Mechanism, Raney nickel

Active Raney nickel induces desulfurization of many sulfur-containing heterocycles thiazoles are fairly labile toward this ring cleavage agent. The reaction occurs apparently by two competing mechanisms (481) in the first, favored by alkaline conditions, ring fission occurs before desul-, furization, whereas in the second, favored by the use of neutral catalyst, the initial desulfurization is followed by fission of a C-N bond and formation of carbonyl derivatives by hydrolysis (Scheme 95). 

In 1974, Gassman et al. reported a general method for the synthesis of indoles. For example, aniline 5 was reacted sequentially with r-BuOCl, methylthio-2-propanone 6 and triethylamine to yield methylthioindole 7 in 69% yield. The Raney-nickel mediated desulfurization of 7 then provided 2-methylindole 8 in 79% yield. The scope and mechanism of the process were discussed in the same report by Gassman and coworkers as well. 

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

B. Ethyl l-Amino-S-methylbenzoate. A 1-1., three-nocked, round-bottomed flask equipped with a mechanical stirrer, a condenser, and a nitrogen-inlet tube is charged with 11.25 g. (0.050 mole) of ethyl 4-amino-3-(methylthiomethyl)benzoate, 300 ml. of absolute ethanol, and 17 teaspoons (ca. 50 g.) of W-2 Raney nickel (Note 7). The reaction mixture is stirred at 25° for one hour, then stirring is discontinued, and the ethanolic solution is decanted from the catalyst (Note 8). The catalyst is then washed with one 300-ml. portion of absolute ethanol and one 500-ml. portion of dichloromethane, the solvent being removed... 

Among the earlier studies of reaction kinetics in mechanically stirred slurry reactors may be noted the papers of Davis et al. (D3), Price and Schiewitz (P5), and Littman and Bliss (L6). The latter investigated the hydrogenation of toluene catalyzed by Raney-nickel with a view to establishing the mechanism of the reaction and reaction orders, the study being a typical example of the application of mechanically stirred reactors for investigations of chemical kinetics in the absence of mass-transfer effects. 

The exact mechanisms of the Raney nickel reactions are still in doubt, though they are probably of the free radical type. It has been shown that reduction of thiophene proceeds through butadiene and butene, not through 1-butanethiol or other sulfur compounds, that is, the sulfur is removed before the double bonds are reduced. This was demonstrated by isolation of the alkenes and the failure to isolate any potential sulfur-containing intermediates. 

That the desulfurizing action of Raney nickel is retained even in the absence of hydrogen has been demonstrated by Hauptmann, Wladislaw and Camargo16 who found that Raney nickel which had been deprived of its hydrogen through heating in vacuum at 200° converted, for example, benzaldehyde dibenzyl thioacetal into a mixture of stilbene and bibenzyl. That this type of reaction proceeds by a free-radical mechanism appears... 

UHV surface analysis, apparatus designs, 36 4-14 see also Ultrahigh vacuum surface analysis mechanisms, 32 313, 319-320 Modified Raney nickel catalyst defined, 32 215-217 hydrogenation, 32 224-229 Modifying technique of catalysts, 32 262-264 Modulated-beam mass spectrometry, in detection of surface-generated gas-phase radicals, 35 148-149 MojFe S CpjfCOlj, 38 352 Molar integral entropy of adsorption, 38 158, 160-161... 

A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with nonprecious, relatively inexpensive metals. To date, most low cost catalyst development work has been directed towards Raney nickel powders for anodes and silver-based powders for cathodes. The essential characteristics of the catalyst structure are high electronic conductivity and stability (mechanical, chemical, and electrochemical). 

Alkyl bromides and especially alkyl iodides are reduced faster than chlorides. Catalytic hydrogenation was accomplished in good yields using Raney nickel in the presence of potassium hydroxide [63] Procedure 5, p. 205). More frequently, bromides and iodides are reduced by hydrides [505] and complex hydrides in good to excellent yields [501, 504]. Most powerful are lithium triethylborohydride and lithium aluminum hydride [506]. Sodium borohydride reacts much more slowly. Since the complex hydrides are believed to react by an S 2 mechanism [505, 511], it is not surprising that secondary bromides and iodides react more slowly than the primary ones [506]. The reagent prepared from trimethoxylithium aluminum deuteride and cuprous iodide... 

A 2-1. two-necked round-bottomed flask fitted with a mechanical stirrer and a reflux condenser is charged with 40.0 g. (0.22 mole) of p-cyanobenzenesulfonamide (Note 1), 600 ml. of 75% (v/v) formic acid, and 40 g. of Raney nickel alloy (Note 2). The stirred mixture is heated under reflux for 1 hour (Note 3). The mixture is filtered with suction through a Buchner funnel coated with a filter aid (Note 4), and the residue on the funnel is washed with two 160-ml. portions of 95% ethanol. The combined filtrates are evaporated under reduced pressure with a rotary evaporator (Note 5). The solid residue (Note 6) is heated in 400 ml. of boiling water and freed from a small amount of insoluble material by decantation through a plug of glass wool placed in a filter funnel. The filtrate is chilled in an ice bath and the precipitate is collected by filtration with miction, washed with a small amount of cold water and dried at 50°... 

General Comments. The formation of deoxy sugars by hydrogenation over Raney nickel often leads to the abnormal isomer (namely, that formed by diequatorial opening of the oxirane ring) as the major product, in contrast to the product afforded by lithium aluminum hydride this suggests that a different mechanism is involved in the nickel-catalyzed reaction. 

Raney-nickel catalysts are barely sensitive to catalyst poisoning (as are Pt-activated cathodes), e.g., by iron deposition, but they deteriorate due to loss of active inner surface because of slow recrystallization—which unavoidably leads to surface losses of 50% and more over a period of 2 years. A further loss mechanism is oxidation of the highly dispersed, reactive Raney nickel by reaction with water (Ni + 2H20 — Ni(OH)2 + 02) under depolarized condition, that is, during off times in contact with the hot electrolyte after complete release of the hydrogen stored in the pores by diffusion of the dissolved gas into the electrolyte. 

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