Osmium microencapsulated

Grafting a modified cinchona alkaloid to hexagonal mesoporous molecular sieve SBA-15 afforded catalyst (27) with excellent activity. 1-Phenyl-1-propene was converted to the corresponding diol in 98% yield (98% ee), while trans-stilbene yielded the desired product in 97% yield (99% ee) [92]. Other examples in this field are the utilization of microencapsulated osmium tetroxide by Kobayashi [93] and the application of continuous dihydroxylation mns in chemzyme membrane reactors described by Woltinger [94]. 

The first heterogeneous osmium catalyst applicable for asymmetric dihydroxylation reactions was described by Kobayashi and coworkers (Table 9, entry 1) [38, 39]. Osmium tetroxide was enveloped in a polymer capsule by microencapsulation techniques [40,41]. The asymmetric dihydroxylation of transmethylstyrene with poly(acrylonitrile-co-butadiene-co-styrene) microencapsulated (ABS-MC) osmium tetroxide as catalyst, NMO as the cooxidant, and (DHQD)2PHAL as the chiral ligand completed in 88% yield with 94% ee [38]. The catalyst and the chiral ligand were reused in five consecutive runs without loss of activity. However, the use of NMO as cooxidant required the slow... 

Polymer-microencapsulated scandium and osmium catalysts have recently been developed by Kobayashi and coworkers [ 119-121]. This protocol has been... 

Several groups have described the fabrication of microcapsules loaded with a catalyst. Catalysts encapsulated include palladium(II) acetate (86) and osmium tetraoxide (87). Microencapsulated catalysts are described as effective, easily isolated from a reaction system by filtration, and reusable. Shchukin and co-workers (88) recognized that chemical processes can be performed within microcapsules to produce imique products that are retained within the microcapsules. They produced crystalline WO3 nanoparticles inside microcapsules with a polyelectrolyte shell. 

A new strategy (at the time) of microencapsulated osmium tetroxide was published by Kobayashi and coworkers in 1998 [38]. The metal is immobilized onto a polymer on the basis of physical envelopment by the polymer and on electron interactions between the n electrons of the benzene rings of the polystyrene-based polymer and a vacant orbital of the Lewis acid. Using cydohexene as a model compound it was shown that this microencapsulated osmium tetroxide (MC OSO4) can be used as a catalyst in the dihydroxylation with NMO as stoichiometric oxidant (Scheme 1.15). 

A study on the rate of conversion of the starting material showed that the reaction proceeds faster using OSO4 than using the microencapsulated catalyst. This is ascribed to the slower reoxidation of the microencapsulated osmium ester with NMO, compared to simple OSO4. 

Scheme 1.15 Dihydroxylation of cyclohexene using microencapsulated osmium tetroxide...

Later, Kobayashi and coworkers reported on a new type of microencapsulated osmium tetroxide using phenoxyethoxymethyl-polystyrene as support [40]. With this catalyst, asymmetric dihydroxylation of alkenes has been successfully performed using (DHQD)2PHAL as a chiral ligand and K3[Fe(CN)6] as a cooxidant in H20/acetone (Scheme 1.16). This dihydroxylation does not require slow addition of the alkene, and the catalyst can be recovered quantitatively by simple filtration and reused without loss of activity. With a divinylbenzene-cross-linked polystyrene microencapsulated OSO4 and a nonionic phase transfer catalyst (Triton X-405), this system can be run in an aqueous system [41]. 

Recently Kobayashi and coworkers reported on a new type of microencapsulated osmium tetroxide using phenoxyethoxymethyl-polystyrene as the support [39]. With this catalyst, asymmetric dihydroxylation of olefins has been successfiiUy performed using (DHQD)2PHAL as a chiral ligand and K3[Fe(CN)6] as a cooxidant in H20/acet-one (Scheme 1.11). 

---