Rhenium catalysis I. Hydrogenation and hydroformylation using rhenium carbonyl compounds ; II. Hydrogenation using catalysts obtained from the reduction of perrhenates with metals in aqueous solution (1957)

The purpose of this work was to investigate the catalytic activity in both hydrogenation and hydroformylation reactions of rhenium preparations which have not been previously characterized. Rhenium pentacarbonyl was prepared in good yield from rhenium heptoxide and carbon monoxide. The optimum conditions for preparation were 25 hours per gram of dry rhenium heptoxide at 250° under 3000 psig. (initial) of carbon monoxide. Rhenium chloropentacarbonyl was prepared in 62% yield from potassium chlororhenite and carbon monoxide at high temperatures and pressures. The iodopentacarbonyl was prepared in 29% yield from potassium perrhenate, methyl iodide, and carbon monoxide. The preparation of rhenium hydrocarbonyl was attempted using sever approaches; however, no indication of the hydrocarbonyl was observed. Hydrogenative decomposition of rhenium pentacarbonyl in benzene solutions yielded an active catalyst which upon analysis proved to be metallic rhenium. Other solvents besides benzene were used but in each case the catalyst appeared as a rhenium mirror which was difficult to remove from the container. The use of rhenium pentacarbonyl as a homogeneous hydrogenation catalyst failed in attempts to reduce hexene-1 and cyclohexanone. The hydrogenation was accomplished only when temperatures were used which were high enough to decompose the carbonyl (200-250°). The metallic rhenium catalysts were characterized against a variety of substrates. With the exception of styrene, the substrates were all reduced in the temperature range 160-200°. Comparatively, the most successful reductions were those of benzene (179/3350 psig. for 22 hours) and acetic acid (2000/4190 psig. for 16 hours). Notably, nitrobenzene required 198° for complete reduction. When activated charcoal was added to the rhenium pentacarbonyl-benzene solution, a slightly more active catalyst was obtained. However, milder conditions of hydrogenative decomposition were not achieved. The addition of iron, zinc, or tin to acidified solutions of ammonium perrhenate resulted in the formation of a "rhenium black" which exhibited catalytic activity in hydrogenation reactions. This catalyst was obtained in a quantitative yield when an excess of reductant was present at all times. Analysis of this catalyst (both quantitatively and qualitatively) indicated that this catalyst was a hydrated rhenium oxide, probably ReO_2•3H_2O or ReO_2•2½H_2O. The activity of these hydrated rhenium oxide catalysts was greater than that of the metallic rhenium catalysts in all cases. Using the hydrated rhenium oxide catalyst, the carbon-carbon double bonds of hexene-1, cyclohexene, and styrene were reduced at 90-130°/3400-3900 psig. Interestingly, the presence of this catalyst did not effect the reduction of cycloheptanone until a temperature of 167° was reached. The reduction of 2-propyn-1-ol at 163° yielded both saturated and unsaturated alcohol. Adam's catalyst resulted in total decomposition of this substrate at 250° with no reduction occurring at lower temperatures. Benzene was also reduced at relatively mild conditions (177°/3935 psig. for 14 hours) using a hydrated rhenium oxide catalyst; pyridine was reduced at 230°/4520 psig. in 22 hours. Acetic acid was reduced at a mild 156° in the presence of a hydrated rhenium oxide catalyst. This is comparable with other rhenium catalysts and much better than Adam's catalyst which will not reduce acetic acid at 250° and better than any other reported catalyst except those of rhenium. The catalysts prepared using iron as a reductant were more active than those which were obtained using zinc; however, this difference was not great in the reduction of most substrates. The hydrated rhenium oxide catalysts were used in the reduction of a series of bifunctional substrates which contained the possible combinations of carbon-carbon double bond, carbonyl, carboxyl, and nitro groups. These reductions were compared with Adam's catalyst in many cases. The olefinic bonds in allylacetone and 2-allylcyclohexanone were preferentially reduced using both the hydrated rhenium oxide catalyst and Adam's catalyst. However, the rhenium catalyst reduced crotonaldehyde to n-butanol while Adam's catalyst yielded n-butyraldehyde. The olefinic bonds in vinyl-acetic, maleic, crotonic, and undecylenic acids were reduced in preference to the carboxyl group using the rhenium oxide catalyst. Under milder conditions, Adam's catalyst also reduced vinylacetic acid to n-butyric acid as expected. The carbonyl group was reduced completely in the presence of the carboxyl group in levulinic acid using the rhenium oxide catalyst. The nitro group was reduced (in the presence of the rhenium catalyst) in preference to the carbon- carbon double bond, carboxylic, or carbonyl groups in m-nitrostyrene, p-nitrophenyl-acetic acid, and m-nitroacetophenone, respectively. The same results were obtained using Adam's catalyst in the reduction of m-nitroacetophenone. The ease of reduction of different groups using the hydrated rhenium oxide catalyst was in the order: aromatic ring < carboxyl < carbonyl < carbon-carbon double bond < nitro. The order was the same using Adam's catalyst except that the carboxylic acid group and aromatic system were not reduced at all in the latter case under conditions of 250°/4500 psig. for 24 hours. However, the order of ease of reduction using the rhenium catalyst in the reduction of mono-functional substrates was aromatic ring < carboxyl < nitro < carbon-carbon double bond < carbonyl. Thus, the nitro group exhibited a poisoning effect when present in bifunctional substrates. Generally, the activity of the catalysts prepared in this study are comparable with previously characterized rhenium catalysts. This applies especially to the reduction of benzene and acetic acid. Rhenium pentacarbonyl, rhenium iodopentacarbonyl, and rhenium hepta-sulfide were used as catalysts in the attempted hydroformylation of cyclohexene and hexene-1 . However, in the temperature range 30-260° no hydroformylation was observed other than that which resulted from iron impurities. However, increased hydrogenation of substrate and products occurred when a rhenium compound was added as a catalyst. Dicobalt octacarbonyl was prepared and used in the hydroformylation reaction for comparison. As reported, the yields of hydroformylated products was excellent.