Asymmetric hydrogenation catalyzed by a rhodium complex of (R)-(tert-butylmethylphosphino)(di-tert-butylphosphino)methane:: Scope of enantioselectivity and mechanistic study
Asymmetric hydrogenation catalyzed by a rhodium complex of (R)-(tert-butylmethylphosphino)(di-tert-butylphosphino)methane:: Scope of enantioselectivity and mechanistic study
复制标题
DOI：
10.1021/ja076542z
复制
发表时间：
2008-02-27
影响因子：
15
通讯作者：
Takahashi, Hidetoshi
中科院分区：
文献类型：
--
作者：
Gridnev, Ilya D.;Imamoto, Tsuneo;Takahashi, Hidetoshi
铑配合物(R)-（叔丁基甲基膦）（二叔丁基膦）甲烷用于铑催化代表性底物的不对称加氢3-14具有高的催化活性和广泛的对映选择性和近乎完美的对映选择性。机理研究（NMR和DFT计算）是为了研究(Z)- α -乙酰氨基乙酸酯(3)不对称氢化反应中对映体选择的机理。虽然在-100℃溶剂化物配合物2中加入3作为动力学产物形成了在非“手性”磷原子附近有双键的催化剂-底物配合物15a、b，但它们迅速重排成在“手性”磷原子附近有双键的更稳定的异构体15c、d。通过核磁共振测定了15c与15d之间相互转化的热力学和动力学参数；相互转化主要通过非螯合的催化剂-底物配合物在分子内发生。在-100 ~ -40℃温度范围内，通过核磁共振线形变化直接观察到15d ~ 16d之间的平衡，而在15c温度下没有观察到这种平衡。这一结果是通过计算确定甲醇插入到15c，d的相应过渡态来解释的。三组不同催化剂-底物配合物的低温加氢实验表明，即使在具有重配双键的15c在不异构为15d的条件下加氢，其对映体选择性的顺序和意义也相同（97% ee (R)）。结果表明，15c，d的氢化不是直接发生的，而是在双键解离之前发生的，从而产生了反应性更强的物质16。这表明对映体选择必须发生在催化循环的后期阶段。结合和迁移插入步骤的DFT计算表明，当螯合二氢化物19d-MeOH由非螯合二氢化物18d形成时，在结合步骤中发生了对映体选择。
The rhodium complex of (R)-(tert-butylmethylphosphino)(di-tert-butylphosphino)methane used in Rh-catalyzed asymmetric hydrogenation of representative substrates 3-14 demonstrated high catalytic activity coupled with wide scope and nearly perfect enantioselectivity. Mechanistic studies (NMR and DFT computations) were carried out in order to investigate the mechanism of the enantioselection in the asymmetric hydrogenation of (Z)-alpha-acetamidocinnamate (3). Although catalyst-substrate complexes 15a,b with the double bond coordinated near the non-"chiral" phosphorus atom were formed as kinetic products upon the addition of 3 to solvate complex 2 at -100 degrees C, they rapidly rearranged to more stable isomers 15c,d with the double bond coordinated near the "chiral" phosphorus atom. The thermodynamic and kinetic parameters of the interconversion between 15c and 15d were determined by NMR; mainly, the interconversion occurred intramolecularly via nonchelating catalyst-substrate complexes 16. The equilibrium between 15d and 16d was directly observed from NMR line shape changes at temperatures ranging from -100 to -40 degrees C, whereas no such equilibrium was observed for 15c. This result was accounted for computationally by determining the corresponding transition states for the methanol insertion into 15c,d. Three sets of experiments of the low-temperature hydrogenation of different catalyst-substrate complexes gave the same order and sense of enantioselectivity (97% ee (R)) even in the case when 15c, having Re-coordinated double bond, was hydrogenated under the conditions precluding its isomerization to 15d. It was concluded that the hydrogenation of 15c,d does not occur directly, but is preceded by the dissociation of the double bond to result in the more reactive species 16. This indicates that enantioselection must occur at a later step of the catalytic cycle. DFT computations of association and migratory insertion steps suggest that enantioselection takes place during the association step when chelating dihydride 19d-MeOH is formed from nonchelating dihydride 18d.