<i>C</i><sub>2</sub>-Symmetric Copper(II) Complexes as Chiral Lewis Acids. Scope and Mechanism of the Catalytic Enantioselective Aldol Additions of Enolsilanes to Pyruvate Esters
作者:David A. Evans、Christopher S. Burgey、Marisa C. Kozlowski、Steven W. Tregay
DOI:10.1021/ja982983u
日期:1999.2.1
regioselectivity (98:2), diastereoselectivity (93:7), and enantioselectivity (97% ee) are also observed in the aldol addition to 2,3-pentanedione. In all instances, the aldol adducts are generated in high yield and in excellent enantiomeric excess using as little as 1 mol % of the chiral complex 1a. Mechanistic insight into the pyruvate aldol reaction has also been gained. Silyl crossover experiments
C2 对称 (S,S)-叔丁基-双(恶唑啉基)Cu(OTf)2 配合物 (1a) 已被证明可催化 α-酮酯与甲硅烷基烯酮缩醛或烯醇硅烷之间的对映选择性羟醛反应,其对映选择性范围为93% 到 99%。使用取代的甲硅烷基乙烯酮缩醛,观察到 90:10 至 98:2 的顺式反应非对映选择和 93% 至 98% 的对映选择性。在 2,3-戊二酮的醛醇加成中也观察到高水平的羰基区域选择性 (98:2)、非对映选择性 (93:7) 和对映选择性 (97% ee)。在所有情况下,使用低至 1 mol% 的手性配合物 1a 即可以高产率和极好的对映体过量生成羟醛加合物。对丙酮酸醛醇反应的机理也有了深入的了解。甲硅烷基交叉实验证明甲硅烷基转移步骤是分子间的。基于这些结果,TMSOTf 已被确定为加速这些反应的附加物。此外,溶剂显示...