Abstract
[Ru(II)Cp((R)-Cl-Naph-PyCOOH)]PF6 ((R)-1) catalyzes the dehydrative cyclization of (E)-hept-2-ene-1,7-diol (2) to 2-vinyltetrahydro-2H-pyran (3) with a 97:3 S/R enantiomer ratio. Complex (R)-1 is in equilibrium between two diastereomers (R,RRu)-1 (AR) and (R,SRu)-1 (AS). A difference of turn over efficiency between the AS and AR cycles is thought to be the origin of the high enantioselectivity. The AS gives a major enantiomeric product (S)-3, according to the results of detailed mechanistic investigation via i) X-ray crystallographic analysis of related complexes, ii) NMR experiments using allylic alcohol 2, OH-lacking 2-mimic 4, d-labeled (S)-4-1d, enantiomerically enriched hept-6-ene-1,5-diol (6) as branched isomer of 2, and OH-lacking 6-mimic 5, iii) substrate structure/reactivity and selectivity relationships, iv) deuterium-labeling experiment, v) kinetics via calorimetric analysis, and vi) ligand structure/reactivity and selectivity relationships. AS captures 2 via hydrogen and halogen bonds. Oxidative addition in an H2Oin mode leads to a macrocyclic σ-allyl intermediate. Here, an efficient nC(7)OH/π*C(3)=C(2) trans-annular (TA) interaction facilitates an SN2′ nucleophilic addition of OH in an OHTA manner to furnish (S)-3. Contrary to the AS, AR cannot capture 2 using the halogen bond and slowly operates to give (R)-3. A conventional π-allyl-complex-involved mechanism is ruled out by a contradiction in the result of ii) and iii).
摘要
[Ru(II)Cp((R)-Cl-Naph-PyCOOH)]PF6((R)-1)催化(E)-庚-2-烯-1,7-二醇(2)脱水环化成 2-乙烯基四氢-2H-吡喃(3),S/R 对映体比例为 97:3。复合物 (R)-1 在两种非对映异构体 (R,RRu)-1 (AR) 和 (R,SRu)-1 (AS) 之间处于平衡状态。AS 和 AR 循环之间的翻转效率差异被认为是高对映选择性的根源。详细的机理研究结果表明,AS 产生了一种主要的对映体产物 (S)-3,具体方法包括 i) 相关复合物的 X 射线晶体学分析;ii) 使用烯丙基醇 2、缺乏 OH 的 2-模拟物 4、d 标记的 (S)-4-1d、对映体富集的庚-6-烯-1、5-二醇(6)作为 2 的支链异构体,以及缺乏羟基的 6-模拟物 5;iii)底物结构/反应性和选择性关系;iv)氘标记实验;v)通过量热分析进行动力学分析;以及 vi)配体结构/反应性和选择性关系。AS 通过氢键和卤素键捕获 2。H2Oin 模式的氧化加成导致产生大环的σ-烯丙基中间体。在这里,有效的 nC(7)OH/π*C(3)=C(2) 反环状(TA)相互作用促进了 OH 以 OHTA 方式进行 SN2′ 亲核加成,生成 (S)-3。与 AS 相反,AR 不能利用卤素键捕获 2,而是缓慢地生成 (R)-3。由于 ii) 和 iii) 的结果存在矛盾,因此排除了传统的 π-烯丙基络合物参与机制。