Transfer hydrogenation, nitro compounds

Hydrogenation of aromatic nitro compounds is very fast, and the rate is limited often by the rate of hydrogen transfer to the catalyst. It is accordingly easy to use inadvertently more catalyst than is actually necessary. Aliphatic nitro compounds are reduced much more slowly than are aromatic, and higher catalyst loadings (6,11) or relatively lengthy reduction times may be... 

Hatziantoniou, V., Andersson, B., and Schbon, N.-H., Mass transfer and selectivity in liquid phase hydrogenation of nitro compounds in a monolithic catalyst reactor with segmented gas-liquid flow. Ind. Eng. Chem. Process Des. Dev. 25, 964-970 (1986). 

While donor substituents assist in ortho and meta protonation, acceptor substituents direct protonation of the primary anion-radicals to the ipso and para positions. It should be emphasized that water treatment of the naphthalene anion-radical in THF leads to 1,4-dihydronaphthalene. Notably, the same treatment of this anion-radical, but o-bound to rhodium, leads to strikingly different results. In the rhodium-naphthalene compound, an unpaired electron is localized in the naphthalene, but no protonation of the naphthalene part takes places on addition of water. Only evolution of hydrogen was observed (Freeh et al. 2006). Being a-bound to rhodium, naphthalene acts as an electron reservoir. The naphthalene anion-radical part reacts with a proton according to the electron-transfer scheme similar to the anion-radicals of aromatic nitro compounds (see Scheme 1.14). 

The reaction has been suggested as involving a one-electron transfer from sodium hydrogen telluride to the nitro compound followed by the detachment of a nitrite anion from the resulting radical anion. 

Formic acid, anhydrous (M.W. 46.03, m.p. 8.5°, b.p. 100.8°, density 1.22), or a 90% aqueous solution, is an excellent hydrogen donor in catalytic hydrogen transfer carried out by heating in the presence of copper [77] or nickel [77]. Also its salt with triethylamine is used for the same purpose in the presence of palladium [72, 73], Conjugated double bonds, triple bonds, aromatic rings and nitro compounds are hydrogenated in this way. 

The reduction of aromatic nitro compounds to the corresponding amines was catalyzed by [Ru3(CO)i2] in combination with aliphatic amine cocatalysts (95). A mixture of diglyme and water was used as a solvent, turnover frequencies were about 5000 h-1, and a CO partial pressure of 20-50 atm was necessary. The reaction is highly selective for aromatic amines. It was speculated that the reaction proceeds via an intramolecular hydrogen transfer in a hydrido-metal-nitrene intermediate without prior formation of H2 in the water gas shift reaction. 

Considerable interest has been shown in the photochemistry of aromatic nitro-compounds, especially those reactions that involve hydrogen transfer to the nitro-group. Nitrobenzene is photoreduced... 

The photoinitiated colour change of aromatic nitro compounds such as the dinitrobenzyl-pyridine (140) is due to a hydrogen transfer from the methylene bridge to the oxygen of the nitro group (aci-nitro tautomerism, Scheme 14) (60JCP(32)1888). 

SELECTIVE HYDROGENATION OF AROMATIC AND ALIPHATIC NITRO COMPOUNDS BY HYOROGEN TRANSFER OVER MgO... 

The possibility of using of aliphatic alcohols as hydrogen donors for the catalytic transfer reduction of nitro group over MgO was examined. Catalytic hydrogen transfer was found to be effective and selective method for reduction of nitrobenzene, A-nitrotoluene, A-chloronitrobenzene, 4-nitro-m-xylene, 3-nitro-styrene, 3-nitrobenzaldehyde, 1-nitropropane, and 1-nitrobutane. Conversion of starting nitro compound into desired product depended on the alcohol used as a donor. Adsorption of reactant and catalyst deactivation were studied by esr. New aspects of a role of one-electron donor sites in hydrogen transfer over MgD were demonstrated. 

The Kp values for the reactions of hydrogen transfer between primary and secondary alcohols and nitro compounds were calculated using Van Krevelen and Chermin procedure (ref. 4). Similarly as the simple reduction with hydrogen the catalytic hydrogen transfer from alcohols to nitro derivatives is strongly favored thermodynamically. For all studied reactions with primary alcohols the K... 

Strong centres, forming anion radical even from nitrobenzene molecule are poisoned irreversibly, however, their presence is not necessity for the preservation of catalytic activity. Taking into consideration that regenerated MgO which is not able to ionize nitrobenzene molecule is still active in its reduction by hydrogen transfer and that only a few from reduced nitro compounds form ion radicals on catalyst surface one can ascertain that ion radicals formation is not necessary step in nitroarenes (or nitroparaffins) activation. Probably, one-electron donor sites take part only in activation of alcohol what was demonstrated by us earlier. 

Polymerization inhibitors stop or slow down polymerization by reacting with the initiator or growing-chain radicals. A wide variety of substances can behave as inhibitors quinones, hydroquinones, aromatic nitro compounds, aromatic amines, and so on. In cases where the inhibitor is a hydrogen donor (symbolized here by InH), then for inhibition to occur, the radical resulting from hydrogen transfer (In-) must be too stable to add to monomer. If it does add to monomer and starts a new chain, chain transfer occurs instead of inhibition. For perfect inhibition, the In- radicals must combine with themselves (or initiator radicals) to give inert products ... 

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