Synthetic Organic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
J Org Chem. 2023 Jun 16;88(12):7764-7773. doi: 10.1021/acs.joc.2c02732. Epub 2023 Feb 22.
Chiral Ni complexes have revolutionized both asymmetric acid-base and redox catalysis. However, the coordination isomerism of Ni complexes and their open-shell property still often hinder the elucidation of the origin of their observed stereoselectivity. Here, we report our experimental and computational investigations to clarify the mechanism of β-nitrostyrene facial selectivity switching in Ni(II)-diamine-(OAc)-catalyzed asymmetric Michael reactions. In the reaction with a dimethyl malonate, the Evans transition state (TS), in which the enolate binds in the same plane with the diamine ligand, is identified as the lowest-energy TS to promote C-C bond formation from the face in β-nitrostyrene. In contrast, a detailed survey of the multiple potential pathways in the reaction with α-keto esters points to a clear preference for our proposed C-C bond-forming TS, in which the enolate coordinates to the Ni(II) center in apical-equatorial positions relative to the diamine ligand, thereby promoting face addition in β-nitrostyrene. The N-H group plays a key orientational role in minimizing steric repulsion.
手性 Ni 配合物彻底改变了不对称酸碱和氧化还原催化。然而，Ni 配合物的配位异构和其开壳性质仍然常常阻碍对观察到的立体选择性起源的阐明。在这里，我们报告了我们的实验和计算研究，以阐明 Ni(II)-二胺-(OAc)-催化不对称迈克尔加成反应中β-硝基苯乙烯面选择性切换的机制。在与二甲基丙二酸酯的反应中，确定 Evans 过渡态（TS），其中烯醇盐在与二胺配体相同的平面内结合，是促进β-硝基苯乙烯从 C 面形成 C-C 键的最低能量 TS。相比之下，对与α-酮酯的反应中多种潜在途径的详细调查表明，我们提出的形成 C-C 键的 TS 明显占优，其中烯醇盐相对于二胺配体以顶-底位置与 Ni(II)中心配位，从而促进β-硝基苯乙烯的 C 面加成。N-H 基团在最小化空间排斥方面起着关键的定向作用。
