 mV/SCE)... 

Thus the electroreduction of benzaldehyde [ 196] can lead to either the hydrodimer in a one-electron per substrate molecule process, or to the benzyl alcohol in a two-electron process, as indicated in Scheme 7. 

Table 7. Electroreductions of Benzaldehyde, p-Methylbenzaldehyde, Dimethylmaleate, and Benzyl Bromide3...

Ionic liquids are also effective for electrochemical reductive coupling reactions. For example, the cathodic reduction of benzaldehyde in [bmpyr][NTf2] [bmpyr = 1-butyl-1-methylpyrrolidinium NTf2 = bis(trifluoromethylsulfonyl) imide] gave rise to dimerization [9]. Electroreductive coupling of acetophenone in ionic liquids gave the corresponding pinacol as a mixture of diastereomers (Equation 12.3) [10]. 

Electrochemically reduced W species tend to transform carbonyl compounds into olefins after dimerization. For example, benzaldehyde (387) can be quantitatively converted into stdbene (388) by electroreduction in a THF-BU4NCIO4/WCI6-(Al/Pt) system at a potential of —1.9 V (SCE) (Scheme 140) [501]. 

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). ... 

Matsuda K, Atobe M, Nonaka T (1994) Ultrasonic effects on electroorganic processes. Part 1. Product-selectivity in electroreduction of benzaldehydes. Chem Lett 23 1619-1622... 

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