Copper chromite rings

For more selective hydrogenations, supported 5—10 wt % palladium on activated carbon is preferred for reductions in which ring hydrogenation is not wanted. Mild conditions, a neutral solvent, and a stoichiometric amount of hydrogen are used to avoid ring hydrogenation. There are also appHcations for 35—40 wt % cobalt on kieselguhr, copper chromite (nonpromoted or promoted with barium), 5—10 wt % platinum on activated carbon, platinum (IV) oxide (Adams catalyst), and rhenium heptasulfide. Alcohol yields can sometimes be increased by the use of nonpolar (nonacidic) solvents and small amounts of bases, such as tertiary amines, which act as catalyst inhibitors. 

Ruthenium is excellent for hydrogenation of aliphatic carbonyl compounds (92), and it, as well as nickel, is used industrially for conversion of glucose to sorbitol (14,15,29,75,100). Nickel usually requires vigorous conditions unless large amounts of catalyst are used (11,20,27,37,60), or the catalyst is very active, such as W-6 Raney nickel (6). Copper chromite is always used at elevated temperatures and pressures and may be useful if aromatic-ring saturation is to be avoided. Rhodium has given excellent results under mild conditions when other catalysts have failed (4,5,66). It is useful in reduction of aliphatic carbonyls in molecules susceptible to hydrogenolysis. 

Copper chromite (Adkins catalyst) does not catalyze hydrogenation of benzene rings. 

Open-chain aliphatic ethers are completely resistant to hydrogenolysis. Cyclic ethers (for epoxides, see p. 83) may undergo reductive cleavage under strenuous conditions. The tetrahydrofuran ring was cleaved in vigorous hydrogenations over Raney nickel [420] and copper chromite [420] to give, ultimately, alcohols. 

Copper chromite tends to catalyze the reduction of the heterocyclic ring and any readily reduced substituents, whereas rhodium on alumina is an effective catalyst for the hydrogenation of the pyrrole ring under very mild conditions (B-77MI30505, B-77MI30507,79CJC1977). 

Partial hydrogenation of either the ortho or meta terphenyl (33) over platinum or copper chromite results in the saturation of the internal ring (Eqn. 17.29) but hydrogenation of para terphenyl (34) takes place primarily on a terminal ring (Eqn. 17.30). ... 

Unlike furfuryl alcohol, tetrahydrofurfuryl alcohol cannot be made successfully with a copper chromite catalyst as the latter has essentially no effect on the fiiran ring, its action being limited to the reduction of the carbonyl group. 

Monochloroanilines are made by reduction of chloronitrobenzenes with either iron/acid or, nowadays, mainly catalytic hydrogenation. Catalysts include platinum, copper chromite and rhenium in conjunction with palladium38. The chloroanilines are used in the manufacture of colorants, agricultural products, pharmaceuticals and polymers. For example, o-chloronitrobenzene (29) is a source of o-nitroanilinc, o-phenylenediamine (1,2-benzenediamine) (30), o-aminophenol (19b), o-chloroaniline and 3,3 -dichlorobenzidine (31a). The o-phenylenediamine (30) is a particularly versatile intermediate, used to prepare thioureidoformates. Ring-substituted o-phenylenediamines with cyanoesters yield benzimidazoles that, on condensation with an aldehyde, followed by treatment with H2S, give a range of thioureas. 

The main synthetic route to cyclopentapyrazines involves formation of the six-membered ring by condensation of appropriate 1,2-diamines and a-dicarbonyl compounds. For example, reaction of diacetyl with cyclopentane-1,2-diamine yielded the tetrahydro compound 2, which was dehydrogenated to the dihydroderivative 3. Suitable dehydrogenation methods include heating under reflux in xylene with palladium on charcoal or heating to 300° over copper chromite. Condensation of ethylenediamine or 1,2-propanediamine with cyclopentanediones such as 8 afforded tetrahydro compounds of formula 9, which on dehydrogenation with copper chromite at 300° afforded the dihydro derivatives 10. As expected, mixtures of isomers are formed when appropriate unsymmetrical starting materials are used. 

Although 1,5-dihydro-l-oxo derivatives of this ring system are quite stable to reduction, reductive cyclization of quinoxalinylpropionic acids yields hexahydro-l-oxo compounds. Thus reduction with hydrogen and Raney nickel of the acids 53 yields the hexahydro compounds 54. The earliest preparation of the ring system involved hydrogenation of the keto ester 55 at high temperature and pressure over copper chromite. The perhydro compound 56 was obtained by this approach. The use of hydrogen and Raney nickel resulted in the synthesis of compound 57. ... 

It is remarkable that furfuraldehyde can be converted into furfuryl alcohol in yields of up to 95% in presence of a copper chromite catalyst stabilized by barium,344b whereas it is hydrogenated simultaneously in the ring when the usual noble-metal catalysts are used. 

Already in the thirties, copper-chromite promoted by addition of an alkaline earth oxide, was a favourite commercial catalyst for various hydrogenations. In the gas-phase hydrogenation of furfural copper catalysts have been used mainly to avoid hydrogen addition on the furan ring. However, an imdesired further reduction of the furfuryl alcohol to 2-methyl-furan can sometimes occur. Bremner et al. [9] studied in details the reaction of furfural over a number of copper catalysts. These studies showed that in the hydrogenation of furfural, high temperatures (> 300°C) or the addition of chromite to the copper catalyst favoured the formation of 2-methyl-furan, whereas low temperatures (< 200°C) or the addition of alkali-metal containing compoimds favoured the formation of furfuryl alcohol. 

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