Catalyst, general promoters

The ease of hydrogenolysis of benzyl amines depends on the nature of the substituents on the nitrogen atom. The debenzylation of tertiary benzyl amines takes place over palladium on charcoal at 25°-50°C and 2-4 atmospheres of hydrogen (Eqn. 20.43). 2 Platinum and rhodium catalysts generally promote... 

Hydrogenation Catalysts. The key to catalytic hydrogenation is the catalyst, which promotes a reaction which otherwise would occur too slowly to be useful. Catalysts for the hydrogenation of nitro compounds and nitriles are generally based on one or more of the group VIII metals. The metals most commonly used are cobalt, nickel, palladium, platinum, rhodium, and mthenium, but others, including copper (16), iron (17), and tellurium... 

Zinc chloride is a Lewis acid catalyst that promotes cellulose esterification. However, because of the large quantities required, this type of catalyst would be uneconomical for commercial use. Other compounds such as titanium alkoxides, eg, tetrabutoxytitanium (80), sulfate salts containing cadmium, aluminum, and ammonium ions (81), sulfamic acid, and ammonium sulfate (82) have been reported as catalysts for cellulose acetate production. In general, they require reaction temperatures above 50°C for complete esterification. Relatively small amounts (<0.5%) of sulfuric acid combined with phosphoric acid (83), sulfonic acids, eg, methanesulfonic, or alkyl phosphites (84) have been reported as good acetylation catalysts, especially at reaction temperatures above 90°C. 

The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier. The two types of sites are necessary for aromatization and isomerization reactions. 

The intramolecular asymmetric Stetter reaction of aliphatic aldehydes is generally more difficult to achieve due to the presence of acidic a-protons. Rovis and co-workers have demonstrated that the NHC derived from pre-catalyst 130 promotes the intramolecular Stetter cyclisation with enoate and alkyhdene malonate Michael acceptors 133. Cyclopentanones are generally accessed in excellent yields and enantioselectivities, however cyclohexanones are obtained in significantly lower yields unless very electron-deficient Michael acceptors are employed... 

Generally speaking, (and this coincides with an opinion of Morrison [8]) today there are four most general approaches to solve the problem regarding selectivity of semiconductor sensors. They entail a) the use of catalysts and promoters, b) the application of the method of temperature control, c) the control of specific surface additives ensuring development of specific adsorption, and, finally, d) the implementation of different filters. 

This procedure is based upon a study 1 of the method outlined in the patent literature.2 The procedure is a general one and may be used for the condensation of succinic anhydride with naphthalene and with the mono- and dimethylnaphthalenes, although in no other case are the purification and separation of isomers so easily accomplished. In this particular type of condensation, as well as in certain other types of Friedel-Crafts reactions, nitrobenzene is far superior to the solvents which are more frequently employed. This is partly because of its great solvent power and partly because it forms a molecular compound with aluminum chloride, and so decreases the activity of the catalyst in promoting side reactions. 

Use of catalysts, generally ferric or ferrous salts, promotes radical reactions with hydrogen peroxide the oxidizing action produced by the ferrous ions is more vigorous. 

A basic amine catalyst may promote the self-aldol reaction of the aldehyde, having an enolizable carbonyl. This reaction can be particularly important in the case of slowly reacting hindered aldehydes. In order to avoid this secondary reaction, a number of trialkylphosphines were tested and, in non-asymmetric reactions, tributylphosphine was generally found to be the most effective [32, 33],... 

Chiral N-oxides have also been employed as catalysts to promote aldol addition [62], but their true potential remains to be realized. Catalysis by N-oxides follows the same general trends that were established for the phosphoramide activators, though with reduced enantioselectivity. Thus, Nakajima [62] has demonstrated that the reaction of aldehydes 1 with silyl enol ethers 55, catalyzed by bidentate... 

Among the more important catalysts are metals, which may be promoted by other metals, or by oxides and oxides, which are usually rendered more effective by mixing with other oxides. It is usual to distinguish between supported catalysts, generally metals in a finely divided condition on the surface of silicate minerals, and promoted catalysts, where an oxide, or occasionally some other compound, is mixed with the metal the mixture being sometimes also supported on an inert refractory support. The distinction is not, however, absolutely sharp. 

Metathesis of Cycloolefins. The same general catalysts which promote the ring-opening polymerization of cycloolefins are also effective in the olefin metathesis reaction in which acyclic internal olefins undergo a unique redistribution process (1, 2, 5, 6). 

Promoters are often needed in addition to the actual catalyst itself. In general promoters increase the rate of a desired reaction this can occur because of a general increase in reaction rates or because of an increase in selectivity towards one product by comparison to others. Most promoters tend to be specific to a particular reaction and catalyst but some promoters can accelerate different reactions. For example potassium (usually added as K2O) promotes the silver catalyzed ethylene oxidation to ethylene oxide (Section 2.4), and also promotes carbon monoxide hydrogenation over Fe catalysts (Section 4.8). 

Comrrrerci al methanol catalysts generally consist of zinc and copper oxides, cither alone or together, pins a promoter which is usually chromium oxide. Mechanistic details involved have not been fully elucidated, probably due to the high pressure needed to carry out investigations under reaction conditions. Basically, there have been three mechanisms proposed (Schemes 1 -3). 

With alkenes having internal C=C bonds, hydroalumination is disfavored by both kinetic and thermodynamic factors, and the uncatalyzed reaction is generally unfeasible. The hydroalumination of internal alkenes can be catalyzed by the addition of titanium(IV) alkoxides but the same catalysts also promote the isomerization of the secondary aluminum alkyls generated into their primary isomers (equation 16). ... 

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... 

While the hydrogenation of unactivated aliphatic aldehydes and ketones does not generally take place over palladium, this catalyst readily promotes the hydrogenolysis of aryl aldehydes and ketones to the methylene at room temperature and 1-4 atmospheres pressure. The use of an acidic solvent facilitates this reaction2 -29 but is not essential for obtaining good to excellent yields of the desoxy product (Eqn. 18.8). 0 With a basic substrate such as a 2- or 4-acyl-pyridine, however, the alcohol product was obtained (Eqn. 18.9).31... 

---