Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
Chem Asian J. 2011 Sep 5;6(9):2550-9. doi: 10.1002/asia.201100246. Epub 2011 Jul 20.
Asymmetric [3+2] cycloaddition of α-aminoester Schiff bases with substituted olefins is one of the most efficient methods for the preparation of chiral pyrrolidine derivatives in optically pure form. In spite of its potential utility, applicable substrates for this method have been limited to Schiff bases that bear relatively acidic α-hydrogen atoms. Here we report a chiral silver amide complex for asymmetric [3+2] cycloaddition reactions. A silver complex prepared from silver bis(trimethylsilyl)amide (AgHMDS) and (R)-DTBM-SEGPHOS worked well in asymmetric [3+2] cycloaddition reactions of α-aminoester Schiff bases with several olefins to afford the corresponding pyrrolidine derivatives in high yields with remarkable exo- and enantioselectivities. Furthermore, α-aminophosphonate Schiff bases, which have less acidic α-hydrogen atoms, also reacted with olefins with high exo- and enantioselectivities. The stereoselectivities of the [3+2] cycloadditions with maleate and fumarate suggested that the reaction proceeded by means of a concerted mechanism. An NMR spectroscopic study indicated that complexation of AgHMDS with the bisphosphine ligand was not complete, and that free AgHMDS, which did not show any significant catalytic activity, existed in the catalyst solution. This means that significant ligand acceleration occurred in the current reaction system.
α-氨基酸酯席夫碱与取代烯烃的不对称[3+2]环加成是制备手性吡咯烷衍生物的最有效方法之一。尽管该方法具有潜在的应用价值，但该方法适用的底物仅限于具有相对酸性α-氢原子的席夫碱。在这里，我们报告了一种用于不对称[3+2]环加成反应的手性银酰胺配合物。由银双(三甲基甲硅烷基)酰胺（AgHMDS）和（R）-DTBM-SEGPHOS 制备的银配合物在手性[3+2]环加成反应中表现良好，可与几种烯烃反应，以高收率和显著的非对映选择性和对映选择性得到相应的吡咯烷衍生物。此外，具有较少酸性α-氢原子的α-氨基膦酸酯席夫碱也与烯烃反应，具有高的非对映选择性和对映选择性。马来酸和富马酸的[3+2]环加成的立体选择性表明反应是通过协同机制进行的。NMR 光谱研究表明，AgHMDS 与双膦配体的络合不完全，而在催化剂溶液中存在没有任何显著催化活性的游离 AgHMDS。这意味着在当前的反应体系中发生了显著的配体加速。
