Catalysis Limitations and Challenges

The major advantage of the use of two-phase catalysis is the easy separation of the catalyst and product phases. However, the co-miscibility of the product and catalyst phases can be problematic. An example is given by the biphasic aqueous hydroformylation of ethene to propanal. First, the propanal formed contains water, which has to be removed by distillation. This is difficult due to formation of azeotropic mixtures. Secondly, a significant proportion of rhodium catalyst is extracted from the reactor with the products, which prevents its efficient recovery. Nevertheless the reaction of ethene itself in the vrater-based Rh-TPPTS system is fast. It is the high solubility of water in the propanal that prevents the application of the aqueous biphasic process [5]. 

To overcome these limitations, there has been much investigation of novel methods, one of them focused on the search for alternative solvents [6,7]. Table 5.4-1 fists 

Further progress in multiphasic catalysis will rely on the development of alternative techniques that allow the reactivity of a broader range of substrates, the efficient separation of the products, and recovery of the catalyst, while remaining economically viable. 

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Despite an ever-growing interest in iron catalysis, the mechanistic understanding of iron-catalysed processes is veiy limited, and key challenges for the future development of the field must certainly include greater mechanistic elucidation. However, it is almost certain that iron catalysis will, in the future, provide access to numerous new synthetic vectors, building upon established reactivity and expanding the portfolio of iron-catalysed processes. 

Until 2006, a severe limitation in the field of chiral Brpnsted acid catalysis was the restriction to reactive substrates. The acidity of BINOL-derived chiral phosphoric acids is appropriate to activate various imine compounds through protonation and a broad range of efficient and highly enantioselective, phosphoric acid-catalyzed transformations involving imines have been developed. However, the activation of simple carbonyl compounds by means of Brpnsted acid catalysis proved to be rather challenging since the acid ity of the known BINOL-derived phosphoric acids is mostly insufficient. Carbonyl compounds and other less reactive substrates often require a stronger Brpnsted acid catalyst. 

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