Ketones, electrolytic reduction

NH2-C0-NH NH2,CH5N30. Colourless crystalline substance m.p. 96" C. Prepared by the electrolytic reduction of nitrourea in 20% sulphuric acid at 10 "C. Forms crystalline salts with acids. Reacts with aldehydes and ketones to give semicarbazones. Used for the isolation and identification of aldehydes and ketones. 

MethyloU2-Butanol (2-Methyl-l,3-butanediol or a >Dioxy-/3-methylbutane). CH3.CH(OH).CH(CH2OH).CH3 mw 104.15, viscous oil, bp 200° 98—99° at 9mm. Sol in w, v sol in ale and eth. Can be prepd either by reduction of the corresponding aldehyde, 2-methylbutanol(3)-al-(l) with A1 amalgam (Ref 1), or by electrolytic reduction in 10% sulfuric acid of the corresponding ketone ale. In the latter case, methyl-2-butanone-3-ol-(l), obtained by the condensation of methylethylketone with formaldehyde, can be used. On nitration, it yields an expl dinitrate Refs 1) Beil 1,482,(250) 2)L.P.Kyria-kides, JACS 36, 535(1914)... 

Several methods are employed for the reductive defluorination of 2-fluoro-l-phenyl-ethanones. Electrolytic reduction at a mercury cathode selectively replaces fluorine in 2-fluoro-l -phenylethanone by hydrogen.94 The hydrodefluorination of fluoroacetophenones, e.g. 7, to dithioacetals of the corresponding defluorinated ketones with aluminum chloride and ethanethiol take place readily by stirring the mixture in dichloromethane at 0°C under nitro-... 

The electronics and structure of the acceptors, especially a, 3-unsaturated ketones, is also a determinant in 1,2- vs. 1,4-addition processes. In general, substitution of aryl or large groups at the carbonyl unit increases the preference for 1,4-addition,16ab while a, 3-unsaturated aldehydes afford exclusive 1,2-addition and [3,(3-disubstitution suppresses 1,4-addition,l6c presumably due to steric hinderance. House and Seyden-Penne have established good correlations between chemoselectivity and either the half-wave electrolytic reduction potentials,17 or the energy levels of the LUMO of various a, 3-unsaturated ketones.17b... 

Ketones are converted by electrolytic reduction into secondary alcohols or pinacones.2 A mercury cathode gives a good yield of the former class, but other cathodes give a mixture8 of the two compounds. 

Electrolytic reduction of benzylic (18) and allylic halides (equation 17) in the presence of anhydrides affords the corresponding ketones in good yields. The electrolysis was conducted in an undivided cell using aluminum or magnesium anode and under constant-current conditions. Similarly, benzylic halides were reported to react with acid chlorides under controlled potential conditions, in acetonitrile or DMF as solvent as shown in equation 1841. 

Brucinonic acid (2,3-dimethoxy XXXVIII) is reduced to two isomeric dihydrobrucidinonic acids (XLII), C-21 epimers, in which the Na lactam carbonyl, but not the Nb lactam carbonyl, has been reduced the ketone carbonyl is concurrently reduced to the epimeric alcohols (60, 109). It would seem that electrolytic reduction to CH2 is a special property of the Na lactam carbonyl group. 

Mixed Electrolytic Reduction of Pyridine Bases and Ketones... 

Preparation of secondary (or tertiary) carbinols from pyridines and an aldehyde (or a ketone) in the presence of magnesium or aluminum and mercuric chloride is known in pyridine chemistry as the Emmert reaction. 7 70 For example, dimethyl-2-pyridylcarbinol is obtained in this way from pyridine and acetone. When a mixture of pyridine and acetone is subjected to an electrolytic reduction in dilute sulfuric acid at lead electrodes, a mixture of two main products results, namely, 2-(2-hydroxy-2-propyl)-3-piperideine and 4-(2-hydroxy-2-propyl)piperidine. Analogous compounds are obtained with the use of methyl ethyl ketone as the reactant. The mixed electrolytic reduction of 2-methylpyridine and acetone affords 2-(2-hydroxy-2-propyl)-6-methyl-3-piper ideine (74) and 2-methyl-4-(2-hydroxy-2-propyl)-piperidine.71... 

Trimethylpyridine, with all the critical positions blocked, does not undergo mixed electrolytic reduction with ketones.61... 

The mixed electrolytic reduction has been performed also with quaternary salts of pyridine71 and its homologs72 and acetone or some other ketones. l,4-Dimethyl-2-(2-hydroxy-2-propyl)-3-piperideine (76) was formed from 1,4-dimethylpyridinium methosulfate and acetone.71... 

The corresponding methyl hetaryl ketones are much less hydrated,285,266 as would be expected, and their electrochemical behavior resembles more that of acetophenone. Depending on pH, the products are the pinacols, the carbinol, or mixtures thereof207-269 in acid solution mostly carbinols, at high pH mostly pinacols, are formed. The proportion of + -pinacol to meso-pinacol obtained in the electrolytic reduction of 2-acetopyridine is, however, different from that from acetophenone.2fl8,269 The /meso ratio in alkaline solution is about 2.8 for acetophenone and about 0.3 for 2-acetopyridine. 

In the case of aryl aldehydes and ketones, benzaldehyde afforded benzyl alcohol as the major product, but acetophenone and its para-substituted derivatives carrying such groups as OMe, Cl or OH provided ethylbenzene derivatives in good yields. As with Clemmensen reduction, the alcohol produced in this reduction cannot be further reduced, and the alcohol is not therefore an intermediate. Still uncertain in the reaction mechanism of electrolytic reduction, however, is the role of adsorbed hydrogen. ... 

Dehalogenation has provided routes to cyclopropanones which may then be trapped in the form of hemiketals or cycloaddition products. Thus, electrolytic reduction of l,3-dibromo-2-propanones served as a method for preparing the derivatives 6 and 7 as shown in equation 2. In a related reaction, treatment of a,a -dibromobenzyl ketone with sodium iodide in acetone in the presence of furan yielded the cycloadduct 8 (equation 3) . 

A special type of ring enlargement is found in the electrolytic reduction of bicyclic a-amino ketones at a lead cathode in 30% sulfuric acid at 60°C to monocyclic products... 

Of more interest than the saturation of double bonds is the hydrodimerization reaction (Chapter 21) sodium amalgam hydrodimerizations of a, -unsaturated ketones were reported early [28], and the product distribution resembled that obtained from direct electrolytic reduction (Chap. 10). 

On reaction with Na(Hg), saturated aldehydes or ketones may form either alcohols or pinacols [82-84] the reaction is similar to the electrolytic reduction (Chapter 10). 

The use of hexamethylphophoramide (HMPA) in the electrolytic reduction of substrates, such as benzene derivatives, aliphatic ketones, amides, and olefins, involves the intermediate formation of solvated electrons, since very negative potentials must be reached. The substrates are normally difficult to reduce. However, it is important not to add so much proton donor to the solvent that the rate of its reaction with the solvated electrons becomes too high. Generally, ethanol can be used as a cosolvent [311], and the best stability of the intermediate solvated electron combined with an optimum protonating efficiency was found to be the mxiture of HMPA-ethanol (33-66 mol moP ) the relatively high concentration of EtOH does not protonate ens because HMPA appears to be selectively adsorbed on the cathode interface and the reaction of Cys with the substrated is fast. With such a ratio of ethanol, a maximum current efficiency was found [312]. 

Reduction of the hydroxyimino group with acetylation of the resulting amino group can be effected by zinc dust in the presence of acetic acid and acetic anhydride diethyl (acetylamino)malonate, which is important for amino acid syntheses, has been prepared in this way.97 Zinc dust reduces oximes of diaryl and arylalkyl ketones better in concentrated ammoniacal than in acid solution, giving the amines as very pure free bases whilst hardly any higher alkylated product is formed.98 Aminoacetone," diaminoacetone,100 and 2-afnino-3-pentanone" have been obtained by reducing oximes with tin or tin(n) chloride in alcoholic hydrochloric acid solution. In individual cases oximes have been reduced by sodium dithionite or aluminum amalgam.198 Further, the patent literature contains examples of electrolytic reduction of oximes.198... 

It can be seen that the activities of the amorphous alloys are lower than those of the polycrystalline catalysts. Formation of the corresponding diol was not observed on the amorphous catalysts, while the crystalline catalysts either produced the diol selectively, or a mixture of the diol and the hydroxy ketone was formed. The fundamental reason for the lower activity and higher selectivity of the amorphous alloys is their rather small surface area. Of the amorphous alloys studied, Ni-B and Ni-P alloy powders prepared by chemical reduction exhibited higher activities than those of Ni-P alloys prepared by electrolytic reduction or rapid quenching. This difference in activity can be attributed to an oxide layer covering the surface of m-P foils [Ij. It is necessary to point out, however, that the comparison of activities is based on unit catalyst weight. Obviously, this comparison does not take into account the real surface area of the nickel samples, nor active site densities. 

The electrolytic reduction of brucinonic acid at a lead cathode reduces Ng-CO —> Na-CHi-and the ketone to an alcoholic hydroxyl the resulting stereoisomeric brucidinolic acid and dihydrobrucidinonic acid show typical brucidine reactions (122). 

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