Origin of enantioselectivity in asymmetric hydrovinylations catalyzed by phosphoramidite nickel catalysts:: An experimentally supported density functional study
Origin of enantioselectivity in asymmetric hydrovinylations catalyzed by phosphoramidite nickel catalysts:: An experimentally supported density functional study
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DOI：
10.1021/om040107l
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发表时间：
2004-11-08
期刊：
影响因子：
2.8
通讯作者：
Leitner, W
中科院分区：
文献类型：
--
作者：
Hölscher, M;Franciò, G;Leitner, W
通过包括阴离子和溶剂效应在内的 DFT 计算，研究了手性亚磷酰胺配体存在下镍催化苯乙烯不对称氢化乙烯基化的机理。理论研究得到了配体晶体结构分析和核磁共振研究的支持。所得结果表明，催化活性体系的性能很大程度上受到亚磷酰胺配体的一个苯环的半不稳定配位模式的影响，该配位模式仅允许活性氢化镍催化剂中苯乙烯配位的两种化学上合理的取向。这两种非对映异构体是氢化物转移到苯乙烯的两条反应路径的起点。一条路径显然优于另一条路径，这可以从对应于反应的对映体区别步骤的显着较低的活化能（+17.57 与+26.92 kcal/mol）推断出来。乙烯对配体的半稳定配位部分的置换引发了随后的C-C键形成反应路径，分别产生S和R构型的反应产物。活化能表明C-C键的形成不是对映选择性进行的。在阴离子存在下的计算表明，含有 BF4 的体系的活性低于含有 BARF(-)（四-（3,5-双（三氟甲基）苯基）硼酸盐）的体系。获得的结果与实验数据一致并有助于合理化实验数据。
The mechanism of the nickel-catalyzed asymmetric hydrovinylation of styrene in the presence of chiral phosphoramidite ligands was investigated by means of DFT calculations including anion and solvent effects. The theoretical studies are supported by crystal structure analysis and NMR investigations of the ligand. The results obtained indicate that the performance of the catalytically active system is largely affected by a hemilabile coordination mode of one phenyl ring of the phosphoramidite ligand allowing only for two chemically plausible orientations of styrene coordination in the active nickel hydride catalyst. These two diastereomers are the starting points of two reaction paths for hydride transfer to styrene. One path is clearly preferred over the other, as can be deduced from the significantly lower activation energy (+17.57 versus +26.92 kcal/mol) corresponding to the enantiodiscriminating step of the reaction. The displacement of the hemilabily coordinated part of the ligand by ethylene initiates the subsequent reaction paths for C-C bond formation, yielding the Sand the R-configurated reaction products, respectively. The activation energies show that C-C bond formation does not proceed enantioselectively. Calculations in the presence of anions showed BF4--containing systems to be less active than systems with BARF(-) (tetrakis-(3,5-bis(trifluoromethyl)phenyl)borate). The results obtained are in accordance with and help to rationalize experimental data.