Development of Catalysts for Epoxidation of Olefins

制备了Nb(V)和Mo(V)卟啉。钼配合物具有八面体结构，轴位上有一个氧代和一个烷氧基配体，而Nb(V)的三u-氧代二聚体具有顺式七配位的二聚体。在可见光照射下，络合物催化苯中烯烃的光化学环氧化反应。利用可见光吸收光谱和电子自旋共振光谱研究了上述配合物的光化学反应。在可见光照射下，Nb(V)和Mo(V)卟啉在苯中被还原，生成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物种被释放出来。还原的络合物在苯中被分子氧氧化，曝气生成超氧化物络合物。超氧化物络合物将一个氧原子转移到加在溶液中的烯烃上形成相应的环氧化物，通过时间分辨ESR光谱揭示了络合物的光还原过程。光还原是轴向配位键的均裂，通过电荷转移三重态和自由基和还原中心金属的三重态进行。超氧化物络合物是由分子氧与三线对反应形成的。络合物轴位配位分子的加入干扰了超氧化物络合物的形成，在光环氧化过程中没有发生顺反异构化反应，表明中间体具有离子性质。具有末端双键的烯烃比具有内部双键的烯烃显示出较低的活性。烷基取代基较多的烯烃比取代基较少的烯烃具有更高的活性。在卟啉配体中引入栅栏型取代基，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.