Electroreduction carbonyl compound

The same electrochemical process was also used for the coupling between aldehydes or ketones and activated alkyl halides such as a-chloroesters, -nitriles, and -ketones as well as aya-dichloroesters.334 Electroanalytical studies have shown initial electroreduction of Fe(n) to Fe(i) and subsequent formation of an iron organometallic intermediate (e.g., a 7t-allyliron complex in Equation (27)) before reaction with the corresponding carbonyl compounds.335... 

Since the electroreduction of ketones shown in Scheme 29 has been well established [1-3, 12, 62-65], one more recent interest in the electroreduction of carbonyl compounds is focused on the stereo-selective reduction of ketones. For example, the diastereo-selective cathodic coupling of aromatic ketones has been reported. In the presence of a chiral-supporting electrolyte, a low degree of enantioselectivity has been found [66] (Scheme 30). 

Electroreduction of carbonyl compounds under irradiation of ultrasound is another... 

Anion radical species formed by electroreduction of carbonyl compounds show interesting reactivities. In some cases, the... 

The electroreduction of disulfides R2S2 (R = Ar, Aik), in the presence of carbonyl compounds and MesSiCl, includes the formation of intermediate thiosilanes and results in trimethylsilyl ethers of hemithioacetals of ketones and aldehydes or in full thioacetals depending on whether a two-compartment (a) or an undivided (b) cell was used (Scheme 48) [218]. 

Imines can be stereoselectively hy-drodimerized to 1,2-diamines with rac/ meso rahos of 0.9 to 1.1 similar to the cathodic coupling of carbonyl compounds to pinacols in an acidic medium [304]. With 4,6-dimethylpyrimid-2-one only the meso-diamine was obtained [305]. Electroreduction of diimines prepared from... 

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

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

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]. 

Antimony(III) trichloride can be used as a catalytic mediator in the electroreduction of carbonyl compounds. The advantage of the procedure is illustrated by the selective reduction of acetophenones (497), leading to the corresponding benzylic alcohols (498) (Scheme 173) [575]. The electroreduction is performed in an EtOH/BuOH(l/1.5)-aq.HCl-(Pt/Pb) system in the presence... 

Contrary to earlier interpretations of reduction of numerous carbonyl compounds, the role of keto-enol equilibria in electroreductions of carbonyl compounds is rather limited. This is caused by a rapid - acid- or base-catalyzed - conversion of any enol present in the solution or formed in the course of electrolysis into the more... 

Another method for reductive amination of carbonyl compounds is based on the electroreduction of oximes in aqueous media. Thus, sugar oximes were reduced to their corresponding glycamines in good yields (Hg, KCl, acetate buffer). Similar reductions of oximes derived from furfural, salicylaldehyde, benzophenone and cyclohexanone have also been described, " but reports of preparative electrolyses in strictly aprotic media seem to be absent in the literature. 

Cross-Coupling with Carbonyl Compounds. The electroreductive cross-coupling of olefins with carbonyl compounds, which are initially reduced to radical or anion intermediates to attack the olefins, is synthetically useful. The stereochemistry of this type of intermolecular reaction has lately been examined less [70-72] than that of intramolecular cyclization [73-77]. 

The reductive cyclization of unsaturated carbonyl compounds can also be achieved by using chemical reducing reagents, such as Al-amalgam, MgTiCl4, and NaNH3, but in general not only chemical yields of cyclized products but also stereoselectivity are lower than those of the electroreductive cyclizations. 

The ease of obtaining glycols and pinacols by the electroreduction of carbonyl compounds offers advantages over catalytic methods due to easier control of the potential of the cathode to maximize yields. 

In the presence of Cr" /3-ionone, /3-ionylideneacetaldehyde (160), and retinal undergo electroreduction to pinacols (161)—(163). A mechanism involving reduction of a complex between Cr " and the carbonyl compound is proposed. ... 

In other cases a chemical process such as dimerization, hydrogen abstraction, or disproportionation may either precede the initial electron transfer or follow the electron-proton uptake step. The particular reaction pathway and the intermediates formed will depend upon such factors as (i) the substrate itself, (ii) the electrode potential, (ill) the electrode material, (iv) the supporting electrolyte and solvent, and (v) the pH of the environment. The importance of the overall reaction conditions can be effectively illustrated by the electroreduction of carbonyl compounds and nitro compounds. 

Considerable attention has been focused upon the electroreduction of the aromatic carbonyl compounds, the ease of reduction making them ideal candidates for studies of the effects of molecular structure, electrode potential, electrode material, pH, and the nature of the electrolyte upon the overall electrode process. For example, a detailed experimental study of the electroreduction of a series of aromatic aldehydes and ketones, perhaps the most complete to date, has recently been published by Nadjo and Saveant. " From this data, the authors have constructed diagrams showing the distribution of the thermodynamically favored species as a function of pH and electrode potential. 

Certainly polarographic and cyclic voltammetric techniques " have been used effectively to elucidate the mechanisms of the electroreduction of both aromatic and aliphatic carbonyl compounds (and indeed a wide variety of electrode processes) but in synthetic studies the very different conditions, particularly with respect to the concentrations of reactants and products, can influence both the reaction kinetics and reaction pathway. 

Again the dimerization or coupling reactions will involve free radicals or radical ions, but the intermediates are generated cathodically. Suitable substrates are electrophiles such as activated olefins and carbonyl compounds. In this section, it is intended to focus only upon the electroreduction of activated olefins, partly because the electrode reactions of aldehydes and ketones has been discussed earlier. However, it is the electrohydrodimerization of an activated olefin that has become a successful, commercial electro-organic process, i.e., the electroreduction of acrylonitrile developed by Monsanto ... 

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