Platinum Metals Rev., 2003, 47, (2), 60
Resin-Supported Palladium Complexes for Catalyses in Water
One of the aims of green chemistry is to switch from organic solvents to water for organic transformations, to reduce environmental problems and contribute to safety. Palladium (Pd)-phosphine complexes are highly active and widely used in organic catalyses, with organic solvents, although Suzuki chemistry can be performed in water/EtOH systems. Water-soluble phosphine catalysts are available, but as many substrates are insoluble in water, they have been received less attention. However, if Pd-phosphine complexes, combined with a solid support, could undertake catalyses in water, it would be a powerful tool in organic synthesis.
Now, Y. Uozumi from the Institute for Molecular Science, Okazaki, Japan, reports the preparation of Pd-phosphine complexes bound to an amphiphilic polymer resin, and their success as catalysts for various reactions in aqueous media (J. Synth. Org. Chem.,Jpn., 2002, 60, (11), 1063–1068).
Polystyrene-poly(ethylene glycol) (PS-PEG) is a commonly used resin, functionalised for attachments. PS-PEG resin beads show relatively uniform swelling in solvents of different polarities. A PS-PEG resin with an amino group was chosen as a support for phosphines. The resin-supported phosphines were then treated with excess [pdCl(η3–C3H5)]2 (Pd:P = 1.2:1) to form a Pd monophosphine complex.
The Pd-phosphine-PS-PEG resin was successfully tested for various transformations in water, such as allylic substitution, hydroxycarbonylation of aryl halides, the Heck reaction, cross-couplings of aryl halides and allyl acetates with arylboron reagents, asymmetric allylic substitution, etc. The catalyst could be recovered and reused without significant loss of activity and selectivity. A novel chiral P,N -ligand was prepared on PS-PEG, and with Pd, in water, gave π-allylic substitutions of cyclic and acyclic substrates of enantioselectivity up to 99% ee.