Solvents in hydroformylation

As a further development in the exploration of the role of solvent in hydroformylation systems, the use of supercritical carbon dioxide (SCCO2), ionic liquids and fluorous systems has been reported. A review by Jessop, Ikariya, and Noyori presents information about the utility of SCCO2 in a variety of homogeneous reactions. ... 

Biphasic reaction conditions can be achieved within a wide range of operating conditions with respect to co-solvents. The most common co-solvents are the lower alcohols the purpose is to improve substrate solubility and as a consequence to increase reaction rate. Recent work with ethanol as a co-solvent shows that this is very effective at improving reaction rates [3]. It is estimated for example that the solubility of 1-octene in a 50 50 mixture of ethanol and water is 104 times greater than in water alone [3], In a comparison of several co-solvents - ethanol, methanol, acetone, and acetonitrile - it was found that ethanol was the most effective at improving reaction rates in the two-phase hydroformylation of 1-octene [4], Generally, though, the use of co-solvents in hydroformylation reactions with Rh/TPPTS catalysts is not advisable, because of diminished reaction selectivity and the possibility of acetal formation (see below). 

Also carbon dioxide, which is frequently used as supercritical solvent in hydroformylation, is able to oxidize phosphines slowly in the presence of Rh phosphine complexes, such as RhCl(PPh3)3 (Scheme 2.50) [168]. RhCl(CO)(PPh3)2 active in this respect. The rate of the oxidation parallels the basicity of the phosphine. 

In answer to these limitations, research and development efforts of the last 20 years have tried to develop advanced liquid-liquid biphasic hydroformylation catalysis by replacing water by another catalyst solvent. As the number of organic-organic, biphasic systems with a sufficient miscibility gap in the presence of hydroformylation products is very limited, these efforts led to the application of so-called advanced fluids or green solvents in hydroformylation catalysis. Given the restricted scope of this textbook, it will be only possible to describe briefly the most important approaches and their scopes and limitations. The reader interested in the details of these dynamic research fields is referred to a series of recently published monographs (Mathison et al., 2006 Leitner and Jessop, 2010, and Stark and Wasserscheid, 2010). 

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See also in sourсe #XX -- [ ]

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