Mechanistic Consideration of Asymmetric C–N and C–C Bond Formations with Bifunctional Chiral Ir and Ru Catalysts
作者:Yasuharu Hasegawa、Ilya D. Gridnev、Takao Ikariya
DOI:10.1246/bcsj.20110307
日期:2012.3.15
The mechanism of two enantioselective reactions, direct amination of α-cyanoacetates 3 with azodicarboxylates 4 and C–C bond formation reaction of α-cyanoacetates with acetylenic esters 6, catalyzed by chiral bifunctional Ir and Ru complexes, Cp*Ir[(S,S)-N-sulfonated dpen] 1 and Ru[(S,S)-N-sulfonated dpen](η6-arene) 2 (DPEN: 1,2-diphenylethylenediamine) was studied by NMR spectroscopic analysis combined with DFT analysis. Notably, these two reactions using the same chiral amide catalysts 1, 2 and pronucleophile, α-cyanoacetates 3 gave quantitatively the conjugate adducts bearing quaternary chiral carbon centers in excellent enantiomeric excess albeit with the opposite absolute configuration depending on the acceptor molecules 4 and 6. NMR investigation of the reactions between Ir complexes 1a–1c with α-cyanoacetates 3 showed that a stereoselective deprotonation reaction takes place to give an equilibrium mixture of N-bound amine complexes 8 and 9, the former with intramolecular hydrogen bonding and the latter without it, respectively. Computational study revealed the full details of the mechanism of the asymmetric C–N and C–C bond forming reactions catalyzed by the chiral Ir catalyst 1b. In the C–N bond forming reaction, the dimethyl azodicarboxylate 4a undergoes productive bifunctional activation by a non-hydrogen-bonded N-bound complex 9b(re) resulting in the formation of the R-product through the energetically favorable transition state. On the other hand, the linear geometry of the acetylenic ester molecule 6 allows its bifunctional activation with both types of the N-bound complexes: 8b and 9b with and without the intramolecular hydrogen bond respectively. The hydrogen-bond stabilized transition state for the C–C bond formation leading to the S-enantiomer is significantly lower in energy than the corresponding non-hydrogen-bonded transition state leading to the R-enantiomer. Thus, chiral induction of these two reactions is determined by the structures of the acceptor molecules.
在手性双功能 Ir 和 Ru 配合物 Cp*Ir[(S,S)-N-sulfonated dpen] 1 和 Ru[(S,S)-N-sulfonated dpen](η6-arene) 2(DPEN:2 (DPEN:1,2-二苯基乙二胺)进行了核磁共振光谱分析和 DFT 分析。值得注意的是,这两个反应使用了相同的手性酰胺催化剂 1、2 和亲核剂 α-氰基乙酸酯 3,定量生成了带有季手性碳中心的共轭加合物,对映体过量,但绝对构型相反,取决于受体分子 4 和 6。对 Ir 复合物 1a-1c 与 α-氰基乙酸酯 3 反应的核磁共振研究表明,发生了立体选择性去质子化反应,生成了 N 键胺复合物 8 和 9 的平衡混合物,前者分别具有分子内氢键,后者则没有。计算研究揭示了手性铱催化剂 1b 催化的不对称 C-N 键和 C-C 键形成反应的全部机理细节。在 C-N 键形成反应中,偶氮二甲酸二甲酯 4a 经过非氢键 N 结合复合物 9b(re)的双功能活化,通过能量有利的过渡态形成 R 产物。另一方面,乙炔酯分子 6 的线性几何形状允许它与两种类型的 N 结合复合物进行双功能活化:8b 和 9b,分别有分子内氢键和无分子内氢键。氢键稳定的 C-C 键形成过渡态导致 S-对映体的能量明显低于导致 R-对映体的相应非氢键过渡态。因此,这两个反应的手性诱导是由受体分子的结构决定的。