Regio- and enantio-selective catalytic epoxidation of conjugated dienes

Abstract

The regio- and enantio-selective catalytic epoxidation of conjugated aliphatic dienes have been studied using a variety of achiral and chiral manganese salen complexes and sodium hypochlorite or iodosylbenzene as the terminal oxidant. The catalysts show a preference for the less substituted alkene in most of the dienes studied, and in some cases a regioselectivity of 100% is found. The regioselectivity is dependent on the terminal oxidant applied. The enantiomeric excess (ee) obtained varies for the different conjugated dienes and the ee is generally highest for internal alkenes, where an ee of up to 71% is observed, whereas 48% is the highest observed ee for the less substituted alkenes. The ee is also dependent on the terminal oxidant applied. The regio- and enantio-selectivity have also been studied for different 1-(para-substituted phenyl)buta-1,3-dienes, but no regio- and enantio-selectivity dependence on the different substituents are observed. A competitive epoxidation experiment with styrene and 1-phenylbuta-1,3-diene shows that the latter is the most reactive and the difference in reactivity is discussed on the basis of the frontier orbitals of the two systems. The electronic structure of the oxo–manganese salen intermediate is investigated using INDO/1 calculations and it is found that the triplet state is the most stable state of the intermediate. Based on the electronic structure of the oxo–manganese salen intermediate a mechanism for the oxygen transfer step to the conjugated diene is proposed.