Raney system

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

Amino-4-phenylthiazole when heated with Raney Ni is reported to yield acetophenone (469). In the course of a general study on reductive cleavage in heterocyclic systems Hoff et al. studied the reaction of 2-amino-4-methylthiazole with Na in liquid ammonia. Two equivalents of Na are necessary to obtain a mixture of 4-methyl-3-thiazoline (240) and... 

Reduction. Just as aromatic amine oxides are resistant to the foregoing decomposition reactions, they are more resistant than ahphatic amine oxides to reduction. Ahphatic amine oxides are readily reduced to tertiary amines by sulfurous acid at room temperature in contrast, few aromatic amine oxides can be reduced under these conditions. The ahphatic amine oxides can also be reduced by catalytic hydrogenation (27), with 2inc in acid, or with staimous chloride (28). For the aromatic amine oxides, catalytic hydrogenation with Raney nickel is a fairly general means of deoxygenation (29). Iron in acetic acid (30), phosphoms trichloride (31), and titanium trichloride (32) are also widely used systems for deoxygenation of aromatic amine oxides. 

The thiones are readily desulfurized with Raney nickel to give the corresponding unsubstituted compounds in bicyclic systems in the 2-, 4- and 7-positions, and in tricyclic systems such as (95). The 2-methylthio derivatives may be similarly desulfurized. Thione groups in the 4-position, but not the 2-position, in pyrido-[2,3- f ]- and -[3,2- f]-pyrimidines may be replaced directly with ammonia or amines. 

The perhydroisoindole system can be prepared by high-pressure hydrogenation of the isoindole over nickel on alumina at elevated temperatures. The use of Raney nickel with dioxane in the reduction of l,3-diphenyl-2-methylisoindole (47) gives the perhydro product (96), accompanied by the isoindoline (97). An alternative route to partially hydrogenated isoindoles has been described in Section III, D. 

A special technique was necessary to obtain good yields of ethyl pyrrole-3-acetate from ethyl pyrrole-3-glyoxalate. Reduction over W-7 Raney Ni in 50% aq ethanol was accompanied by major ring reduction and tarring. By use of a two-phase system, toluene and 50% aq ethanol, these side reactions could be curtailed. Apparently the desired product was removed effectively from the aqueous layer into the toluene as soon as it was formed (26). 

To accelerate the polymerization process, some water-soluble salts of heavy metals (Fe, Co, Ni, Pb) are added to the reaction system (0.01-1% with respect to the monomer mass). These additions facilitate the reaction heat removal and allow the reaction to be carried out at lower temperatures. To reduce the coagulate formation and deposits of polymers on the reactor walls, the additions of water-soluble salts (borates, phosphates, and silicates of alkali metals) are introduced into the reaction mixture. The residual monomer content in the emulsion can be decreased by hydrogenizing the double bond in the presence of catalysts (Raney Ni, and salts of Ru, Co, Fe, Pd, Pt, Ir, Ro, and Co on alumina). The same purpose can be achieved by adding amidase to the emulsion. 

In contrast to the resistance of cycloheptatriene systems (e.g., tropolone) towards hydrogenation, 3-benzothiepin 3,3-dioxide is readily hydrogenated at atmospheric pressure in the presence of Raney nickel to give l,2,4,5-tetrahydro-3-benzothiepin 3,3-dioxide in 78% yield.82... 

R. G. Herman et al. (8) studied these catalyst systems In great detail and suggested a Cu-fl solution In ZnO as active phase where Cu- - non-dlssoclatlvely chemisorbs and activates CO and ZnO activates H2. In the range of 15 to 85Z CuO In the catalyst, up to 16% Cu+1 became dissolved In the ZnO (9) and Cu+1 has been widely accepted as active site (10). Recently, however, Raney Cu-Zn catalysts have been shown to be very active methanol synthesis catalysts (11). The active component for these Raney catalysts was found to be metallic Cu with an activity maximum at 97 wt% Cu (12). 

Traditionally, monoqrclic arene hydrogenation is carried out in drastic conditions with heterogeneous catalysts [9-18] such as Rh/Al203 and Raney Nickel or metal sulfides. Nevertheless, some pure homogeneous systems have been reported [19-23]. 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

The selectivity of RNH2 on M/A1203 and Raney catalysts decreased in the order Co Ni Ru>Rh>Pd>Pt. This order corresponds to the opposite sequence of reducibility of metal-oxides [8] and standard reduction potentials of metalions [9], The difference between Group VIII metals in selectivity to amines can probably been explained by the difference in the electronic properties of d-bands of metals [3], It is interacting to note that the formation of secondary amine, i.e. the nucleophilic addition of primary amine on the intermediate imine can also take place on the Group VIII metal itself. Therefore, the properties of the metal d-band could affect the reactivity of the imine and its interaction with the amine. One could expect that an electron enrichment of the metal d-band will decrease the electron donation from the unsaturated -C=NH system, and the nucleophilic attack at the C atom by the amine [3], Correlation between selectivity of metals in nitrile hydrogenation and their electronic properties will be published elsewhere. 

In the most effective, chirally modified catalytic systems, Pt/cinchonidine and Raney-Ni/tartaric acid, the enantioselectivity was also sensitive to the method of catalyst preparation and on support properties (5, 6). 

Under relatively mild conditions the Ru/C catalyst poisoned with Sn (lines 1 and 2), the Ir/C catalyst (lines 14 and 15), and the Raney-cobalt catalyst modified with CoCl2 (line 19) seem likely systems to try when initiating a search for an effective method for selectively hydrogenating the C=0 bond in an a, 3-unsaturated aldehyde. 

Benton, W.J. Raney, I.H, Miller, C.A. Enhanced Videomicroscopy of Phase Transitions and Diffusional Phenomena in Oil-Water-Nonionic Surfactant Systems, paper presented at the National AIChE Meeting, March 1985, Houston, Texas. 

The choice of solvent can also be beneficial in another respect. This possibility was highlighted by the findings of Cioffi on the Raney Nickel catalyzed hydrogen-deuterium exchange of a model carbohydrate [l-0-methyl-/l-D-galactopyranoside] but under ultrasonic irradiation (Tab. 13.2) [43], Extensive deuteration at C-4 position occurred for a series of ethereal solvents, the C-3 position was deuterated by seven solvent systems and the C-2 position deuterated less extensively, also by seven solvent systems. For l,4-dioxane-D20 no labeling at the C-2 position occurred and for l,2-dimethoxyethane-D20 no C-3 labeling was observed. 

The reverse reaction, namely hydrogenation, has also frequently been used to decrease the degree of unsaturation present in macrocyclic systems - typically converting imine linkages to amine groups. Such hydrogenations have usually been performed catalytically (for example, using H2 in the presence of Raney nickel or a precious metal catalyst) or by means of chemical reductants such as sodium borohydride. 

The enantioselective hydrogenation of prochiral substances bearing an activated group, such as an ester, an acid or an amide, is often an important step in the industrial synthesis of fine and pharmaceutical products. In addition to the hydrogenation of /5-ketoesters into optically pure products with Raney nickel modified by tartaric acid [117], the asymmetric reduction of a-ketoesters on heterogeneous platinum catalysts modified by cinchona alkaloids (cinchonidine and cinchonine) was reported for the first time by Orito and coworkers [118-121]. Asymmetric catalysis on solid surfaces remains a very important research area for a better mechanistic understanding of the interaction between the substrate, the modifier and the catalyst [122-125], although excellent results in terms of enantiomeric excesses (up to 97%) have been obtained in the reduction of ethyl pyruvate under optimum reaction conditions with these Pt/cinchona systems [126-128],... 

A diastereoselective dipolar cycloaddition of chiral nitrone 80 with alkene dipolarophiles afforded imidazo[ 1,2-3]-isoaxazole (Scheme 9). The conversion via N-O reduction of this ring system with Raney-Ni in methanol gave the corresponding pyrrolo[l,2-A imidazole in 66% yield. The structure has been confirmed by X-ray diffraction crystal stmcture analysis <2000SL967>. 

As previously reported in CHEC-II(1996) <1996CHEC-II(8)249>, this ring system is fairly resistant to reduction, and, under more forcing conditions, the six-membered ring is reduced preferably. Desulfurization with Raney-Ni of 7-SMe derivatives was reported <1999T7645> to occur efficiently, as shown in Scheme 2. 

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