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Osmium tetroxide, OsO₄, is an efficient oxidation catalyst, well known for olefin hydroxylation and dihydroxylation reactions. For the catalytic asymmetric dihydroxylation (AD) of olefins, addition of a chiral ligand, such as (DHQD)₂PHAL, to the OsO₄ reaction mixture, gives access to a wide range of enantiomerically pure vicinal diols. The chiral ligand is based on a biscinchona alkaloid with a phthalazine core (1).

However, OsO₄ is highly toxic and volatile and difficult to use, and to avoid hazards OsO₄ has sometimes been bound to polymers. Replacing organic solvents by water for chemical reactions is an additional interest.

Now, a team from The University of Tokyo, Japan, have developed a microencapsulated OsO₄ system, based on poly(phenoxyethoxymethylstyrene-co-styrene) (PEM-MC OsO₄) (2) which can perform AD of olefins with water as the sole solvent without catalyst leaching. The system uses a H₂O-acetone solvent and K₃Fe(CN)₆ cooxidant, with (DHQD)₂PHAL ligand added to PEM-MC OsO₄ catalyst; acetone is then removed. Addition of a non-ionic surfactant, Triton^® X-405 (octylphenoxypolyethoxyethanol) was found to increase yield and ee and reduce Os leaching. Issues around catalyst solubility were resolved by adding the ligand directly to the reaction mixture. Product diols were obtained in better yield without catalyst deactivation even after three runs. Os leaching was completely suppressed by neutralising with aqueous H₂SO₄. Other olefins behave similarly and the PEM-MC OsO₄ can be separated by filtration, recovered and reused without loss of activity.