Hydrogen donors hydrazine

Two hydrogen-transfer systems have been developed that also give good yields of hydroxylamines. One uses 5% palladium-on-carbon in aqueous tetrahydrofuran with phosphinic acid or its sodium salt as hydrogen donor the other uses 5% rhodium-on-carbon in aqueous tetrahydrofuran and hydrazine as donor. These systems are complementary and which is the better may depend on the substrate (36). The reductions cannot be followed by pressure drop, and both require analysis of the product to determine when the reduction should be terminated. 

Hydrogen donors that function poorly with homogenous catalysts include hydrazine hydrate, alkenes (e.g., cyclohexene), and ascorbic acid. This is somewhat surprising as they can be very effective in heterogeneous transfer hydrogenation. 

Hydrazine is a very effective hydrogen donor in the catalytic transfer of hydrogen achieved by refluxing compounds to be reduced with hydrazine in the presence of Raney nickel [277,278,279] (p. 13). 

In the presence of the hydrogen donor tri-Af-butyltin hydride, only hydroxamic ester 195 is formed and the reaction is presumed to involve N—O bond homolysis to give the resonance-stabilized Af-alkoxyamidyl radicals (198), which in the presence of BusSnH are reduced to hydroxamic ester according to Scheme 30. In the absence of BusSnH hydride, formation of esters probably involves alkoxyamidyl dimerization to hydrazines (199) as intermediates (see Section Vl.C, Scheme 34), although HERON reactions of 194 with production of methoxynitrene cannot be discounted. Mesityl radicals, formed by the... 

In the absence of a laboratory or industrial pressure vessel, catalytic hydrogentransfer is a viable alternative. Here, the nitro compound is stirred in a solvent with a catalyst, commonly 1% platinum on carbon, in the presence of a hydrogen donor such as hydrazine, sodium formate (sodium methanoate) or methylcyclohexene (Scheme 7.6). In the case of hydrazine, interaction at the catalyst surface produces the reductant, hydrogen, and nitrogen. Toluene is the by-product when methylcyclohexene is used. 

Selective reduction to hydroxylamine can be achieved in a variety of ways the most widely applicable systems utilize zinc and ammonium chloride in an aqueous or alcoholic medium. The overreduction to amines can be prevented by using a two-phase solvent system. Hydroxylamines have also been obtained from nitro compounds using molecular hydrogen and iridium catalysts. A rapid metal-catalyzed transfer reduction of aromatic nitroarenes to N-substituted hydroxylamines has also been developed the method employs palladium and rhodium on charcoal as catalyst and a variety of hydrogen donors such as cyclohexene, hydrazine, formic acid and phosphinic acid. The reduction of nitroarenes to arylhydroxyl-amines can also be achieved using hydrazine in the presence of Raney nickel or iron(III) oxide. ... 

He also pointed out 12 possible N—H- -F—C hydrogen bonds [2]. The hydrogen bond donors (N—H) among these could roughly be classified into four N—H of amides (6) (Scheme 4.5), Lewis acid coordinated amine (8), aniline (10), and ammonium (9). Howard et al. showed another hydrazine proton donor (7) [23]. All these have acidic amino protons and thus could protonate negatively charged organic fluorine atoms. 

II Other molecules containing both active hydrogen atoms and donor atoms (0, N, and F) Alcohols, acids, phenols, primary and secondary amines, oximes, nitro and nitrile compounds with o-hydrogen atoms, ammonia, hydrazine, hydrogen fluoride, and hydrogen cyanide... 

The nitro groups of polynitrostyrene are reduced by phenyl hydrazine that acts as a hydrogen donor [225] ... 

Accordingly, the test involves a redox reaction analogous to that in the tests for hydrazine, hydroxylamine, sodium hydrosulfite, Rongalite, which function as hydrogen donors in alkaline solution. 

A general method has been developed for the rapid, metal-catalysed, transfer reduction of nitro-compounds to N-substituted hydroxylamines. High yields of hydroxylamines are obtained by reduction with hydrazine as hydrogen donor and a rhodium-charcoal catalyst, or alternatively using phosphinic acid as hydrogen donor and a two-phase solvent system, with a palladium-charcoal catalyst. Some hydroxylamines were too labile for isolation in a pure state and were further characterized by oxidation to the corresponding nitroso-compounds with ferric chloride. 

In recent years, some new hydrazinium salts of organic acids have been prepared by neutralization of organic acids with hydrazine hydrate. The reaction of hydrazine hydrate and formic acid in an ice water bath has led to the isolation of a monoformate salt of hydrazine. This hydrazinium monoformate is reported to be a hydrogen donor with Raney nickel [35]. 

Weak donor acceptor Alcohol, organic acid, phenol, primary and secondary amines, oxime, nitro and nitrile compounds having hydrogen at a-position, hydrazine, HF, and HCN... 

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