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Reducing amides to amines
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'Electrolytic Reduction of Organic Compounds' - Chemical Reviews, Vol 62, pp 19-40 (1962) (https://www.thevespiary.org/rhodium/Rhodium/pdf/electroreduction-chem.rev.62.19-40.1962.pdf)
A. Introduction to Electroorganic Reduction
1. Cathode reactions
The passage of electricity through two electrodes immersed in an electrolyte results in the deposition of a metal or the evolution of hydrogen from the cathode. Reducing conditions exist at the cathode of sufficient power to enable the electrolytic hydrogen to add to unsaturated organic molecules or to remove oxygen from organic molecules. Unlike the reduction of most inorganic substances, the reduction of organic compounds is sometimes reversible (e.g., quinone-hydroquinone, azobenzene-hydrazobenzene, nitrosobenzenephenylhydroxylamine). Definitions of many of the terms used are included in earlier reports on electrochemistry (323) and are not discussed here.
2. Theories of reduction at the cathode
Several theories have attempted to account for the reducing action of the cathode. The most reasonable theory, first proposed by Haber (130) and later supported by Creighton (55), states that hydrogen atoms are released at the cathode by the discharge of a proton with an electron. The atomic hydrogen thus formed reacts with the organic compound.
The hydride ion has been considered to be the active reducing agent (109). Other workers (51, 368) believe electrolytic reduction to be a phenomenon of simultaneous electrolytic and chemical reduction. Certainly, the electrolytic reduction of an organic compound in a basic solution at a mercury cathode could conceivably be due to the chemical activity of the electrolytically formed amalgam. There may also be the phenomenon of simultaneous electrolytic and catalytic reduction; the activity of platinized platinum and nickelized nickel cathodes probably is due to the catalytic effect of the finely divided cathodic surface. Evidence has been presented (171, 198) that reduction may take place by direct transfer of electrons from the electrode to the substance being reduced; this hypothesis presumes the addition of the highly mobile proton to a negative center. It has been observed (342) that an unstable, benzene-soluble, reducing substance is formed at platinum or lead cathodes. This unknown substance was shown not to be a gas, tritium, or ferrous ion; it was not, however, further identified. Some investigators (115, 186, 337) have stated that the cathode may enter into the electroorganic reduction; they have reported the formation of organometallic compounds, particularly when carbonyl groups were reduced at lead or mercury cathodes.
It appears that to a large extent the mechanism of electroreduction depends upon many factors. In the case of the electroreduction of diphosphopyridine nucleotide (167), it appears that the process involves proton transfers, rather than a direct transfer of hydrogen atoms, since no dependence is found on the use, of a variety of electrodes of different overvoltages. Antropov (16) has presented a mathematical treatment which supports the theory that nonpolar groups of compounds such as phenol, acetylene, vinylacetylene, crotonic acid, oleic acid, and sorbic acid are reduced on low-overvoltage cathodes by hydrogen atoms adsorbed on the electrode surface, while polar groups of acetone, acetaldehyde, 1-acetyl 2-propanol, acetophenone, benzophenone, salicylaldehyde, and oxalic acid are reduced on cathodes of high overvoltage by hydrogen ions activated in the double layer. Many other papers have appeared which discuss the mechanism of electroorganic reduction in specific systems (140, 218).
3. Disadvantages and advantages of electroorganic reduction
The two main disadvantages which have been offered for electroorganic reductions (116, 320) are that the reductions are usually of a low velocity and that the process requires careful control of many conditions.
There are several apparent advantages of electroorganic reduction:
(1) impurities from a reducing agent are not present in the reaction mixture;
(2) partial reduction may be achieved by altering the reduction conditions;
(3) selective reduction of certain unsaturated groups in the presence of other groups has been acomplished (208);
(4) electroorganic reductions have been successful in the presence of compounds that usually poison catalytic reductions, and such substances as mercury, halides, hydrogen sulfide, and arsenic sulfide have been demonstrated to be electrocatalytic (163). Electroorganic reduction can also be developed into procedures suitable for large-scale preparative work (80).
B. Factors That Influence Electroorganic Reduction
1. The cathode potential
It has been stated (208) that the cathode potential is the factor that determines the success or failure of electroorganic reductions. This fact has long been recognized and conceded by responsible investigators, but very few have described the cathode potential used during reduction. Rather, a host of less important factors have been enumerated and described.
Haber (129, 130) first recognized the importance of controlled cathode potential for electroorganic reductions. He showed that in an alkaline solution at a platinum cathode nitrobenzene was reduced in good yield to pure azoxybenzene when the cathode potential was held at 0.9 v.; a potential of -1.3 v. gave good yields of pure hydrazobenzene. This use of cathode potential has been found to lead to purer products in many cases. In the electroreduction of 8-chlorotheophylline (353), to theophylline the cathode potential was varied until the potential which gave the purest product was found. At all other potentials mixtures of products were obtained. Acetylene has been selectively reduced (32) to ethylene or ethane at a platinum cathode by controlling the cathode potential. Other workers also have stated (10, 12, 80, 207) that the cathode potential was of primary importance for controlled electroorganic reductions, and additional examples will be found laterin this review. The electroreduction of nitrobenzene at an empirically established cathode potential of about -1.5 v. has been studied at a number of different cathodes (210). In all cases about 60 per cent yields of azoxybenzene and 40 per cent yields of aniline were obtained at zinc, copper, platinum, tin, and nickel cathodes.
The above work has been reiterated and pushed even further by the determination, polarographically, of the optimum cathode potential for the reduction of 9-(o-iodophenyl)acridine to 9-(o-iodophenyl)dihydroacridine at a mercury cathode (208). Although all previous methods and attempts to carry out this reduction had failed, because of an excessive reduction that removed the iodine atom, the use of the polarographically determined, constant cathode potential gave the desired product quantitatively. Although other workers (165, 256) also have shown that the potential found in polarographic work is confirmed by electroreductions, it has been pointed out (80, 92) that polarographic data alone do not rigidly demonstrate the efficacy of a preparative method; therefore polarographic data have not been included in this review (see reference 360 for a review of such data).
2. The cathode overvoltage
Two definitions of overvoltage have been presented (58):
(1) the overvoltage of a gas upon a specific electrode is the polarization involved in the evolution of that gas on that cathode at a specified current density;
(2) the overvoltage of a gas on a specified electrode is the minimum polarization at which visible evolution of gas occurs, or at which there is a marked increase in current density.
The higher the overvoltage value of any cathode, the greater is the reducing power of that cathode toward any organic depolarizer. Tafel (333, 335) was the first to recognize the relation between high overvoltage and a high reducing power. One of the early examples of this relationship was the inability of a smooth platinum cathode of low overvoltage to reduce cinnamic acid to hydrocinnamic acid; this reduction was accomplished, however, at the high-overvoltage lead and mercury cathodes.
The concept of overvoltage is important, since in an electroorganic reduction when a depolarizer is present, the potential at which hydrogen is evolved as a gas from the cathode marks that potential as the critical point above which the hydrogen is not absorbed by the depolarizer. Below this potential the atomic hydrogen reacts with the depolarizer as fast as it is formed.
Various theories of overvoltage have been discussed in great detail (118). The most acceptable view is that the metals of lower overvoltage function as catalysts for the reaction H+ + e- H0 by decreasing the energy of activation for the process. Metals of high overvoltage have a large energy of activation, and hydrogen atoms formed at a metal of high overvoltage have a much greater potential energy than those liberated at metals of low hydrogen overvoltage.
Considerable study of overvoltages of various metals has demonstrated that the absolute values obtained have little import; only the relative differences in hydrogen overvoltages are of significance. Many of the existing data on overvoltage have been summarized (58, 183). The average overvoltage, obtained by averaging various literature values (49, 58, 183, 335), of hydrogen on various cathodes. It should be reemphasized that the absolute values given are of no significance; only the approximate order of arrangement of the cathodes should be considered.
The following factors have been listed as those which affect the overvoltage of hydrogen on metals (58):
(1) an increase in current density decreased the overvoltage;
(2) an increase in temperature decreased the overvoltage;
(3) an increase in pressure decreased the overvoltage;
(4) the overvoltage increased with time;
(5) an alternating current superimposed on a direct current lowered the overvoltage;
(6) the overvoltage (116) is less in an alkaline solution than in an acid solution.
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3. Cathode material and condition
The cathode material and its condition are rather intimately connected with the cathode potential and cathode overvoltage, but several observations on cathode materials properly belong by themselves. In addition to the more specific property of cathode overvoltage, the cathode material and condition have an effect upon electroorganic reductions.
The purity of the cathode is of prime importance. It has been demonstrated (333, 334) that when lead is used as a cathode, its reducing power is nil when there is present as little as 3 mg. of copper, 0.5 mg. of silver, or 0.04 mg. of platinum per square decimeter of lead surface. The difference between 99.9 per cent lead and 99.99 per cent lead is often enough to cause failure in an electroorganic reduction (320). All leads, tabs, and connections of a metal different from the cathode were shown to be deleterious to the cathode activity.
The physical state of the cathode may often play a leading role in an electroreduction. Generally a rough cathode has a lower overvoltage than a smooth, one; but since it also has a greater surface area, the depolarizer has more opportunity to react with atomic hydrogen on it. Cinnamic acid failed to undergo reduction at a smooth platinum cathode but reacted at a platinized platinum cathode. It was found (203) that the rate of cathodic reduction of compounds such as aconitic acid and quinoline was increased by continuous renewal of the mercury surface, while the rate of reduction was decreased with naphthalene and anthracene. In these experiments, the surface of the mercury was renewed by motion, and at a high rate of motion reduction was stopped in many cases. It was concluded that if the mercury had a catalytic effect, renewal accelerated the reduction; if the reduction were due to
electrolytically produced amalgam, then renewal counteracted the reduction; and finally that if renewal had no effect, as is the case of sorbic acid, then the reduction was truly electrolytic and depended only on the cathode potential.
The history of a cathode is of importance. A case has been cited (320) in which commercial cadmium, presumably of a microcrystalline character, was unfit for use as a cathode until melted and recast. Cathodes which have been used previously as anodes have been shown to have special activity as cathodes.
In addition to the above observations, it has been ascertained that certain cathodes appear to have a definite catalytic effect upon certain reactions which is unexplained in terms of cathode potential or overvoltage. Only platinized platinum, the cathode of lowest overvoltage, reduced aromatic nuclei (24, 25, 102). At a copper cathode, unlike other cathodes, nitrobenzene was reduced completely to aniline in both acid and alkaline solutions.
4. Current density
Current density is usually defined as the amperes of current applied per square decimeter of cathode surface. Too much importance has been placed upon the influence of current density in electroorganic reductions. Its importance lies primarily in the fact that it is a means of controlling the cathode potential. Usually the current density is the major experimental point in the literature (it is sometimes even listed in papers which do not mention what the cathode material is), but it has been shown (10) that the current density is of little value in reproducing work unless everything else is reproduced exactly, and that quite often electroorganic reductions are somewhat independent of current density and applied voltage.
5. Solvent, pH, and concentration
Because of the water insolubility of most organic compounds, a solvent must be used that conducts an electric current and also dissolves organic compounds. Generally this end is attained by adding acetone, alcohol, or lower-molecular-weight organic acids to a water solution of the electrolyte. Organic solvents raise the resistance of an electrolyte and lower the overvoltage of the cathode (116). Another disadvantage is the possibility of reaction of the organic solvent either at the cathode, with the depolarizer, or with the reduction product.
Use of concentrated water solutions of sodium and potassium aromatic sulfonates to increase the solubility of the depolarizer has been found to be of advantage (214, 215). Suspensions of the depolarizer in the catholyte have in many instances been employed successfully. Dispersion has been aided by use of a cellulose ether (278). It has been demonstrated (260) that surface-active agents, as emulsifying aids, decrease current efficiencies during electroorganic reductions.
When the reducible group forms the negative part of a dipole the lowering of the dielectric constant of the solvent, as in the addition of dioxane to water (108), forces the reducible group away from the cathode and thus lowers the reduction efficiency. This has been found to be true with p-nitrophenol and nitrobenzene; however, with benzaldehyde and phthalimide the increase in solubility overbalances the change of dielectric constant and the reduction efficiency is increased (108). Other workers (358) have pointed out the importance of the polarity of the organic compound being reduced and emphasized that only the dipole moment of the group being reduced is important. A compound can be reduced if its dipole moment is greater than that of the solvent.
The hydrogen ion concentration of the catholyte plays an important role in electroorganic reductions. The course of a reduction may be controlled by altering the pH, and the formation of intermediate condensation products may be accelerated with certain pH conditions. As examples of the first effect:
(1) it has been shown (268) that the reduction of carbon dioxide to formic acid takes place only in a very strong acid;
(2) aromatic carboxylic acids are reduced to benzyl alcohols in acid solution (224), whereas in alkaline medium a nuclear reduction occurs; and
(3) reduction of levulinic acid gave the hydroxy acid in acid solution and valeric acid in a basic solution (338).
The reduction of nitrobenzene in a basic solution leads to products with two aromatic rings (89, 90, 137, 211), presumably formed by a base-catalyzed condensation of such intermediates as nitrosobenzene and phenylhydroxylamine. The formation of bimolecular products from the reduction of carbonyl compounds in acid solutions may also be cited (91, 116, 117, 164, 184, 185) as examples of the effect of the pH of the reduction solution.
Generally it has been observed (116, 118) that the rate of reduction is higher in a high concentration of depolarizer than in a low one. High concentrations of depolarizer often give a more rapid reduction, but no increase in yields (84). The effects of the type of compound being reduced can be seen in Section III of this review; compounds which contain unsaturated nitrogen are generally the easiest to reduce, while aromatic benzenoid compounds are by far the most difficult to reduce.
6. Temperature
Temperature influences three factors (116): the cathode overvoltage, the rate of reduction, and the rate of diffusion of the depolarizer to the cathode. In addition, certain side reactions can be influenced by the temperature. The cathode overvoltage loss in most
metals amounts to about 0.02-0.03 v. for every tendegree rise in temperature. Despite this drawback, the rate of reduction in some instances is so low at room temperature that heat is necessary to bring about reduction (267, 336, 339, 359). The increase in reduction, or velocity of reduction, with heat probably is caused by an increase in the rate of diffusion of the depolarizer to the cathode (184). The heating of the cathode itself has been found to be of no value in at least one case (228).
7. Catalysts
Catalysts for electroreduction can be divided into three types (116, 119):
(1) salts of metals such as copper, tin, lead, and mercury which plate out on the cathode, thereby altering its nature and catalytic action;
(2) salts of metals such as titanium and vanadium, which are reduced at the cathode from a higher to a lower valence state and in this latter state reduce the organic material;
(3) finely divided metals with a true catalytic surface, either suspended in the catholyte or fixed to the cathode surface. The influence of catalyst poisons has been studied (295), and it has. been found that the addition of arsenic trioxide increased the yields of aniline, but decreased the yields of glyoxalic acid from oxalic acid in electroreduction. From this it was deduced that the reduction of nitrobenzene was based on atomic hydrogen, but that the latter reduction depended on the hydrogen overvoltage.
8. Agitation and miscellaneous factors
As has been mentioned, the rate of diffusion of a depolarizer to the cathode during reduction limits the velocity of reduction. If electrons are supplied to protons faster than the atomic hydrogen is used to saturate the depolarizer, a state of concentration polarization may be attained and gaseous hydrogen evolved. It has been shown (184, 229) that the reaction velocity depends upon the diffusion of the depolarizer to the cathode. In a stirred solution, however, one can reach a point where the velocity of reduction is independent of the rate of diffusion (184).
In a few cases (242, 251) electroorganic reductions have been carried out under a pressure of either hydrogen or carbon dioxide. In many cases the yields are increased at increased pressure, but the results have been variable.
The use of ultrasonics (181) in the reduction of nitrobenzene has been found to give a ten to fifteenfold increase in the limiting current.
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C. METHODS AND APPARATUS FOR ELECTROORGANIC REDUCTIONS
1. Use of diaphragms
The organic depolarizer, cathode, and catholyte must be isolated from the anode section of the electrolytic cell; otherwise the organic compound could diffuse into the anolyte and be oxidized. In most cases this end is accomplished by enclosing or separating the cathode section of the electrolytic cell from the anode section with a partition which conducts an electric current but prevents diffusion of the depolarizer to the anode. Partitions have been constructed of porous clay, alundum, wood, sintered glass, and paper. Because of the high resistance of partitions and their tendency to become impervious, as a result of the formation of tar, such diaphragms are but a poor solution to the problem of anodic oxidation. Many attempts to solve this problem by rendering the anode passive to the depolarizer have met with failure. However, several investigations (95, 96, 265, 277, 298, 299) have been reported wherein an effective separation of the catholyte was accomplished by using either two immiscible solutions, or solutions of considerably different specific gravity. In these cases, of course, the cathode and anode are arranged one above the other. It has been reported (230) that no diaphragm was needed for the reduction of fumaric acid, whereas reduction of maleic acid required a diaphragm.
2. Cathode materials
Cathodes may be prepared from metallic sheets, wire, gauze, rods, or wool. The materials from which cathodes have been made are platinized platinum, platinum, tungsten, antimony, gold, nickelized nickel, nickel, tantalum (386), palladium, silver, carbon, tin, copper, iron, graphite, bismuth, aluminum, mercury, zinc, lead, lead dioxide, gallium (371), cadmium, thallium (385), and amalgams of several of these metals. Platinum, mercury, and lead appear to be the most widely used cathodes. In many instances the pH of the catholyte governs the choice of the cathode. Although, as mentioned earlier, the purity of the cathode has been shown to be of prime importance, there have been several instances reported in which alloys, amalgams, and plated cathodes have proven of value. The use of the following cathodes has been reported: iron-nickel alloys (115), cadmium-bismuth alloys (263), Monel (215), bronze (215), and amalgamated cadmium. Cathodes have been studied in which-, tin was plated on copper and lead; lead, copper, zinc, nickel, iron, and silver were plated onto porous graphite; and nickel, tin, and copper were plated onto nickel.
S. The catholyte
The catholyte may consist of either an aqueous or a nonaqueous medium. In the event water is the catholyte, insoluble organic compounds are brought into solution by adding such solubilizers as methanol, ethanol, formic acid, or acetic acid. Concentrated aqueous solutions of sodium or potassium benzene-, toluene-, or xylenesulfonates have been shown (215) to be excellent hydrotropic agents. Some instances have been reported in which the insoluble organic compounds were emulsified in water by adding Igepon T, Nekol BX, sodium lauryl sulfate, cetylpyridinium bromide (260), or a cellulose ether (278).
In order to make aqueous solutions more conductive to an electric current, acids such as sulfuric acid, hydrochloric acid, hydrocyanic acid, and ammonium hydrogen sulfate have been utilized. A basic conducting medium has been attained by adding ammonium hydroxide, sodium hydroxide, or sodium carbonate to water. Nearly neutral aqueous solutions have been prepared by adding ammonium chloride, ammonium sulfate, or sodium acetate to water. Methanol, ethanol, formaldehyde, formic acid, acetic acid, ethyl acetate, hydrogen cyanide, liquid ammonia, and fuming sulfuric acid have served as nonaqueous catholytes. Sodium and potassium acetates have been added to many of the above catholytes to increase the conductivity of the electrolyte.
4. The anode and the anolyte
The anode should be a nonattackable electrode such as platinum, carbon, lead, nickel, or iron. Generally, the anolyte should be of the same material and concentration as the catholyte, for this reduces to a minimum complications that may arise because of diffusion through the diaphragm.
5. Source, measurement, and control of current and potential
Six-volt storage cells are the best source of power for electroorganic reduction. The consumption of current may be measured by an ammeter, and the potential drop across the cell by a voltmeter. A variable resistance must be included in the circuit in order to control the current density at the cathode. Many varieties of power packs and converters are also now available.
The determination of the quantity of electricity (coulombs) passed through a cell may be accomplished by recording accurately the time intervals and the amperes. An easier method is the utilization of silver, copper, or gas coulometers. The silver or copper coulometers are less desirable for routine work, for the gas coulometer visibly evolves gas in direct proportion to the current passed through the reduction cell, and may be referred to with ease as the reduction proceeds. In some investigations the cathode of the reduction cell itself is enclosed so that any gas escaping from it may be measured. If the gas evolved by the coulometer and by the cathode is known, the absorption of hydrogen by the depolarizes may be determined easily. Almost all references contain some description of the apparatus and many include wiring diagrams. The above apparatus has been described in- an excellent manner (260).
A determination of cathode potential requires the use of a probing electrode, potentiometer, and standard cell; a description of this apparatus has been presented (208). Designs of several pieces of apparatus which can be used to maintain constant cathode potential have been described (3, 107, 207). An apparatus to maintain constant cathode potentials as high as -6 v. has been described in both a micro and a macro form (10). A simple potentiostat, which requires no preliminary adjustment or calibration and which is sensitive to ±0.01 v. up to 25 v., has been reported (207).
6. Control of temperature
The temperature of a cell may be controlled by placing the entire cell in an ice bath, on a steam cone, or in a constant-temperature bath. A glass coil in the catholyte may be used to circulate cold or hot water; occasionally the cathodes themselves are hollow tubes through which cold or hot water circulates.
7. Agitation
Stirring may be accomplished by the usual types of paddle stirrers, by rotating a cylindrical cathode, or by circulating the liquid by means of a small pump. In some cases bubbling of air or other gas through the catholyte provides sufficient agitation.
Reduction of carbon-carbon unsaturated compounds
Ethylenic bonds
Carbon-carbon unsaturation, which is found in isolated ethylenic bonds, is very resistant to electroorganic reduction. The only successful attempts to reduce isolated ethylenic bonds have utilized cathodes with a spongy surface (for example see reference 258). It is to be questioned whether these reductions are bona fide electrolytic reductions, or merely reductions by molecular hydrogen at a catalytic surface.
Carbon-carbon double bonds conjugated with either another ethylenic group, a carbonyl group, or a nitro group appear to be easier to reduce than isolated ethylenic bonds. Either the use of high-overvoltage cathodes, such as lead or mercury, or of an activated cathode surface, such as platinized platinum or nickelized nickel, is necessary to bring about this type of electrolytic reduction. The diene acid B,y-diphenylmuconic acid, however, is reduced only to B,y- The olefinic group in acids with a double bond between a benzene ring and the carboxyl group, or between two carboxyl groups, can be reduced using lead or mercury cathodes. In acidic solutions at a mercury cathode certain acids undergo bimolecular reduction to adipic acids (172), but this is not general (374). The ,electrolytic reduction of 3-nitrocinnamic acid to B3-nitrophenylpropionic acid (293) was very unusual, for the nitro group is generally more easily reduced at the cathode than is the ethylenic group. Reduction of dimethylmaleic and dimethylfumaric acids at a mercury cathode gave racemic acid and meso-a, a'-dimethyl.succinic acid, respectively. This is apparently the first clear case of trans addition at an electrode (266).
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2. Aromatic unsaturation
A number of carbon-to-carbon double bonds in nitrogen heterocyclic compounds, including some alkaloids, have been reduced. Generally, but not always, either high-overvoltage cathodes or especially activated cathodes were necessary to bring about these electroreductions. Aromatic rings containing a nitrogen atom were reduced with relative ease at high-oversoltage cathodes, particularly lead. A comparison of the activity of heterocyclic and homocyclic rings may be noted in the reduction of quinoline to 1,4-dihydroquinoline and 1,2,3,4-tetrahydroquinoline (200).
Platinized platinum cathodes were required for the complete reduction of the benzene nucleus. Claims that aromatic carboxylic acids were completely reduced in the nucleus under basic conditions at lead or mercury cathodes (223) appear to be ill-founded (224). Several 1-alkoxynaphthalenes have been reduced to 3,4dihydronaphthalenes by use of a tetrabutylammonium iodide catholyte at a mercury cathode maintained at a constant potential (80).
3. Acetylenes
Nickelized nickel, platinized platinum, and silverplated copper cathodes served adequately for the reduction of acetylenes to ethylenes or saturated hydrocarbons. Acetylene can be reduced to ethylene by controlling the cathode potential (32). In a number of cases ethynes have been reduced to cis olefins (47).
B. REDUCTION OF CARBON-NITROGEN UNSATURATED COMPOUNDS
Carbon-nitrogen unsaturation has been reduced under electrolytic conditions in a variety of compounds. Ketoximes and aldoximes have been reduced to amines in acid solution, generally at a lead cathode.
The reduction of imino ethers to amines involves not only the saturation of a carbon-to-nitrogen double
bond, but also the hydrogenolysis of an ethoxy group to give ethanol (362).
Reduction of Schiff bases to secondary amines and related compounds has been carried out at high-overvoltage cathodes of lead or mercury in an acid solution. With some Schiff bases derived from ketones the use of a lead cathode and an alkaline solution has been reported (149). The reduction of a nitro compound in the presence of a carbonyl-containing compound may also lead to a secondary amine, the Schiff base probably being an intermediate.
Carbon-nitrogen unsaturation in heterocyclic compounds has also been reduced electrolytically, in many cases at the same time as carbon-carbon unsaturation.
A lead cathode is generally necessary for the reduction of the carbon-to-nitrogen triple bond. Unlike the other carbon-nitrogen compounds, a neutral solution appeared to be the most favorable for the reduction of cyanides to amines. It has been shown (235) that yields of amines by electroreduction of cyanides fell off as the catholyte was made either acidic or basic. In some cases, however, an acidic solution has been used successfully (239). A cyano group attached directly to a benzene ring gives good yields of a benzylamine upon electroreduction, but benzyl cyanide is hardly reduced at all to the corresponding amine (235) (however, see reference 166).
C. REDUCTION OF CARBONYL COMPOUNDS
1. Aldehydes
The electrochemical reduction of carbonyl compounds has been studied almost as extensively as has that of the nitro group. The reduction of saturated aliphatic aldehydes may take several routes. Monomolecular reduction appeared to be favored by an acidic catholyte. Use of a mercury or lead cathode generally gave an alcohol, while at a cadmium cathode, of highest overvoltage, reduction of an aldehyde to a hydrocarbon may take place. An example of bimolecular reduction is the reduction of glyoxylic acid to tartaric acid at a lead or mercury cathode in alkaline, neutral, or acid solution (42). Conjugated unsaturated aldehydes were more easily reduced than the isolated carbonyl groups.
As with the aliphatic aldehydes, the electrolytic reduction of aromatic aldehydes may take several courses: conditions for the electroreduction of aromatic saturated aldehydes. An acidic catholyte and high-overvoltage cathodes appeared to be most desirable, although good yields also were reported from 'basic solutions. It has been stated (287) that copper, the' cathode of lower overvoltage, favors bimolecular reduction.
2. Ketones
The electrolytic reduction of keto groups to methylene groups in aliphatic compounds has been reviewed by Swann (324). This reduction generally takes place only in acid solution; an amalgamated zinc cathode appears to be the most active. Reduction of aliphatic ketones at cathodes of high overvoltage, generally in alkaline solution, tends to lead to pinacol formation, although this reaction is not general for all aliphatic ketones. A host of
other products have been isolated from the electroreduction of aliphatic ketones. Generally the reduction of aldehydes can be accomplished a little more readily than the reduction of ketones.
Acetone has been reduced to isopropyl alcohol and pinacol in aqueous solution at a mercury cathode (141). It is believed that the acetone is first adsorbed on the mercury, forming a covalent bond. This may then dissociate as a free radical to form pinacol, or may first add a proton and then dissociate from the mercury surface as an ion, which adds a second proton to give the alcohol.
In certain cases an electroreduction of an ethylenic bond in an aliphatic unsaturated ketone may occur selectively. Also, with a, l3-unsaturated ketones, a bimolecular reduction may occur (187).
Reduction of saturated aromatic ketones at a lead cathode in slightly basic solutions gave high yields of secondary alcohols. An acidic catholyte generally produced a mixture of the pinacol and the benzhydrol; if the catholyte was strongly acidic, rearrangement to the pinacolone sometimes occurred. o-Nitrobenzophenone was reduced in alkaline solution at a lead cathode to o-hydroxylaminobenzohydrol, which loses water to form 1,2-dihydroanthranil (23.) To reduce aromatic ketones to hydrocarbons it is generally necessary to use amalgamated zinc cathodes. The reductive coupling of 4-methoxyacetophenone with 4-amino- and 4dimethylaminoacetophenones to yield mixed pinacols is achieved by electrolysis at a mercury cathode (14). It is of interest to note that 2,3-bis(p-aminophenyl)2,3-butanediol can be obtained only by electrolytic reduction of the appropriate ketone in an aqueous hydrochloric acid medium at a mercury cathode (10, 11). The usual chemical methods gave only the carbinol. It is also of interest to note in regard to the reduction of p-aminoacetophenone that using an aqueous hydrochloric acid solution at a mercury cathode with a reference potential of -1.1 v. gave the carbinol, while a reference potential of -1.5 v. (10, 11) or a tin cathode (196) gave the pinacol. A number of Mannich bases also have been reduced to pinacols (12). Depending upon the reference potential used, either a low-melting or a high-melting form of the pinacol was obtained from p-dimethylaminoacetophenone (5).
An acid solution with a high-overvoltage cathode, such as lead or mercury, is best for the reduction of cyclic ketones. Alcohols, pinacols, pinacolones, and hydrocarbons have been isolated from the electroreduction of cyclic ketones. Reduction of 2-methylcyclohexanone at a mercury or lead cathode gave pure trans-2-methylcyclohexanol, at a nickel cathode a mixture was obtained, at a copper cathode pure cis-2methylcyclohexanol was obtained, and at a platinum cathode no reduction took place (18). The reduction of 2-ethyl-l-methyl-3-piperidone has been reported to give products such as 1-methyl-2-propylpyrroline, 2-ethyl-3-hydroxy-l-methylpiperidine, N-methylheptylamine, and 1-methyl-2-propyl-2-pyrroline (193) depending on the cathode and the temperature. Thus reductions, and rearrangements of the Clemmensen type, can be obtained by electrolysis. Reduction of keto acids and keto esters may take several courses. Generally an acidic catholyte allowed complete reduction to hydrocarbons with hydrogenolysis of the ester group. A basic catholyte usually gives a less reduced product than an acidic catholyte.
Quinones are reduced easily at the cathodes of lowest overvoltage in acid solution. For example, the following gave the corresponding hydroquinones: anthraquinone (244), 3,6-bis(1,4-dihydroxyphenyl)-1,4benzoquinone (127), naphthoquinone (243), and quinone (1, 170).
3. Carboxylic acids, esters, and lactones
An acid solution at a lead or mercury cathode was preferred for the electroreduction of aliphatic carboxylic acids to aldehydes, alcohols, or hydrocarbons. With these cathodes, ethyl oxalate in ethanolic sulfuric acid has been reduced to ethyl glyoxylate ethyl hemiacetal (252). T
Aromatic carboxylic acids are reduced to alcohols in a sulfuric acid catholyte using a lead or mercury cathode. It is possible to reduce certain aromatic acids to aldehydes in alkaline solution in the presence of the borate ion at a mercury cathode (227). Isonicotinic acid has been reduced to 4-picoline at an amalgamated lead cathode (364). Aromatic carboxylic acid esters have been reduced to the corresponding ethers or alcohols in an acid catholyte at mercury or lead cathodes.
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4. Amides, imides, and related compounds
The reduction of amides to amines has been carried out in acidic catholytes (some aliphatic amides have been reduced in basic solutions) at lead cathodes. The more substituted the amide nitrogen, the better the yield of amine; a phenyl group attached to the amide group increases the yield of substituted benzylamine. In fact, any aryl group linked directly to the carbonyl function gave good results, indicating that the greater the electron-donating effect of substituents on the amide nitrogen and the a-carbon atom, the greater the ease of reduction. A few instances of hydrogenolysis of the nitrogen compound to give ammonia or a simple amine have been reported. Attempts to reduce p-aminobenzanilide have been without success at many cathodes (38).
Imides of dibasic acids have been reduced to cyclic lactams and cyclic amines in acid solution. Generally a lead cathode is used, but cadmium and amalgamated zinc have been useful in some cases. Occasionally the ring has been opened by hydrogenolysis. In the case of many aliphatic imides the product from the reduction of only one carbonyl group may be isolated in good yields, while electroreduction of both carbonyl groups gave poor yields of the corresponding cyclic amine. N-substituted tetrachlorophthalimide can be reduced to a hydroxyphthalimide at a palladium cathode and a potential of -0.68 v. This hydroxyphthalimide can then be reduced further to an isoindoline at a reference potential of -1.10 v. (13). The mechanism of this reduction has been discussed in some detail (8).
D. HYDROGENOLYSIS OF ALCOHOLS
sAside from the possible reduction of carbonyl groups to methylene groups that may conceivably proceed through the alcohol, there are very few examples known of the hydrogenolysis of an alcohol to the methylene compound. In the case of leucopterin the hvdrogenolysis of all the hydroxyl groups does not take place, and the product is 2-amino-7-hydroxypteridine (366). The hydrogenolysis of hydroxysparteine to sparteine (112) may also be cited in this ection.
E. REDUCTION OF NITROGEN-OXYGEN UNSATURATED COMPOUNDS
1. Hydroxylamines
A nickel cathode and a basic catholyte were preferred for the reduction of aromatic hydroxylamines to szoxy compounds or to amines. Liquid ammonia has been used as a solvent in the reduction of phenylhydroxylamine to benzenediazoic acid (289).
2. Nitroso compound
The reduction of a nitrogen-nitroso compound to the corresponding hydrazine was best carried out in an acidic catholyte at cathodes of copper, lead, mercury, and cadmium, while a nickel or platinum cathode and a basic catholyte were preferred for the reduction of a carbon-nitroso compound to the corresponding amine. For example, 1,1-dimethylhydrazine was obtained in 90 per cent yield by the electroreduction of N-nitroso dimethylamine in aqueous sulfuric acid at a lead cathode (152) and 2,4,5-triamino-6-hydroxypyrimidine was obtained in 80-85 per cent yield by the reduction of 2,4-diamino-6-hydroxy-5-nitrosopyrimidine in dilute alkali (383), or by the reduction of the 5-isonitroso compound in aqueous hydrochloric acid or 1 per cent sodium hydroxide (28).
3. Azoxy compounds
The reduction of azoxy compounds to azo, hydrazo, or amino compounds took place quite readily in an acid solution. In this manner, azoxybenzene gave azobenzene, hydrazobenzene, benzidine, and aniline (30).
4. Aliphatic nitro compounds
The reduction of aliphatic nitro compounds was carried out in an acidic catholyte at a wide variety of cathodes. Generally, the cathodes of lower overvoltage such as copper, tin, or nickel, permitted partial reduction of the hydroxylamine (which can be reduced further under certain conditions as mentioned above), whereas the cathodes of higher overvoltage, such as mercury and lead, favored the complete reduction of the nitro group to the amino group.
A number of 2-aminoethyl compounds have been prepared by the reduction of a,o-unsaturated nitro compounds at a lead cathode in hydrochloric acid.
5. Aromatic nitro compounds
The aromatic nitro group is perhaps the easiest of all unsaturated groups to reduce electrolytically. The platinum cathode has been principally used to bring about this reduction, but procedures have been described for the reduction of aromatic nitro compounds at almost all other known cathodes. The pH of the catholyte is of prime importance in controlling the product of the electroreduction of an aromatic nitro compound; in fact, several chapters of a book (42) have been devoted to the reduction of aromatic nitro compounds in basic, neutral and acidic catholytes.
Briefly, the effect of the pH of the catholyte upon the reduction of aromatic nitro compounds may be divided into four classes. In a basic catholyte bimolecular products, the azo-, azoxy-, and hydrazo-compounds, predominate. Presumably this is due to the base-catalyzed condensation of such intermediate
reduction products as the nitroso- and hydroxylamine compounds.
The reduction to azoxy compounds is general except for o- and p-nitrophenols, nitroanilines, and nitro-Nand -N,N-substituted anilines, and certain hindered nitro compounds. Generally a cathode of low hydrogen overvoltage, such as nickel, and an alkaline solution are used. The low hydrogen overvoltage prevents the azoxy compound from being reduced. The azoxy compound is insoluble in the eatholyte and precipitates out. In order to prepare the azo compound the azoxy compound must be kept in solution. Generally the addition of alcohols or salts of aromatic sulfonic acids aids this solubility. A nickel cathode is most generally used at a temperature near the boiling point of the catholyte for the formation of azo compounds. In order to form hydrazo compounds the conditions for the formation of an azo compound are followed and then the cathode current density is lowered. In a neutral or weakly acidic medium it is sometimes possible to isolate the aromatic hydroxylamine and nitroso compounds. The reduction of aromatic nitro compounds to p-aminophenols has been known for some time. At a smooth platinum cathode in sulfuric acid the reduction of aromatic hydroxylamines does not proceed rapidly; rather, in the acid medium they rearrange to the p-aminophenols. In the case of 3-nitrophthalic acid the hydroxylamine, as formed, reacts with the carbonyl group to give benzisoxazolone-4carboxylic acid (120).
In strong acid solutions amines result from the reduction of nitro compounds, while in a very strongly acidic catholyte benzidine may be obtained, probably through the acid-catalyzed benzidine rearrangement of intermediate hydrazo compounds.
An excellent procedure for the preparation of 3chlorotoluquinone has been described (48). 6-Chloro2-nitrotoluene is reduced electrolytically to the p-aminophenol, and the reaction mixture is oxidized directly to the quinone in good yield. It is often possible to reduce the nitro group without further reducing other groups present in the molecule.
F. REDUCTION OF NITROGEN-NITROGEN UNSATURATED COMPOUNDS
1. Azo compounds
Azo compounds were reduced easily to the corresponding hydrazo compounds, or cleaved to two molecules of the corresponding amines, in either basic or acidic catholytes at many different cathodes, including platinum. Azobenzene, for example, has been reduced to hydrazobenzene, benzidine, and aniline (30, 328). Coke has been found to be an efficient cathode for this reduction (328).
2. Diazonium salts
A few diazonium salts have been reduced at a number of high-overvoltage cathodes in acidic catholytes to the corresponding substituted phenyihydrazines. In this manner phenylhydrazine (269) and oand p-methoxyphenylhydrazines (101) have been obtained in good yield; in the case of the methoxy compounds the yields are better than those obtained by conventional chemical methods.
G. REDUCTION OF SULFUR-CONTAINING ORGANIC
COMPOUNDS
Reduction of the sodium or potassium salts of sulfonic acids at a lead cathode resulted in cleavage of the sulfonic acid residue from the organic molecule.
In an acid solution at a lead cathode aromatic sulfonyl chlorides may be reduced to sulfinic acids, disulfides, sulfoxides, or thiophenols, depending upon the duration of the electroreduction (340, 341).
Sodium thiosulfates are reduced to disulfides in neutral or alkaline catholytes at a platinum cathode.
Even with very long periods of electroreduction no mercaptans or thiophenols were ever isolated.
By the use of a lead or an amalgamated aluminum cathode and an acidic catholyte, thioacetamides are reduced to the corresponding amines with complete removal of the sulfur.
Reduction of nitrated aromatic thiocyanates, in which the two groups were adjacent to one another, resulted in interaction of the reduced forms to give a thiazole (104).
Reduction of disulfides in an acid solution at a lead cathode resulted in reductive cleavage of the molecule between the two sulfur groups to give the corresponding mercaptans (291). Homocystine has been converted to homocysteine by use of a mercury cathode and a basic catholyte (15). Thiolactams were reduced to the corsponding cyclic amines at a lead cathode in an acidic catholyte.
H. REDUCTION OF MISCELLANEOUS
ORGANIC COMPOUNDS
Hydrogenolysis of organic halogen compounds has been found to take place in either acidic or basic catholytes at palladium, lead, copper, mercury, and amalmated zinc cathodes. Generally better yields, in a shorter period of time, were obtainable in an acid catholyte. The reduction of 9-(o-iodophenyl)acridine to (o-iodophenyl)acridan and then to 9-phenylacridan at a mercury cathode can be performed selectively by controlling the potential (208). The removal of an aromatic halogen cannot always be predicted (see reference 13).
Arsenic compounds in an acid catholyte were reduced to arseno or arsinic type compounds; generally a high concentration of acid favored the former.
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