Epoxidation of Styrene with Molecular Oxygen Catalyzed by Cobalt(II) Salen Complexes

Abstract

Cobalt(II) complexes with salen Schiff-base ligands derived from ethylenediamine or (S,S)-1,2-diphenylethylenediamine and salicylaldehyde or 3,5-dibromo and 3,5-di-t-butylsalicyladehyde have been tested as catalysts for the epoxidation of styrene with molecular oxygen, in 1,4-dioxane as a solvent. The reaction tests were followed by analyses of gas chromatography and mass spectrometry. Catalytic activities were found to be dependent upon both the electron effects of the substituents on the Co(II) salen complexes and the presence of phenyl substituents on the ethylene moiety. The highest catalytic activity was reached with the Br-substituted complex (2), which was used for further studies on the effects of temperature, time, solvent and additive for epoxidation. The effect of solvent was noted to be critical and the higher reaction temperature (90ºC) was needed. Conversion of styrene reached 97.1 %, with a 59.8 % selectivity of epoxide, when reaction was run at 90ºC for 5 h. Possible reaction mechanism steps were outlined.