Asymmetric Epoxidation of Simple Olefins by Chiral Bitetralin-Linked "Twin-Coronet" Porphyrin Catalysts.
作者:Yoshinori Naruta、Nobuo Ishihara、Fumito Tani、Kazuhiro Maruyama
DOI:10.1246/bcsj.66.158
日期:——
Catalytic and asymmetric epoxidation of styrenes and related aryl substituted olefins with the iron complexes of chiral bitetralin (Bitet)–linked “twin–coronet” porphyrins was performed with iodosylbenzene as an oxidant. Among two topological isomers of the catalyst, the eclipsed one (5b) showed higher enantioselectivity than the staggered (6b). With 5b, the resulting epoxides, except for the olefins bearing an electron–donating substituent, were obtained in good to excellent enantioselectivity (54—96% ee), especially for the styrenes with electron–withdrawing substituent(s). Being different from other porphyrin–based chiral catalysts, the catalyst 5b is robust enough under the applied oxidation conditions to exhibit chiral epoxidation with the same ee and the same rate as those of the initial period of the reaction even after about 500 turnovers. The Bitet catalyst is superior in the epoxide enantioselectivity than the corresponding chiral binaphthalene (Binap)–linked catalyst (3b). In the reactions with the catalysts 3b and 5b, good correlation in epoxide ees was observed. Increase of the epoxide ee in the reaction with the Bitet catalyst was elucidated by the shape and size of the reaction cavities of the Bitet were tighter than those of the latter. The observed ees of the substituted styrene oxides showed good correlation with Σσ+ values of their substituent(s). In the reaction with the electron–deficient olefins, π–π* interaction between the HOMO of the electron–rich Bitet auxiliary ring and the LUMO of the electron–deficient aryl ring of the substrate are pointed out as the key for the realization of high ees. Some nitrostyrenes, however, gave rather lower ees in spite of rather higher degree of their electron deficiency. This deviation was elucidated by the mismatching of their frontier orbitals.
使用碘代氧化苯作为氧化剂,通过手性双四氢萘(Bitet)连接的“双冠”卟啉铁配合物对苯乙烯及相关芳香取代烯烃进行催化和不对称环氧化反应。在两种拓扑异构体的催化剂中,重叠式(5b)表现出比交错式(6b)更高的手性选择性。使用5b催化剂,除了带有供电子取代基的烯烃外,所得环氧化物均在良好的手性选择性(54—96% 对映体过量)下获得,尤其是带有吸电子取代基的苯乙烯。与其他基于卟啉的手性催化剂不同,催化剂5b在应用的氧化条件下足够稳定,即使在约500次周转后,也能在相同对映体过量和相同速率下展示不对称环氧化反应,如同反应初期一样。Bitet催化剂在环氧化物的对映选择性上优于相应的双萘(Binap)连接的催化剂(3b)。在使用催化剂3b和5b的反应中,环氧化物的对映体过量呈现出良好的相关性。通过Bitet催化剂反应腔的形状和大小比后者更紧密,解释了环氧化物对映体过量的增加。取代苯乙烯氧化物的观测对映体过量与其取代基的Σσ+值显示出良好的相关性。在电子缺乏烯烃的反应中,指出富电子Bitet辅环的HOMO与电子缺乏基质的芳香环的LUMO之间的π-π*相互作用是实现高对映体过量的关键。然而,一些硝基苯乙烯尽管具有更高的电子缺乏程度,却给出了较低的对映体过量。这种偏差通过它们前沿轨道的不匹配得到了解释。