X-ray structural studies of highly enantioselective Mn(salen) epoxidation catalysts
X-ray structural studies of highly enantioselective Mn(salen) epoxidation catalysts
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DOI：
10.1002/chem.19960020812
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发表时间：
1996-08-01
影响因子：
4.3
通讯作者：
Jacobsen, EN
中科院分区：
文献类型：
--
作者：
Pospisil, PJ;Carsten, DH;Jacobsen, EN
考察了一系列手性Mn(Salen)络合物(1-10)不对称环氧化未功能化烯烃反应中催化剂结构与对映体选择性的关系。测定了六配位9([6，6‘=-TBU；4，4’=-TBU](+)ClO4-)和10(6，6‘=-TBU；4，4’=-BR)的五配位配合物5，8的X射线结构。催化剂1-9由(R，R)-1，2-二氨基环己烷衍生，催化剂10由(S，S)-1，2-二苯乙二胺衍生。催化剂1-9的水杨醛部分邻位(6，6‘)和PMA(4，4’)的立体电子取代不同。结构5、8和9的比较表明，在五配位和六配位的环己二胺衍生的配合物中，金属中心和手性二亚胺主链周围的配体几何构型保持显著不变；相反，配合物的水杨亚基区域显示出广泛的构象。研究了配合物1-10催化吲哚和6-氰基-2，2-二甲基色烯与氯化钠的不对称环氧化反应。系统地增加邻位上的立体体积，然后以1(6，6‘=-H；4，4’=-H)、2(6，6‘=-CH3；4，4’=-CH)、3(6，6‘=-TBU；4，4’=-H)、4(6，6‘=-TBU；4，4’=-CH3)、5(6，6‘=-TBU，4，4’=-TBU)和6(6，6‘=-TBU；4，4’=-三叔丁基)的顺序增加对位体，并对7(6，6‘=-TBU；4.4‘=-OME)和8(6，6’=-TBU；4，4‘=-OTIPS)导致目标环氧化物的对映选择性增强。在固体中观察到的构象变化可能反映了可获得的溶液构象，并可能有助于解释这些催化剂在非官能化烯烃的不对称环氧化反应中获得高水平的立体诱导。
The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(salen) complexes (1-10) was examined. The X-ray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 ([6,6' = -tBu; 4,4' = -tBu](+) ClO4-), and 10 (6,6' = -tBu; 4,4' = -Br) were determined. Catalysts 1-9 were derived from (R,R)-1,2-diaminocyclohexane and catalyst 10 from (S,S)-1,2-diphenylethylenediamine. Catalysts 1-9 differ in the stereoelectronic substitution of the ortho (6,6') and pma (4,4') positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five- and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1-10 was effected. Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6' = -H; 4,4' = -H), 2 (6,6' = -CH3; 4,4' = -CH,),3 (6,6' = -tBu; 4,4' = -H), 4 (6,6' = -tBu; 4,4' = -CH3), 5 (6,6' = -tBu, 4,4' = -tBu), and 6 (6,6' = -tBu; 4,4' = -trityl), and electronically modifying the para substituents in 7 (6,6' = -tBu; 4.4' = -OMe) and 8 (6,6' = -tBu; 4,4' = -OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.