REGIOSELECTIVE AND ENANTIOSELECTIVE CATALYTIC EPOXIDATION OF CONJUGATED DIENES

The regio- and enantio-selective catalytic epoxidation of conjugated a liphatic dienes have been studied using a variety of achiral and chira l 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 o n the terminal oxidant applied. The enantiomeric excess (ee) obtained varies for the different conjugated dienes and the ee is generally hig hest for internal alkenes, where an ee of up to 71% is observed, where as 48% is the highest observed ee for the less substituted alkenes. Th e ee is also dependent on the terminal oxidant applied. The regio- and enantio-selectivity have also been studied for different 1-(para-subs tituted phenyl)buta-1,3-dienes, but no regio- and enantio-selectivity dependence on the different substituents are observed. A competitive e poxidation experiment with styrene and 1-phenylbuta-1,3-diene shows th at the latter is the most reactive and the difference in reactivity is discussed on the basis of the frontier orbitals of the two systems. T he electronic structure of the ore-manganese salen intermediate is inv estigated using INDO/1 calculations and it is found that the triplet s tate is the most stable state of the intermediate. Based on the electr onic structure of the oxomanganese salen intermediate a mechanism for the oxygen transfer step to the conjugated diene is proposed.