Low Pressure Asymmetric Hydrogenation of Quinolines using an Annulated Planar Chiral N-Ferrocenyl NHC-Iridium Complex
Low Pressure Asymmetric Hydrogenation of Quinolines using an Annulated Planar Chiral N-Ferrocenyl NHC-Iridium Complex
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DOI：
10.1002/adsc.201500105
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发表时间：
2015-06-15
影响因子：
5.4
通讯作者：
Metallinos, Costa
中科院分区：
文献类型：
--
作者：
John, Joshni;Wilson-Konderka, Cody;Metallinos, Costa
环状平面手性n -二苯基甲醇衍生物经酸介导环化得到的n -二茂铁咪唑酮类化合物，可以用二异丁基氢化铝（DIBAL-H）还原得到一系列稳定且可分离的半胺醚类化合物。其中两种衍生物可与三苯基四氟硼酸碳氧化生成n -杂环碳（NHCs）的咪唑盐前体。这些盐在（环二烯）氯化铱二聚体{[Ir(COD)Cl](2)}存在下的去质子化反应提供了具有独特刚性四环框架的n -二茂铁酰NHCs的手性配位配合物。这些配合物的阳离子类似物在非常温和的条件下（1 mol%配合物，1 mol% PPh3, 1-5 atm H-2，甲苯，25℃）催化2-取代喹啉的不对称加氢，具有明显的对映选择性（高达90:10 er）。氢化过程对磷化氢添加剂变化的敏感性表明，可能涉及外球反应机制，正如Crabtree及其同事报道的非手性NHC-Ir配合物所提出的那样。
Annulated planar chiral N-ferrocenylimidazolones, obtained by acid-mediated cyclization of diphenylmethanol derivatives, may be reduced with diisobutylaluminium hydide (DIBAL-H) to afford a series of surprisingly stable and isolable hemiaminal ether aminals. Two of these derivatives can be oxidized with triphenylcarbenium tetrafluoroborate to imidazolinium salt precursors of N-heterocyclic carbenes (NHCs). Deprotonation of these salts in the presence of (cyclooctadiene) iridium chloride dimer {[Ir(COD)Cl](2)} provides chiral coordination complexes bearing N-ferrocenyl NHCs with unique rigid tetracyclic frameworks. Cationic analogues of these complexes catalyze the asymmetric hydrogenation of 2-substituted quinolines under very mild conditions (1 mol% complex, 1 mol% PPh3, 1-5 atm H-2, toluene, 25 degrees C) in appreciable enantioselectivity (up to 90:10 er). The sensitivity of the hydrogenation process to changes in the phosphine additive suggests that an outer-sphere reaction mechanism may be involved, as proposed for a related achiral NHC-Ir complex reported by Crabtree and co-workers.