Transition metal catalysis aqueous biphasic systems

As outlined in general in Section 6.3, two distinct engineering concepts can be envisaged for transition metal catalysis in scC02-based biphasic systems. In one case the compressed CO2 phase is used as the compartment for substrates and/or products, whereas the catalyst is contained in the liquid phase, in the present case water. We will refer to this scenario as H2O/SCCO2 biphasic catalysis and examples are discussed in this section. In the other case, carbon dioxide is used as the catalyst-containing phase, whereas substrates and products are retained in the polar aqueous phase. We refer to this system as inverted biphasic catalysis , denoted as SCCO2/ H2O, and it will be subject of the discussion in Section 6.4.4.4. 

Catalysis in liquid-liquid biphasic systems has developed recently into a subject of great practical interest because it provides an attractive solution to the problems of separation of catalysts from products and of catalyst recycle in homogeneous transition metal complex catalysis. Two-phase systems consist of two immiscible solvents, e.g., an aqueous phase or another polar phase containing the catalyst and an organic phase containing the products. The reaction is homogeneous, and the recovery of the catalyst is facilitated by simple phase separation. 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

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