Raney nickel with promoters

This approach should be useful in determining the direction of hydrogenation for molecules in which the carbinol group is replaced by carbon-carbon or carbon-nitrogen double bonds. With an alkene, though, the simple conformational model would have to be used and the hydrogenation should be run under conditions that do not promote double bond isomerization, that is, not with palladium or nickel catalysts. With carbonyl compounds the preferred eonditions for selective reaction involve platinum, rhodium or ruthenium catalysts imder non-diffusion control conditions. The use of nickel catalysts, especially Raney nickel, with its basic components, can cause an equilibration of the alcohol product. 

There are five dihydropyrimidines. The 1,4- and the 1,6-dihydropyrimidines can readily interconvert by tautomerism because of the mobile NH proton. The common metals to effect hydrogenation can be used . Platinum has been the catalyst of choice for the reduction of the 5,6-double bond of uracils, for example, in the addition of deuterium to uracil to produce [5,6- H2]5,6-dihydrouracil. But in the reduction of 2(l/f)-pyrimidinone and its iV-methyl derivative it is the 1,6-dihydro derivatives which are formed. The addition of hydrogen to the 5,6-bond of thymidine and other 5-substituted uridines is stereospecific with rhodium-on-alumina as catalyst. Rhodium-on-charcoal has been useful for hydrogenation of the 5,6-double bond in uracils, uridine, and isocytosine. Raney nickel readily promotes saturation of the 5,6-double bond. Thio derivatives may either be dethiated or taken further to reduced forms by Raney nickel catalysts . 

This reaction is favored by moderate temperatures (100—150°C), low pressures, and acidic solvents. High activity catalysts such as 5—10 wt % palladium on activated carbon or barium sulfate, high activity Raney nickel, or copper chromite (nonpromoted or promoted with barium) can be used. Palladium catalysts are recommended for the reduction of aromatic aldehydes, such as that of benzaldehyde to toluene. 

In order to synthesize quinolizidine compounds, some authors have used the Parsons method (Bu3SnH/AIBN) to cleave the iV-tosyl group of 2-piperidones such as 144 (AIBN = 2,2 -azobisisobutyronitrile). After detosylation to 145, the intramolecular cyclization of the lactam promoted by sodium hydride gave quinolizidinone 146. T reatment of this compound with Raney nickel both cleaved the C-S bond and reduced the C=C bond to give quinazolinone 147, while the lactam carbonyl was reduced with LiAlH4 to give 148 (Scheme 23) <2005TL8551>. 

Compounds 3 (when R=He) and 5 are available In commercial quantities. Compound 4 may also be purchased or prepared In high yield via hydrogenation of 3 over a promoted Raney nickel catalyst (C. 6. Coe, unpublished results). The condensation of 4 with distilled acryloyl chloride proceeds In over 95X yield. 

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

Surfaces of finely divided nickel also promote the formation of aniline. A practical route to tlie preparation of electrodes coaled with a finely divided metal involves electroplating nickel onto a cathode from a solution containing a suspension of finely divided Raney nickel (Ni 50% A1 50%) or Devarda copper alloy (Cu 50% A1 45% Zn 5%), Some alloy particles stick to the cathode surface which is then activated by leaching out the aluminium using hot aqueous sodium hydroxide... 

Selective catalytic hydrogenation with chromium-promoted Raney nickel is reported (e.g. citral and citronellal to citronellol) NaHCr2(CO)io and KHFe(CO)4 reduction of a/3-unsaturated ketones (e.g. citral to citronellal) has been described (cf. Vol. 7, p. 7). The full paper on selective carbonyl reductions on alumina (Vol. 7, p. 7) has been published." Dehydrogenation of monoterpenoid alcohols over liquid-metal catalysts gives aldehydes and ketones in useful yields. ... 

To promote the activity and selectivity of Raney nickel catalysts, alloying of the starting Ni-Al alloy with metal was often used. For instance, Montgomery (ref. 4) prepared catalysts by activating ternary alloy powders of Al (58 wt %)-Ni (37-42 wt %) - M (0.5 wt %) where M = Co, Cr, Cu, Fe and Mo. All promoted catalysts tested were more active than the reference catalyst, in hydrogenation of butyronitrile. Molybdenum was the most effective promoter. With Cr or Ti, hydrogenation of isophtalonitrile on Raney nickel occurred at lower optimum temperature than with non activated nickel (ref. 5). It was shown that addition of Ti or Co to Raney nickel suppressed the formation of secondary amine (ref. 6). 

An equimolar mixture of 3,4,5-trimethoxy phenyl iodide 157, lithium propargyl alkoxide 158, and diethyl ethoxymethylene malonate 159 was stirred at room temperature in the presence of a palladium catalyst. Then, to the resulting intermediate 161 potassium t-butoxide was added, and the ensuing base-promoted decarboxylative aromatization afforded tetrahydrofuran MCR adduct 162 in good yield. The ester was first reduced and the furan ring was hydrogenated with Raney nickel to furnish a diastereomeric mixture of products 163 in high yield. Further synthetic manipulations then provided a known precursor to the natural product. 

Massive metals themselves are used as unsupported fixed-bed catalysts for example, Raney nickel is used in a variety of hydrogenation reactions. The synthesis of ammonia from N2 and H2 is carried out with reduced massive iron containing minor amounts of promoters. 

Although the action of most additives on a catalytic surface is somewhat obscure, it seems safe to assume that the action of platinic chloride is a promoter action through the formation of metallic platinum under these reductive conditions. The platinum is plated out on the nickel surface. Promoter action has been observed when platinic chloride in amount sufficient to provide only 0.4 mg. of platinum is mixed with as much as 3 g. of Raney nickel catalyst (41). 

TABLE 1.6 Hydrogenation of Organic Compounds with Promoted Raney Nickel Catalysts with Optimum Activity3... 

Raney copper is another Raney type catalyst that is prepared from a copper-aluminum alloy. This catalyst has been used infrequently but does show some reaction selectivity not possible with other catalysts. Of particular interest is its use for the selective hydrogenation of substituted dinitrobenzenes (Eqn. 11.6).2 This catalyst, as well as Raney cobalt, generally promotes fewer side reactions than does Raney nickel. 25... 

Metal cations can also affect the activity of Raney nickel catalysts. The presence of three to ten per cent of chromium or molybdenum in the Raney nickel alloy results in a general increase in the activity of the catalyst prepared from this alloy. Activation of nickel catalysts has also been accomplished by treating them with aqueous solutions of chromium chloride. Another effective promoter for Raney nickel is platinum which increases the activity of this catalyst for the... 

The Raney nickel alloy will react with water at 70°-85°C to give the skeletal nickel in the presence of aluminum hydroxide. The resulting catalyst is active after washing with alcohol . Since no base is present, the extensive water washings are not required. The reaction with water can be promoted by the addition of a small amount of base, and, further, some Bayerite, Al(OH)3, to further facilitate the aluminum removal. ... 

An ESCA analysis of the molybdenum promoted Raney nickel showed that when a low molybdenum content alloy was used, the activated catalyst had greater amounts of nickel on the surface than the unpromoted active catalyst. At a 2% molybdenum content the nickel surface area in the activated catalyst reached a maximum and then decreased with the presence of more molybdenum. These findings correlate with the observed maximum in catalytic activity observed with the 2% molybdenum Raney nickel catalyst. The initial increase in... 

The hydrogenation of aliphatic nitro groups takes place with retention of configuration when the reaction is run under the normal conditions. The basic nature of Raney nickel does not promote epimerization of the nitro group before hydrogenation at low temperatures but the alkaline impurity found in platinum oxide does. The use of platinum oxide in ethanol resulted in the formation of a mixture of product stereoisomers but, in acetic acid, retention of configuration was observed. Higher temperatures also promoted inversion (Eqn. 19.24).5>... 

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