Cardon Joseph M, Coombs James C, Ess Daniel H, Castle Steven L
Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA.
Tetrahedron. 2019 Feb 22;75(8):945-948. doi: 10.1016/j.tet.2019.01.018. Epub 2019 Jan 11.
Attempts to perform the OsO-catalyzed enantioselective base-free aminohydroxylation of β,β-disubstituted enoates are described. Low yields and racemic products were obtained in the presence of standard chiral ligands, suggesting the occurrence of a "Second Cycle" process due to slow hydrolysis of the amino alcohol product from the Os metal center. Support for this hypothesis was provided by the slightly improved enantioselectivity (60:40 er) obtained with an amino alcohol ligand. Based on density functional theory calculations, it is proposed that the lack of significant enantioselectivity is due to a low-energy (3 + 2) oxo/imido cycloaddition transition state without the chiral ligand in the Second Cycle that outcompetes protonolysis in the First Cycle.
本文描述了尝试进行OsO催化的β,β-二取代烯酸酯的对映选择性无碱氨基羟基化反应。在标准手性配体存在的情况下，得到的产率较低且产物为外消旋体，这表明由于氨基醇产物从Os金属中心缓慢水解，发生了“第二循环”过程。使用氨基醇配体时对映选择性略有提高（对映体比例为60:40），这为该假设提供了支持。基于密度泛函理论计算，提出对映选择性不显著是由于在第二循环中没有手性配体时存在低能量的（3 + 2）氧代/亚氨基环加成过渡态，该过渡态在第一循环中比质子解作用更具优势。
