Dissolving metals pyridines

Reduction of isoindoles with dissolving metals or catalytically occurs in the pyrrole ring. Reduction of indolizine with hydrogen and a platinum catalyst gives an octahydro derivative. With a palladium catalyst in neutral solution, reduction occurs in the pyridine ring but in the presence of acid, reduction occurs in the five-membered ring (Scheme 38). Reductive metallation of 1,3-diphenylisobenzofuran results in stereoselective formation of the cw-1,3-dihydro derivative (Scheme 39) (80JOC3982). 

Cationic rings are readily reduced under relatively mild conditions. 1-Methylpyridinium ion with sodium borohydride (in H20, 15°C) gives the 1,2-dihydro derivative (330) at pH > 7 and the 1,2,3,6-tetrahydro derivative (331) at pH 2-5. The tetrahydro compound is probably formed via (332) which results from proton addition to (330). Pyridine cations are also reduced to 1,2-dihydropyridines by dissolving metals, e.g. Na/Hg. 

Chemical reduction of pyridines can be achieved with hydride, dithionite, dissolving metal reagents, or hydrogenation. The pyridine nucleus can be activated to reduction by conversion to a pyridinium species <1984CHEC(2)165, 1996CHEC-II(5)80>. 

Reactions of 1,2,4-thiadiazoles with radicals and electron deficient species are virtually unknown. Catalytic and dissolving metal reductions usually result in S—N bond cleavage. For example, the 5-anilino-3-hydroxy derivative (51) gives a good yield of l-phenyl-2-thiobiuret (52) on Zn-HCl reduction (Scheme 27). Reduction of the diamino derivative (53) gives amidinothiourea (54) from which it may be prepared by oxidation (Scheme 28). Under similar conditions, cleavage of the 3,5-diphenyl derivative (55) results in loss of sulfur and formation of benzylbenzamidine (56 Scheme 29). Reduction of 5-alkylamino-or 5-arylamino-3-alkylthio derivatives (57) with H2S in pyridine-triethylamine or sodium in liquid ammonia yields 1-substituted dithiobiurets (58 Scheme 30). 

Diacetoxymercuri-p-nitroaniline.—An alcoholic solution of 9 grams of p-nitroaniline is boiled with an aqueous solution of 18 4 grams of mercuric acetate. Any monomercuri compound is removed from the residue by washing with hot alcohol containing a little acetic acid, and the product is then dissolved in pyridine to remove any metallic mercury. The addition of acetone precipitates the compound, which is dried in vacuo. It is orange in colour and does not melt at 300° C. 

Radical Anions. Radical anions are common intermediates in organic reactions they are easily prepared from compounds with low-enough LUMOs by the addition of an electron (from a dissolving metal or from a cathode, or the solvated electron itself). Those derived from carbonyl groups (421) dimerize at carbon 339 those derived from a,/ -unsaturated carbonyl compounds (422) dimerize at the /1-position,340 and pyridines dimerize predominantly at the 4-position.341 In each case, the odd electron has been fed into the orbital which was the LUMO of the starting material the site of coupling therefore should, and does, correlate with the site at which nucleophiles attack the neutral compounds. 

In addition to being more selective, dissolved calcium metal functions in a similar way to lithium and sodium metals towards organic functional groups [45]. Tab. 4.2 lists reductions giving the same products by the three dissolved metals. Among these, calcium affords the highest yields for some substrates (entries 1-3). The compounds in Tab. 4.2 include an aldehyde, indole [46], aryl ketone, enone, naphthalene [47], pyridine N-oxide [48], benzyl alcohol, styrene, and buckminster-fullerene. 

Reduction of Pyridine. Highly resonance-stabilized aromatic heterocycles are, like benzene, resistant to catalytic hydrogenation. The conditions for reducing pyridine to piperidine shown in Scheme 6.9 suggest this. However, chemical reduction, as with sodium and alcohol, is a much easier process and accomplishes the same goal under much milder conditions. This type of reduction is referred to as dissolving metal reduction and proceeds with free-radical intermediates. 

Metal Cleaning. About 204 thousand metric tons of HCl (100% basis) was consumed in 1993 for steel pickling, wherein the hydrochloric acid readily dissolves all of the various oxides present in the scale formed during the hot rolling process. Using suitable inhibitors such as alkyl pyridines, HCl reacts very slowly with the base metal rendering the surface so clean that it must be passivated with a mild alkaline rinse. 

DMA in 500 ml ether mix rapidly with 270 ml 0.9 M phenyl-Li, boil fifteen hours and extract as for (VI) or as described previously to get 8 g oily 4-methoxy-indoline (or its 1-methyl derivative) (VII). Alternatively, add 36 g naphthalene to 300 ml tetrahydrofuran and add 11 g Na metal cut in small pieces. Reflux and stir three hours and add 18 g (VI) and 8 g DEA in 200 ml tetrahydrofuran rapidly and boil twelve hours. Evaporate in vacuum, dissolve the oily residue in 2N HCI and extract with ether. Proceed as described to get (VII). 4 g (VII) in 200 ml dry pyridine add to 6 g Cu chloride in 400 ml pyridine and reflux 1 xh hours. Pour on water and extract with ether. Wash extract with 4N HCI and then water and dry and evaporate in vacuum the ether to get 2 g of the indole (VIII). Alternatively, dissolve 4 g (VII) and 9.5 g cinnamic acid in 700 ml mesitylene, add 1 g 5% palladium-carbon and reflux five hours. Filter, wash with HCI and NaHC03 and dry and evaporate in vacuum the mesitylene to get the red, oily (VIII) (can chromatograph on alumina and elute with benzene-petroleum ether). 

Dichloro-tetrapyridino-rhodium Chloride, [Rh py4Cl2]Cl, is prepared by dissolving rhodium zinc in aqua-regia, and, after removal of acid, heating the aqueous solution with pyridine. On cooling, the pyridino-salt is deposited in yellow prisms. It melts when heated, yielding a black oil, and on further heating, metallic rhodium. 

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