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 source, and because of the breadth of opportunity they offer the organic synthetic chemist in terms of the highly selective reactions they undergo, often requiring only very mild conditions. Since most epoxides also bear at least one stereogenic centre the strategic importance of these molecules in synthesis is even higher. The most important asymmetric alkene epoxidation catalyst systems that have been discovered are those reported by Sharpless and his co‐workers utilising tartrate ester complexed Ti(IV) centres¹ and by Jacobsen and his co‐workers utilising chiral Mn(III) salen complexes.² The former system provides high conversions and high enantioselectivity (enantiomeric excess, ee%) in the case of allylic alcohol substrates, while the latter is likewise effective in the case of non‐functional cis ‐internal alkenes, especially cyclic systems. Both catalytic systems are homogeneous and exploitation of both involve rather laborious work‐up 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 analogue have been well rehearsed.³ Suffice to say that on a laboratory scale supported metal complex catalysts considerably facilitate product work‐up and isolation, while on a large scale such heterogeneous species allow processes to be run continuously using packed or fluidised bed columns with considerable financial advantages both in terms of capital expenditure on plant and with regard to recurrent costs.