Phosphines, supported aqueous-phase catalysis

Although the first aim of the use of a water-soluble palladium catalyst in allylic alkylation in a two-phase system was the recycling of the catalyst, this methodology finds quite interesting applications in the deprotection of peptides as well as in the selective alkylation of uracils and thiouracils. More recently, the effective use of supported aqueous-phase catalysis as well as asymmetric alkylation in water in the presence of surfactants or amphiphilic resin-supported phosphines open new applications and developments for the future. 

The most severe dra wback in homogeneous catalysis is the separation of the catalyst from the reaction mixture. The industrial success of the aqueous two-phase hydroformylation ofpropene to n-butanal [1] in Ruhrchemie AG in 1984 represents the considerable progress in this field. However, aqueous/organic biphasic catalysis has its limitations when the water solubility of the starting materials proves too low, as in hydroformylation of higher olefins (see Chapter 1). To solve this issue, a variety of approaches have been attempted. Additions of co-solvents [2] or surfactants [3, 4] to the system or application of tenside ligands [5, 6] and amphiphilic phosphines [7, 8] are ways to increase the reaction rates. Other approaches such as fluorous biphase system (FBS see Chapter 4) [9], supported aqueous phase catalysis (SAPC see Section 2.6) [10], supercritical CO2 (cf. Chapter 6) [11] and ionic liquids (cf Chapter 5) [12] have also been introduced to deal with this problem. 

---