Catalytic asymmetric aziridination with borate catalysts derived from VANOL and VAPOL ligands: Scope and mechanistic studies
Catalytic asymmetric aziridination with borate catalysts derived from VANOL and VAPOL ligands: Scope and mechanistic studies
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DOI：
10.1002/chem.200701558
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发表时间：
2008-01-01
影响因子：
4.3
通讯作者：
Wulff, William D.
中科院分区：
文献类型：
--
作者：
Zhang, Yu;Desai, Aman;Wulff, William D.
描述了由 VANOL 和 VAPOL 配体以及三苯硼酸酯制备的催化剂介导的重氮乙酸乙酯催化亚胺不对称氮丙啶化的范围和机理的扩展研究。对scalemic VANOL和VAPOL的非线性研究揭示了配体的光学纯度和产物之间基本上呈线性关系，表明催化剂包含单分子的配体。 H-1 NMR 研究表明，由 B(OPh)(3) 和 VANOL 或 VAPOL 制备的催化剂中存在两种物质。催化剂混合物的质谱分析表明，其中一种物质涉及一个配体分子和一个硼原子（B1），另一种物质涉及一个配体分子和两个硼原子（B2）。后者可以配制为线性或环状焦硼酸盐，并且 B-11 NMR 谱与线性焦硼酸盐结构最一致。开发了几种新的催化剂制备方案，可以生成比例为 10:1 至 1:20 的 B1 和 B2 催化剂混合物。对富含 B1 和 B2 物质的催化剂的研究表明，B2 催化剂是 VAPOL 催化的不对称氮丙啶化反应中的活性催化剂，其不对称诱导和速率明显高于 B1 催化剂。 VANOL 系列中的差异并不明显。采用最佳催化剂制备程序对一系列 12 种不同的亚胺进行了研究，发现对于 VANOL 和 VAPOL 催化剂，芳香族亚胺的不对称诱导值在 90 年代中期到 90 年代中期，脂肪族亚胺的不对称诱导值在 80 年代中期到 90 年代中期。尽管如此，N-二苯甲基氮丙啶的结晶度使得几乎所有筛选的 12 种氮丙啶产品都可以通过单次重结晶达到 > 99% ee。
An extended study of the scope and mechanism of the catalytic asymmetric aziridination of imines with ethyl diazoacetate mediated by catalysts prepared from the VANOL and VAPOL ligands and triphenylborate is described. Nonlinear studies with scalemic VANOL and VAPOL reveal an essentially linear relationship between the optical purity of the ligand and the product suggesting that the catalyst incorporates a single molecule of the ligand. Two species are present in the catalyst prepared from B(OPh)(3) and either VANOL or VAPOL as revealed by H-1 NMR studies. Mass spectral analysis of the catalyst mixture suggests that one of the species involves one ligand molecule and one boron atom (B1) and the other involves one ligand and two boron atoms (B2). The latter can be formulated as either a linear or cyclic pyroborate and the B-11 NMR spectrum is most consistent with the linear pyroborate structure. Several new protocols for catalyst preparation are developed which allow for the generation of mixtures of the B1 and B2 catalysts in ratios that range from 10:1 to 1:20. Studies with catalysts enriched in the B1 and B2 species reveal that the B2 catalyst is the active catalyst in the VAPOL catalyzed asymmetric aziridination reaction giving significantly higher asymmetric inductions and rates than the B1 catalyst. The difference is not as pronounced in the VANOL series. A series of 12 different imines were surveyed with the optimal catalyst preparation procedure with the finding that the asymmetric inductions are in the low to mid 90s for aromatic imines and in the mid 80s to low 90s for aliphatic imines for both VANOL and VAPOL catalysts. Nonetheless, the crystallinity of the N-benzhydryl aziridines is such that nearly all of the 12 aziridine products screened can be brought to > 99% ee with a single recrystallization.