JACOBSEN ASYMMETRIC RING-OPENING OF EPOXIDES

A variety of ie.so-epoxidcs could be selectively ring-opened this way with e.e. s as high as 97% [28], The azides can be converted to 1,2-amino alcohols, which are very desirable synthetic intermediates. Surprisingly, the mechanism of the ARO (asymmetric ring-opening) was more complicated than expected [29], First, it turned out that the chloride ion in Cr-salen was replaced by azide. Secondly, water was needed and HN3 rather than Me3SiN3 was the reactant nucleophile. Thirdly, the reaction rate was found to be second order in catalyst concentration, minus one in epoxide (cyclopentene oxide), and zero order in HN3 [30], 

The second order in catalyst points to the involvement of two chromium salen molecules in the transition state complex. Therefore several dimeric species were synthesised with suitable linkers the dimeric catalysts gave reaction rates that were one or two orders of magnitude higher than that of the monomeric catalyst. Trimeric species gave still higher reaction rates The 

Ring-opening by azide and the subsequent work-up is not ideal from a point of view of safety and costs and ring-opening by other nucleophiles is attractive. Furthermore, the use of meso compounds restricts the use of the catalyst to a small number of substrates and useful products. 

Covalent attachment chiral Co(salen) complexes to polystyrene and silica gave efficient and highly enantioselective catalysts for the hydrolytic kinetic resolution (HKR) of terminal epoxides, including epichlorohydrin. These systems provide practical solutions to difficulties with the isolation of reaction products from the HKR. Removal of the supported catalyst by filtration and repeated recycling was demonstrated with no loss of reactivity or enantioselectivity. The immobilised catalysts have been adapted to a 

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