Olefin fluorous biphasic system

The aqueous biphasic hydroformylation concept is ineffective with higher olefins owing to mass transfer limitations posed by their low solubility in water. Several strategies have been employed to circumvent this problem [22], e.g. by conducting the reaction in a monophasic system using a tetraalkylammonium salt of tppts as the ligand, followed by separation of the catalyst by extraction into water. Alternatively, one can employ a different biphasic system such as a fluorous biphasic system or an ionic liquid/scC02 mixture (see later). 

The arylbutylselenide 53 readily prepared from the corresponding aryl bromide 54 and lithium butylselenide is an excellent catalyst for the performance of epoxidation reactions in a fluorous biphase system. With 5 mol % of the catalyst 53, various polysubstituted and functionalized olefins 55 are epoxidized in a biphasic system of bromoperfluorooctane and benzene using hydrogen peroxide (60% in water, 1.5-2.0 equiv) leading to epoxides 56 in good to excellent yields Eq. (25). 

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. 

Scheme 3.1 Catalysts for olefin dimerization - standard nickel catalyst 1 and its fluorous analog 2 (n = 3-5). In a Hostinert 216 (3)/toluene biphasic system the catalyst stays in the fluorous phase [5] whereas the product dissolves preferably in toluene [4],...

Similar, fluorous palladium /i-dikctonatc complexes (27) have been employed for Wacker oxidation of olefins to the corresponding ketones in a biphasic system [27] (Scheme 3.10). 

The concept of fluorous biphase hydroformylation of heavy olefins was introduced by Horvath at Exxon in 1994 [42, 43]. Fluorocarbon-based solvents, especially perfluorinated alkanes and ethers, are of modest cost, chemically inert, and nonpolar and show low intermolecular forces. Most of them are immiscible with water and can be therefore used as the nonaqueous phase. Moreover, their miscibility with organic solvents such as toluene, THF, or alcohols at room temperature is quite low. Only at elevated temperature miscibility occurs. These features allow hydroformylation at smooth reaction conditions at 60-120 °C in a homogeneous system [44]. Upon cooling, phase separation takes place. The catalyst is recovered finally by simple decantation. One of the last summaries in this area was given by Mathison and Cole-Hamilton in 2006 [45]. 

The liquid support may be water, supercritical fluids, ionic liquids, organic liquids or fluorous liquids [12]. The Shell higher olefin process (SHOP) and the Oxo synthesis (hydrofomylation) are examples of important industrial processes based on biphasic catalytic systems. 

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