Catalysis of Organic..

Hallett, Pollet, Eckert and Liotta 401Figure 4. Left side: Miscible Solution ofTHF/water at 1 bar. Right side: phaseseparation at 30 bar CO 2 . The partitioncoefficient of the water-soluble dye isgreater than 10 6 ! (14)Supercritical fluids are benign alternatives to conventional organic solvents thatmay offer improvements in reaction rate, product selectivity, and product separation.We reported the first use of SCFs for phase-transfer catalysis (PTC), where thesebenign alternatives also offer greatly improved transport, product separation, catalystrecycle, and facile solvent removal (26-29).PTC is a powerful and widely used technique for conducting reactions betweentwo or more reactants in two or more immiscible phases (30,31). Polar aproticsolvents have traditionally been used to dissolve the reactants into a single phase,however these solvents are frequently expensive, environmentally undesirable, andgreatly complicate product purification. PTC eliminates the need for these solventsby employing a phase-transfer agent to transfer one of the reacting species from onephase into a second phase where reaction can occur. Generally, PTC involves thetransfer of an ionic reactant into an organic phase across a phase boundary. Once thereaction is complete, the catalyst transports the ionic product back to the aqueous orsolid phase to start a new catalytic cycle. The most PTCs are quaternary ammoniumsalts (31), which are inexpensive and separated from the reaction products by waterextraction. In both liquid-liquid and solid-liquid PTC, a three-phase PTC system canoccur if the phase-transfer catalyst has limited solubility in both phases.The utility of SCFs for PTC was demonstrated for several model organicreactions – the nucleophilic displacement of benzyl chloride with bromide ion (26)and cyanide ion (27), which were chosen as model reversible and irreversible S N 2reactions. The next two reactions reported were the alkylation and cycloalkylation ofphenylacetonitrile (28,29). Catalyst solubility in the SCF was very limited, yet therate of reaction increased linearly with the amount of catalyst present. Figure 5shows data for the cyanide displacement of benzyl bromide, and the data followedpseudo-first order, irreversible kinetics. The catalyst amounts ranged from 0.06(solubility limit) to 10% of the limiting reactant, benzyl chloride.