Hydrogenolysis temperature

For the ruthenium catalysts with nitrate as the precursor of promoters, nitrate needs to be reduced by hydrogen, then transform into effective components during the reduction and activation of the catalysts. The existence of ruthenium and the properties of support have great effect on the hydrogenolysis reaction of nitrates. Metal ruthenium can reduce the decomposition temperatures of nitrates significantly. The optimum hydrogenolysis temperature of promoter on different... 

The hydrogenolysis temperature of nitrates has influence on particle size of ruthenium.The ruthenium particle sizes of Ru-M (M=K, Cs, Ba)/TC with different nitrate as promoter, measured after the hydrogenolysis reaction at temperatures in range of 430°C-550°C are shown in Fig. 6.13. The results show that promoters covered on the surface of ruthenium or interface between ruthenium and support. Therefore, the particle size of ruthenium in catalysts with single promoter is obviously bigger than the one (Ru/C) without promoter. 

The sensitivity of different promoters on hydrogenolysis temperature is different. For example CsNOs and KNO3 are suitable for the hydrogenolysis temperature below 475°C, while high temperature reduction will cause ruthenium particle sintering and growth. Due to the protection of promoter, ruthenium particles would... 

Fig. 6.13 Effect of hydrogenolysis temperature on the average Ru peu ticle size in Ru catalyst with different promoters...

Various terminal allylic compounds are converted into l-alkenes at room temperature[362]. Regioselective hydrogenolysis with formate is used for the formation of an exo-methylene group from cyclic allylic compounds by the formal anti thermodynamic isomerization of internal double bonds to the exocyclic position[380]. Selective conversion of myrtenyl formate (579) into /9-pinene is an example. The allylic sulfone 580 and the allylic nitro compound... 

Hydrogenation of trisodium citrate over a Ni catalyst at 8.6 MPa (85 atm) and a temperature of 220—230°C results in hydrogenolysis fragments. 

If hydrogenolysis does not occur, the pressure would be about 7000 lb. when a temperature of 300° is reaehed. If the pressure rises above 6200, starting with 3500 lb. at room temperature, it is evident that the quality of the catalyst or the alcohol is not satisfactory. Further attempts to prepare the glycol should be made with alcohol and catalyst of better quality,... 

Aromatic rings are hydrogenated with a variety of catalysts. However, aromatic alkoxy and hydroxyl substituents are susceptible to hydrogenolysis under most conditions used to saturate the ring. Hydrogenolysis does not occur to any appreciable extent with ruthenium catalysts even though high temperatures and pressures are required. Thus, substituted phenols are... 

Reduction of vinylic and allylic compounds without hydrogenolysis may present a problem. The ratio of olefin saturation to hydrogenolysis depends importantly on catalyst, temperature, solvent, and pH. In both classes of compounds, hydrogenolysis is favored by polar solvents, acid, and elevated temperatures hydrogenation, by nonpolar solvents and low temperatures. 

Hydrogenolysis can be diminished by reduction at low temperature, Hydrogenaiion of asperuloside tetraacetate (28) over 5% Rh-on-C in ethyl acetate at 25 C gave mainly 29 accompanied by several hydrogenolysis products, but by starling at — 30 C and raising the temperature slowly toO C over 3 h, 29 was obtained quantitatively. The catalyst was reusable at least three times (13). 

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. 

Nickel in the presence of ammonia is often used for reduction of nitriles to primary amines. The reaction is done at elevated temperatures and pressures ( 100 C, 1000 psig) unless massive amounts of nickel are used. Cobalt is used similarly but mainly under even more vigorous conditions. Nitriles containing a benzylamine can be reduced over Raney nickel to an amine without hydrogenolysis of the benzyl group (7). A solution of butoxycarbonyl)-3-aminopropyl]-N-<3-cyanopropyl)benzylamine (13.6 g) in 100 ml of ethanol containing 4 g. NaOH was reduced over 3.0 g Raney nickel at 40 psig for 28 h. The yield of A/ -benzyl-Air -(f-butoxycarbonyl)s >ermidine was 95% (7). 

Anilines have been reduced successfully over a variety of supported and unsupported metals, including palladium, platinum, rhodium, ruthenium, iridium, (54), cobalt, and nickel. Base metals require high temperatures and pressures (7d), whereas noble metals can be used under much milder conditions. Currently, preferred catalysts in both laboratory or industrial practice are rhodium at lower pressures and ruthenium at higher pressures, for both display high activity and relatively little tendency toward either coupling or hydrogenolysis,... 

CaLalysl and loading Reduction time (h) Temperature CQ Hydrogenolysis Conversion (%)... 

These data show hydrogenolysis to increase with temperature, a general observation supported by many experiments. Here the influence of temperature is less with the mixed-metal catalysts. 

A key step in the synthesis of 13-membered meta ansa and 14-membered para ansa peptide alkaloids involves catalytic hydrogenolysis of carbobenzyl-oxypeptide pentafluorophenyl esters. The most suitable solvent is dioxane with addition of a catalytic amount of pyrrolidinopyridine and 2% ethanol. Temperature should not exceed 90°C. The authors believe that after deblocking, the amino function remains on the surface until ring formation with the activated carboxylic function is accomplished (/5/). 

Selectivity is influenced by temperature. Hydrogenolysis of 22 to 23 was carried out at 5°C to prevent opening of the cyclopropane ring (S2). 

Extensive hydrogenolysis of vinyl ethers does not occur always over platinum. Reduction of 28 proceeded smoothly to 29 (/09). It is likely that the high pressure and low temperature used in this experiment helped to minimize hydrogenolysis. For effective use of subambient ( —30°C) temperatures in stopping hydrogenolysis of vinyl functions, see (/Oa). 

Asymmetric hydrogenolysis of epoxides has received relatively little attention despite the utility such processes might hold for the preparation of chiral secondary alcohol products. Chan et al. showed that epoxysuccinate disodium salt was reduced by use of a rhodium norbornadiene catalyst in methanol/water at room temperature to give the corresponding secondary alcohol in 62% ee (Scheme 7.31) [58]. Reduction with D2 afforded a labeled product consistent with direct epoxide C-O bond cleavage and no isomerization to the ketone or enol before reduction. 

Many other authors studied the catalytic activity of palladium in more complicated hydrogenation reactions because of being coupled with isomerization, hydrogenolysis, and dehydrogenation. In some cases the temperatures at which such reactions were investigated exceeded the critical temperature for coexistence of the (a + /3)-phases in the other case the hydrogen pressure was too low. Thus no hydride formation was possible and consequently no loss of catalytic activity due to this effect was observed. 

Palladium-catalyzed aminations of aryl halides is now a well-documented process [86-88], Heo et al. showed that amino-substituted 2-pyridones 54 and 55 can be prepared in a two-step procedure via a microwave-assisted Buchwald-Hartwig amination reaction of 5- or 6-bromo-2-benzyloxypyri-dines 50 and 51 followed by a hydrogenolysis of the benzyl ether 52 and 53, as outlined in Fig. 9 [89]. The actual microwave-assisted Buchwald-Hartwig coupling was not performed directly at the 2-pyridone scaffold, but instead at the intermediate pyridine. Initially, the reaction was performed at 150 °C for 10 min with Pd2(dba)3 as the palladium source, which provided both the desired amino-pyridines (65% yield) as well as the debrominated pyridine. After improving the conditions, the best temperature and time to use proved... 

---