Development of Catalysts for Epoxidation of Olefins

制备了铌(V)和钼(V)卟啉。钼配合物具有八面体结构，轴位上有氧和烷氧配体，而铌(V)卟啉的三氧二聚体具有顺七配位结构。配合物在可见光好氧照射下催化苯中烯烃的光化学环氧化反应。利用可见吸收光谱和ESR光谱对上述配合物的光化学反应进行了跟踪。在可见光厌氧照射下，Nionium(V)和molybdenum(V)卟啉在苯中还原得到相应的五配位配合物。在光还原过程中，轴向配位键发生均裂，轴向配体作为相应的自由基释放。被还原的配合物在苯中被曝气后被分子氧氧化生成超氧配合物。超氧化物配合物将氧原子转移到加入溶液中的烯烃上，形成相应的环氧化物。通过时间分辨ESR光谱揭示了配合物的光还原作用。光还原，即轴向配位键的均解，通过电荷转移三重态和自由基与被还原的中心金属的三重态对进行。超氧配合物是由分子氧与三联体对反应形成的。在配合物的轴位上配位的分子的加入干扰了超氧化物配合物的形成。在光环氧化过程中没有发生顺反异构化，表明中间体具有离子性质。具有末端双键的烯烃活性低于具有内双键的烯烃。烷基取代基多的烯烃活性高于取代基少的烯烃。在卟啉配体中引入桩栅栏型取代基，提高了活性和选择性。

Niobium(V) and molybdenum(V) porphyrins were prepared. Molybdenum complexes had an octahedral structure with an oxo and an alkoxo ligand on their axial site, while tri-u-oxo dimer of niobium(V) porphyrins had cis-hepta-coordinated one. Complexes catalyzed photochemical epoxidation of olefins in benzene upon aerobic irradiation of with visible light. Photochemical reactions of above complexes were followed by means of visible absorption and ESR spectroscopy.Nionium(V) and molybdenum(V) porphyrins were reduced in benzene upon anaerobic irradiation with visible light affording corresponding pentacoordinate complexes. Homolysis of an axial coordination bond took place during the photoreduction and axial ligands were released as corresponding radical species. The reduced complexes were oxidized in benzene by molecular oxygen upon aeration resuting superoxide complexes. The superoxide complexes transferred an oxygen atom to an olefin added in the soultion to form corresponding epoxide.The photoreduction of the complexes was revealed by means of time-resolved ESR spectroscopy. The photoreduction, the homolysis of the axial coordination bond, proceeded via a charge transfer triplet state and a triplet pair of a radical and a reduced central metal. The superoxide complex was formed by reaction of molecular oxygen with the triplet pair. Addition of molecules which could coordinate at the axial site of the complexes disturbed the formation of the superoxide complexes.No cis-trans isomerization took place in the photo-epoxidation process suggesting an ionic nature of the intermediate. Olefins having a terminal double bond showed lower activity than ones having an internal duble bond. Olefins with more alkyl substituent groups had higher activity than those with less substituent groups. Introduction of picket fence type substituents into the porphyrin ligand improved both activity and selectivity.