Dissolving metal reductions with zinc

More versatile reactions are being discovered in metal-templated reactions involving C-N bond formation.21-23 Co-condensation of formaldehyde, nitroethane and the aminothioether 26 in the presence of copper(II) yields an intermediate di-imino nitro macrocycle. Subsequent dissolving metal reduction with zinc in HC1 yields the saturated aza-thia macrocycle 27 in which the hydroxy group and amino groups are /rani-related (Scheme 3.10).22,23... 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

The traditional Reformatsky reaction involves the conversion of a a-haloester to a a-organozinc ester in the presence of zinc metal and an initiator such as I2 or 1,2-diiodoethane (the initiator is necessary to remove the layer of zinc oxide). The resulting organozinc reagent then reacts with an aldehyde or ketone to deliver a p-hydroxy ester. Other zinc sources such as Rieke zinc , dissolving lithium reductions with... 

Some platinum is still present as hexachloroplatinate(IV) in the light yellow filtrate. It is recovered by strongly acidifying this solution with hydrochloric acid (pH about 1), adding excess mossy zinc, and allowing the mixture to stand until reduction appears complete (about one hour), with occasional stirring if necessary to remove precipitated platinum from the zinc and expose fresh metal surface. Additional acid is added, if necessary, to dissolve any excess zinc, and the platinum is collected on a Buchner funnel and washed and dried as was the previously recovered platinum. Since the metal produced by reduction with zinc is not as pure as that produced by ignition of ammonium hexachloroplati-nate(IV), the products should be kept separate. 

Ester enolate have been generated under dissolving metal conditions with the reduction of a-halo carbonyl compounds. For example, the use of freshly activated Zn dust following Reformatsky protocol leads to the Claisen rearrangement of zinc enolate derived from 123 to yield acid 124. 

A eutectic mixture of sodium and potassium dissolves in THF to form an intense blue solution when 18-crown-6 is used as a complexing agent, enabling dissolving metal reductions to be conveniently performed in this solvent. Acetylenes are rapidly reduced to olefins in good yields at 0 "C but -stereoselec-tivity E Z = 3 1) is lower than with the classical procedures. Acetylenes are reduced to Z-olefins in almost quantitative yield by a zinc-copper couple in refluxing methanol. ... 

In this process, ores containing copper(II) oxide and copper(II) sulfide are dissolved in sulfuric acid, and then hydrogen is bubbled through the solution. The reduction is thermodynamically favored, because the standard potential of the couple Cu2+/Cu is positive ( ° = +0.34 V). Metals with negative standard potentials, such as zinc ( ° = —0.76 V) and nickel ( ° = —0.23 V), cannot be extracted by reduction with hydrogen. 

The complexing action of cyanide is also important in the metallurgy of silver and gold. Both gold and silver in the elemental state will dissolve in a solution of cyanide if air is present to effect an oxidation both metals form complexes of the type M(CN)7, from which the metals themselves may be recovered by reduction with metallic zinc. 

Nitric acid, UNO 3, and the nitrates are familiar enough so that little space need he devoted to them. It will be recalled that the acid is an excellent oxidizing agent when hot and concentrated, dissolving many metals and metallic sulfides, the latter with the formation of sulfur or sulfate. The reduction products of nitrate in acid solution are almost always the nitrogen(II) or nitrogen(IV) oxides, but nitrate can be reduced to ammonia in basic solutions, using active metals (such as zinc or aluminum). 

The characteristic colours and solubilities of many metallic sulphides have already been discussed in connection with the reactions of the cations in Chapter III. The sulphides of iron, manganese, zinc, and the alkali metals are decomposed by dilute hydrochloric acid with the evolution of hydrogen sulphide those of lead, cadmium, nickel, cobalt, antimony, and tin(IV) require concentrated hydrochloric acid for decomposition others, such as mercury(II) sulphide, are insoluble in concentrated hydrochloric acid, but dissolve in aqua regia with the separation of sulphur. The presence of sulphide in insoluble sulphides may be detected by reduction with nascent hydrogen (derived from zinc or tin and hydrochloric acid) to the metal and hydrogen sulphide, the latter being identified with lead acetate paper (see reaction 1 below). An alternative method is to fuse the sulphide with anhydrous sodium carbonate, extract the mass with water, and to treat the filtered solution with freshly prepared sodium nitroprusside solution, when a purple colour will be obtained the sodium carbonate solution may also be treated with lead nitrate solution when black lead sulphide is precipitated. 

The nickel, cobalt, and zinc in the reduction end solution are precipitated as metal ammonium double salts after solution evaporation to 500 gm/liter ammonium sulfate. The double salts containing the nickel and cobalt centrifuged from the solution are then dissolved in water. Nickel and cobalt are separated by formation of cobaltic pentammine sulfate solution. The cobaltic pentammine solution is reduced at 350°F under hydrogen at 500 psig to produce cobalt powder. The ammonium sulfate by-product is prepared by stripping out the metal values with hydrogen sulfide. 

The reduction of a carbon-carbon multiple bond by the use of a dissolving metal was first accomplished by Campbell and Eby in 1941. The reduction of disubstituted alkynes to c/ s-alkenes by catalytic hydrogenation, for example by the use of Raney nickel, provided an excellent method for the preparation of isomerically pure c -alkenes. At the time, however, there were no practical synthetic methods for the preparation of pure trani-alkenes. All of the previously existing procedures for the formation of an alkene resulted in the formation of mixtures of the cis- and trans-alkenes, which were extremely difficult to separate with the techniques existing at that time (basically fractional distillation) into the pure components. Campbell and Eby discovered that dialkylacetylenes could be reduced to pure frani-alkenes with sodium in liquid ammonia in good yields and in remarkable states of isomeric purity. Since that time several metal/solvent systems have been found useful for the reduction of C=C and C C bonds in alkenes and alkynes, including lithium/alkylamine, ° calcium/alkylamine, so-dium/HMPA in the absence or presence of a proton donor,activated zinc in the presence of a proton donor (an alcohol), and ytterbium in liquid ammonia. Although most of these reductions involve the reduction of an alkyne to an alkene, several very synthetically useful reactions involve the reduction of a,3-unsaturated ketones to saturated ketones. ... 

Numerous examples of homoleptic complexes in high or low formal oxidation states are known. In general, the high oxidation state complexes are best prepared by chemical or electrochemical oxidation of the normal oxidation state compounds, followed by further reaction in situ or precipitation with a suitable inert anion. In this respect, perchlorate is ideal as both oxidant and precipitant, but the complexes obtained are frequently violently explosive. Similarly, the low oxidation state complexes are best obtained by chemical or electrochemical reduction of available compounds (or normal oxidation salts in the presence of an excess of bpy). Commonly used reductants have included dissolving metals (zinc, sodium, lithium, magnesium) and the complexes Li(bpy) and Li2(bpy). Isolated examples are known of the synthesis of low oxidation state complexes by reaction of M(0) complexes with bpy or by metal vapor synthesis. 

Gold, silver, mercury, and platinum metals, as well as Se and Te, can be precipitated from acid solution in the elemental form by reduction with chemical reagents such as zinc, NH2OH, N2H4, SO2, or formic acid. In the trace analysis of high purity mercury the sample (about 100 g) is dissolved in HNO3 and the solution is warmed in the presence of formic acid. First of all, nitric acid, then mercury, is reduced. The mercury forms a separate liquid phase, and the impurities remain in the aqueous solution [102]. In the trace analysis of silver, the sample is dissolved in nitric acid, then formic acid and mercury are added. The silver liberated on reduction dissolves in the mercury to form an amalgam [102]. 

The reduction of furoxans by dissolving metals has frequently been accomplished. Zinc and acetic acid19 87,259 260,272-274 276 277 317 353 419 and tin and hydrochloric acid273 274,270 278,353 are the most commonly used reagents. The reduction products are usually dioximes (in some cases in the expected amphi configuration, in others in a different form), or furazans, probably arising as secondary products by dehydration of the dioximes. Zinc with acetic acid is reported to convert dibenzoylfuroxan into 1,2-dibenzoylethane.431 Stannous chloride in acid has often been used for the reduction to furazans.355,365,421,432... 

The Clemmensen reduction of aldehydes and ketones to methyl or methylene groups takes place by heating with zinc and hydrochloric acid. A non-miscible solvent can be used and serves to keep the concentration in the aqueous phase low, and thus prevent bimolecular condensations at the metal surface. The choice of acid is confined to the hydrogen halides, which appear to be the only strong acids whose anions are not reduced with zinc amalgam. The Clemmensen reduction employs rather vigorous conditions and is not suitable for the reduction of polyfunctional molecules, such as 1,3- or 1,4-diketones, or of sensitive compounds. However, it is effective for simple compounds that are stable to acid (7.38). A modification under milder conditions uses zinc dust and HCl dissolved in diethyl ether (ethereal HCl). Other methods for converting C=0 to CH2 are described in Schemes 7.87 and 7.105. 

The mined zinc ores retrieved from the mines are too low in zinc content for direct reduction to refined metal thus, they are first concentrated. Production of concentrates requires crushing and grinding followed by gravity or magnetic methods of separation or flotation. These processes may be combined, depending on the complexity of the ore. A caustic-leach process is used to decrease the extent of metal loss during the concentration process. In this process, the metal is leached by caustic soda, the resulting electrolyte is purified with zinc dust and lime, and the zinc is electrodeposited. The crude zinc may be dissolved in sulfuric acid and purified by electrodeposition. 

---