Undivided cells electrosynthesis

The Barhier-type reaction of aldehydes and ketones with allyl halides (485) in the presence of Sml2, leading to homoallyl alcohols (486), has received recent interest as a one-step alternative to the Grignard reaction. However, the reactions require the use of stoichiometric amounts of the reducing Sm(III) species. Recently, the electroreductive Barhier-type allylation of carbonyl compounds in an SmH-mediated reaction has been developed [569]. The electrolysis of (485) is carried out in a DMF-SmCl3-(Mg/Ni) system in an undivided cell to give the adduct (486) in 50 85% yields (Scheme 168) [569]. Electrosynthesis of y-butyrolactones has been achieved by the reductive coupling of ethyl 3-chloropropionate with carbonyl compounds in the presence of a catalytic amount of SmCfi [570]. 

Where Na is number of moles of A transformed. The counter electrode reaction must be chosen carefully in undivided cells to prevent reaction with the target product. The use of a sacrificial counter electrode may be satisfactory. The ideal solution is a paired electrosynthesis, i.e., when both cathodic and anodic processes are of interest. In most electrolyses, oxygen, which is electroactive, is a poison and must be removed by bubbling an inert gas through the solution or by vacuum techniques. When the electrolysis is complete, the product must be recovered. Obviously, there is no problem when the product precipitates or electrocrystallizes. The work-up of the solution may be facilitated by an appropriate choice of the experimental conditions. In... 

After the electrolysis the acetals can be hydrolyzed to their aldehydes and methanol is recovered. By this elegant way to avoid overoxidation to the acids, aromatic aldehydes are synthesized from toluene derivatives [11]. The electrosynthesis takes place in good yields for toluene derivatives with electron pushing para-substituents like the tert-bntyl group. It is carried out in an undivided cell developed by BASF the capillary gap cell which contains a stack of bipolar round graphite electrodes. The electrodes are separated by spacers and connected in series [12]. 

A soln. of tri-n-hexylborane in propionitrile containing tetraethylammonium iodide as supporting electrolyte electrolyzed 1 hr. at room temp, under Ng in an undivided cell with Pt-plate electrodes, then oxidized 2 hrs. with 30%-H2O2 and 3 N NaOH 2-methyloctanonitrile. Y 80% based on borane consumed. F. e. s. Y. Takahashi et al., Chem. Lett. 1975, 523 a-alkylation of aliphatic nitro compds. s. Synthesis 1976, 616 organic electrosynthesis, review s. L. L. Miller and E. Kariv, Ann. Reports. Med. Chem. 12, 309 (1977). 

Capillary gap cell — The undivided capillary gap (or disc-stack) cell design is frequently used in industrial-scale electroorganic syntheses, but is also applicable for laboratory-scale experiments when a large space-time yield is required. Only the top and bottom electrodes of c.g.c. (see Figure) are electrically connected to - anode and cathode, respectively, whereas the other electrodes are polarized in the electrical field and act as -> bipolar electrodes. This makes c.g.c. s appropriate for dual electrosynthesis, i.e., pro duct-generating on both electrodes. 

Sacrificial anodes (Al,7,8 Mg9,10) have been proposed for electro-organic processes as they allow one to use simple undivided electrochemical cells. In coordination electrosynthesis, they may provide a means to obtain complexes containing the metal of the sacrificial electrode (see Section 1.44.3.2). 

The examples illustrate the diversity as well as the common features of a paired electrosynthesis. One can start with one or two substrates to generate one or two products. Electrode processes can be mediated or direct. Undivided and divided cells are employed in paired electrosyntheses. But as in the BASF phthahde example, it is crucial for the synthesis of glyoxylic acid, sorbitol, and methyl ethyl ketone that the cathodic process is the reduction of the substrate and not the reduction of protons because in these cases protons are generated at the anode and the electrolysis takes place in aprotic solvent. Therefore effects that minimize the overpotential of hydrogen have to be omitted. Reaction control is important in all described examples, and consequently the cell and the setup have to fit for each case. Work-up and product isolation are significant for a successful synthesis and can be even more challenging in a paired synthesis. 

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