Catalytic hydrogenation, supercritical carbon dioxide

Recent work by several research groups has shown that supercritical fluids can be superior to other solvents for several chemical processes. For example, DeSimone has demonstrated the ability of supercritical CO2 to replace Freons in the free radical polymerization of fluorinatkl acrylate monomers. 34) Noyori has shown that significant rate enhancements can be achieved in supercritical carbon dioxide relative to other solvents for the homogeneous catalytic hydrogenation of carbon dioxide to either formic acid or its derivatives in the presence of triethylamine or triethylamine/methanol respectively, (equation 1). (55-57) As discussed below, we have recently demonstrated that improved enantioselectivities can be achieved in supercritical carbon dioxide for the catalytic asymmetric hydrogenation of several enamides. 5 8)... 

P. G. Jessop, T. Ikariya, R. Noyori, Homogeneous Catalytic Hydrogenation of Supercritical Carbon Dioxide , Nature 1994,368,231-233. 

To study the effect of the Ru/Al Oj catalyst on hydrogen yield for refomung of glucose in supercritical water, the experiments were compared to reactions with and without catalytic runs imder identical conditions. Typical product distributions are shown in Table 6.9 for experiments with and without a Ru/Al Oj catalyst at 973 K with 1 wt.% glucose feed (Byrd et al., 2007). There was a significant reduction in carbon monoxide and methane yields in the presence of the catalyst. The main products of the reaction were hydrogen, methane, carbon dioxide, and carbon monoxide. The low carbon monoxide yield (0.1% by vol.) indicates that the water-gas shift reaction approaches completion. 

The three-phase catalytic hydrogenation of an unsaturated ketone using supercritical carbon dioxide as a solvent was studied in order to simulate the performance of a semi-industrial trickle-bed reactor. It is shown that supercritical CO2 strongly increases the reaction rate (Devetta et al., 1999). 

High catalytic activities have been achieved by the PYRPHOS- [18], PPCP [20], BICHEP- [21], Et-DuPHOS-Rh [19] complexes among others, allowing the reaction with a substrate-to-catalyst molar ratios (S/C) as high as 50,000. With a [2.2]PHANEPHOS-Rh complex, the reaction proceeds even at -45°C [27], Supercritical carbon dioxide, a unique reaction medium, can be used in the DuPHOS and BPE-Rh-catalyzed hydrogenation [43], A highly lipophilic counteranion such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) or trifluoromethanesulfonate is used to enhance the solubility of the cationic Rh complexes. Under the most suitable reaction conditions of 102 atm of carbon dioxide, 1 atm of hydrogen, and 22°C, a-amino acid derivatives are produced with up to 99.7% ee. 

Burk MJ, Feng S, Gross MF et al (1995) Asymmetric catalytic hydrogenation reactions in supercritical carbon dioxide. J Am Chem Soc 117(31) 8277-8278... 

Other non-classical reaction media [96] have, in recent years, attracted increasing attention from the viewpoint of avoiding environmentally unattractive solvents and/or facilitating catalyst recovery and recycling. Two examples, which readily come to mind, are supercritical carbon dioxide and room temperature ionic liquids. Catalytic hydrogenation in supercritical C02, for example, has... 

Catalytic asymmetric hydrogenations have also been performed in supercritical carbon dioxide [79-81]. For example, a-enamides were hydrogenated in high enantioselectivities comparable to those observed in conventional solvents, using a cationic rhodium complex of the EtDuPHOS ligand (Fig. 7.24) [79]. More recently, catalytic asymmetric hydrogenations have been performed in scC02 with... 

Jessop PG, Ikariya T, Noyori IG (1994) Homogeneous catalytic-hydrogenation of supercritical carbon-dioxide. Nature 368 231-233. 

Bianchini, C. Glendenning, L. Catalytic production of dimethylformamide from supercritical carbon dioxide. Methyl formate synthesis by hydrogenation of supercritical carbon dioxide in the presence of methanol. Selectivity for hydrogenation or hydroformylation of olefins by hydridopentacarbonylmanganese(I) in supercritical carbon dioxide. Chemtracts Org. Chem. 1996, 9 (6), 318-321. 

Jessop and co-workers have pointed out that homogeneous catalysis in supercritical fluids can offer high rates, improved selectivity, and elimination of mass-transfer problems.169 They have used a ruthenium phosphine catalyst to reduce supercritical carbon dioxide to formic acid using hydrogen.170 The reaction might be used to recycle waste carbon dioxide from combustion. It also avoids the use of poisonous carbon monoxide to make formic acid and its derivatives. There is no need for the usual solvent for such a reaction, because the excess carbon dioxide is the solvent. If the reaction is run in the presence of dimethy-lamine, dimethylformamide is obtained with 100% selectivity at 92-94% conversion.171 In this example, the ruthenium phosphine catalyst was supported on silica. Asymmetric catalytic hydrogenation of dehydroaminoacid derivatives (8.16) can be performed in carbon dioxide using ruthenium chiral phosphine catalysts.172... 

The catalytic hydrogenation of unsaturated ketone in supercritical carbon dioxide with a supported palladium catalyst, was described by Bertucco et al. [43] (Scheme 13.5). 

Catalytic Hydrogenation of Olefins in Supercritical Carbon Dioxide Using Rhodium Catalysts Supported on Fluoroacrylate Copolymers... 

This chemical and physico-chemical behavior of the binary H2O-CO2 mixture [38] suggests that water is an attractive liquid to be combined with supercritical carbon dioxide in multiphase catalysis. CO2/H2O systems have adequate mass-transfer properties, especially if emulsions or micro-emulsions can be formed ([39] and refs, therein). The low pH of aqueous phases in the presence of compressed CO2 (pH ca. 3-3.5 [40]) must be considered and the use of buffered solutions can be beneficial in the design of suitable catalytic systems, as demonstrated for colloid-catalyzed arene hydrogenation in water-scC02 [41]. 

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CC are presented the copolymerization of CO2 with epoxides, ruthenium>mediated phase transfer oxidation of olefins in a supercritical COa/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2, but no improvement in reactivity over conventional solvents was observed. Hythogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates. 

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