Highly Enantioselective Epoxidation of Styrenes Catalyzed by Proline-Derived C1-Symmetric Titanium(Salan) Complexes
Highly Enantioselective Epoxidation of Styrenes Catalyzed by Proline-Derived C1-Symmetric Titanium(Salan) Complexes
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DOI：
10.1002/anie.200903567
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Katsuki, Tsutomu
中科院分区：
文献类型：
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作者：
Matsumoto, Kazuhiro;Oguma, Takuya;Katsuki, Tsutomu
烯烃的催化不对称环氧化反应是制备对映体富集环氧化物的关键过程，而对映体富集环氧化物是许多化学反应中的重要分子。[1，2]特别地，氧化苯乙烯衍生物在不对称有机转化中是有用的和通用的手性结构单元。虽然已经开发了各种各样的手性催化剂（包括过渡金属配合物和有机催化剂）用于烯烃的不对称环氧化反应，但在苯乙烯衍生物的环氧化反应中，很少有催化剂能够实现高的对映选择性。[3-5]锰（salen）催化体系和Shi的酮催化体系是非官能化烯烃不对称环氧化的成熟方法。然而，当与其它烯烃中的较高水平的对映选择性相比时，在苯乙烯的环氧化中观察到的对映选择性（通常80- 90%ee）是不足的[4-6]，并且在此使用经济上可接受的和环境上可持续的过氧化氢作为氧化剂将是更有利的。[7]事实上，我们的研究小组已经引入了在钛（salalen）1和salan 2催化剂存在下使用过氧化氢水溶液作为氧化剂的非官能化烯烃的不对称环氧化（图1; salen= N，N '-双（亚水杨基）乙二胺阴离子，salan=还原salen，salalen [8]=半还原salen）。[9，10]虽然各种烯烃如末端、顺式二取代和三取代烯烃已有效地转化为高对映体过量的环氧化物，但在苯乙烯衍生物的环氧化中观察到的对映体选择性不足。即使是高度精细的钛（salalen）配合物1，其中配体具有两个手性中心和两个手性轴，氧化苯乙烯的ee值仅为93%。[9a]在此，我们报告了简单和高不对称选择性的钛催化剂的苯乙烯环氧化反应的发展。该催化剂衍生自天然存在的α-氨基酸脯氨酸，并实现高达98%ee的高对映选择性。
Catalytic enantioselective epoxidation of olefins is a pivotal process for providing enantioenriched epoxides, which are very important molecules in a broad range of chemical transformations.[1, 2] In particular, styrene oxide derivatives are useful and versatile chiral building blocks in asymmetric organic transformations. Although a wide variety of chiral catalysts (including transition-metal-based complexes and organocatalysts) have been developed for the asymmetric epoxidation of olefins, there are few catalysts that can achieve high enantioselectivity in the epoxidation of styrene derivatives.[3–5] Manganese (salen)-catalyzed and Shi’s ketone-catalyzed systems are the well-established methods for asymmetric epoxidation of unfunctionalized olefins. However, the enantioselectivity (generally 80–90% ee) observed in the epoxidation of styrenes is inadequate when compared to the higher level of enantioselectivity in other olefins,[4–6] and here the use of economically acceptable and environmentally sustainable hydrogen peroxide as an oxidant would be more favorable.[7] In fact, our research group has introduced asymmetric epoxidation of unfunctionalized olefins using aqueous hydrogen peroxide as the oxidant in the presence of titanium (salalen) 1 and salan 2 catalysts (Figure 1; salen= N, N’-bis (salicylidene) ethylenediamine anion, salan= reduced salen, salalen [8]= half-reduced salen).[9, 10] Although a variety of olefins such as terminal, cis-disubstituted, and trisubstituted olefins have been effectively converted into the epoxides in high enantiomeric excesses, the enantioselectivity observed in the epoxidation of styrene derivatives was insufficient. Even with the highly elaborate titanium (salalen) complex 1, in which the ligand has two chiral centers and two chiral axes, the ee value of styrene oxide was only 93%.[9a] Herein, we report the development of simpler and highly enantioselective titanium catalysts for the epoxidation of styrenes. The catalysts are derived from the naturally occurring a-amino acid proline, and achieve high enantioselectivity of up to 98% ee.