Polymer-supported Ti(IV) and Mn(III) asymmetric alkene epoxidation catalysts

Epoxides represent a very important group of speciality and fine chemicals because they are derived directly from alkenes, a primary petrochemical sou rce, and because of the breadth of opportunity they offer the organic synth etic chemist in terms of the highly selective reactions they undergo, often requiring only very mild conditions. Since most epoxides also bear at leas t one stereogenic centre the strategic importance of these molecules in syn thesis is even higher. The most important asymmetric alkene epoxidation cat alyst systems that have been discovered are those reported by Sharpless and his co-workers utilising tartrate ester complexed Ti(TV) centres(1) and by Jacobsen and his coworkers utilising chiral Mn(III) salen complexes.(2) Th e former system provides high conversions and high enantioselectivity (enan tiomeric excess, ee%) in the case of allylic alcohol substrates, while the latter is likewise effective in the case of non-functional cia-internal alk enes, especially cyclic systems. Both catalytic systems are homogeneous and exploitation of both involve rather laborious workup procedures. Generally no attempt is made to recover and re-use these catalysts. The potential advantages in converting a process catalysed by a homogeneous metal complex into one involving a heterogeneous polymer-supported analogu e have been well rehearsed.(3) Suffice to say that on a laboratory scale su pported metal complex catalysts considerably facilitate product work-up and isolation, while on a large scale such heterogeneous species allow process es to be run continuously using packed or fluidised bed columns with consid erable financial advantages both in terms of capital expenditure on plant a nd with regard to recurrent costs.