RHENIUM HEPTOXIDE

Litde is known concerning the toxicology of rhenium or its compounds. Rhenium heptoxide is said to be an irritant if inhaled. 

Metathesis of oleic acid to produce a C g straight-chain dibasic acid can be carried out at 70°C with a WCl Sn(CH2)4 catalyst or with rhenium heptoxide promoted by Sn(CH2)4 (55,77). 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

Re207 RHENIUM HEPTOXIDE -50918.9674 1.6274E+06 2.0103E+04 -1.5444E+01... 

Rhenium heptoxide (Re O, for example, is explosive. There are at least six different forms of rhenium oxides. 

Although all of the above elements catalyze hydrogenation, only platinum, palladium, rhodium, ruthenium and nickel are currently used. In addition some other elements and compounds were found useful for catalytic hydrogenation copper (to a very limited extent), oxides of copper and zinc combined with chromium oxide, rhenium heptoxide, heptasulfide and heptaselen-ide, and sulfides of cobalt, molybdenum and tungsten. 

Use of ruthenium catalysts [41] and rhenium heptoxide [42] is rare. Their specialty is reduction of free carboxylic acids to alcohols. 

The same is true of rhenium catalysts rhenium heptoxide [42], rhenium heptasulfide [5i] and rhenimn heptaselenide [54] all require temperatures of 100-300° and pressures of 100-300 atm. Rhenium heptasulfide is not sensitive to sulfur, and is more active than molybdenum and cobalt sulfides in hydrogenating oxygen-containing functions [55,55]. 

With the same excess of catalysts hydrogenations of the esters over Raney nickel could be carried out at temperatures as low as 25-125° at 350atm with comparable results (80% yields). However, benzene rings were saturated under these conditions [55]. In addition to nickel and copper, zinc and chromium oxides, rhenium obtained by reduction of rhenium heptoxide also catalyzes hydrogenation of esters to alcohols at 150-250° and 167-340 atm in 35-100% yields [42]. 

Broadbent HS, Campbell GC, Bartley WJ, Johnson JH (1959) Rhenium and its compounds as hydrogenation catalysts. 3. Rhenium heptoxide. J Org Chem 24 1847-1854... 

Prereduced rhenium heptoxide catalyst,65 especially the catalyst poisoned with pyridine, has been found to give high yields of unsaturated alcohols in the hydrogenation of unsaturated aldehydes (Table 5.2).66 A typical hydrogenation with the rhenium catalyst is shown in eq. 5.27. In the vapor phase hydrogenation of acrolein to allyl alcohol, the selectivity of rhenium catalysts has been found to be improved by poisoning with CO and CS2.67... 

TABLE 5.2 Hydrogenation of Unsaturated Aldehydes over Prereduced Rhenium Heptoxide Poisoned with Pyridine1 ... 

Broadbent et al. have found that the rhenium blacks prepared by reducing rhenium heptoxide are highly effective catalysts for the hydrogenation of carboxylic acids to alcohols at 150-170°C for monocarboxylic acids (eq. 10.3) and at 200-250°C for dicarboxylic acids (eq. 10.4) under 13.5-27 MPa H2.10,11 Rhenium heptoxide can be reduced to the active blacks in appropriate solvent (ethanol, 1,4-dioxane, acetic acid, or water) at 120-220°C and 15-21 MPa H2 for 1-2 h, or more conveniently, in situ in... 

Carboxylic acids are one of the most difficult of all functional groups to hydrogenate. They can be hydrogenated to the alcohol, however, over ruthenium 34 or rhenium heptoxide 35,136 t 150°-200°C and 300-400 atmospheres pressure. Activated Cu-Ba-CrO is also effective at 250°-300°C and 300-400 atmospheres (Eqn. 18.31). 3 A Re-Os bimetallic catalyst has been used to hydrogenate carboxylic acids at 100 -120°C and 20-100 atmospheres of hydrogen. High conversions to the alcohol were reported. 38... 

Derivation By reacting rhenium heptoxide with carbon tetrachloride at 400C. 

Perrhenate is also leached with water from a sample melted with Na2C03 or NaOH. Rhenium heptoxide, Rc207, is volatile and can be separated by distillation at 260-280°C from a concentrated sulphuric acid medium [1,17,18]. It may be accompanied by As(III), Hg, Se, and to a lesser degree by Sb, Te and Mo. In the presence of hydrochloric acid, rhenium distils as the oxychloride at a lower temperature, but As, Ge, Hg, Sn, Se, Mo, Te, and T1 are wholly or partly co-distilled. In view of the partial co-distillation of Mo, Re cannot be directly determined in the distillate. In the distillation of Re, carbon dioxide or air is used as the carrier gas. 

Re207 RHENIUM HEPTOXIDE 113.266 6.8782E-01 -3.3492E-03 7.2466E-06 -5.8789E-09 298 318 solid... 

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