Microencapsulated osmium tetroxide

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]. 

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). 

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). 

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... 

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