Lewis Acid-Mediated Acetal Substitution Reactions: Mechanism and Application to Asymmetric Catalysis
Lewis Acid-Mediated Acetal Substitution Reactions: Mechanism and Application to Asymmetric Catalysis
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DOI：
10.1002/adsc.201100346
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发表时间：
2011-08-01
影响因子：
5.4
通讯作者：
Yamashita, Yasuhiro
中科院分区：
文献类型：
--
作者：
Kobayashi, Shu;Arai, Kenzo;Yamashita, Yasuhiro
缩醛与碳原子亲核试剂的取代反应是最基本、最常用的有机反应。我们成功地制备了一种光学活性的缩醛，它与硅烯醇醚顺利地反应，得到所需的外消旋形式的加合物。通过对产物的ee与回收的缩醛的ee的比较，我们得出结论，羟醛型反应不是通过直接置换（S（N）2）或接触离子对（亲密离子对）（S（N）1），而是通过自由氧碳正离子（S（N）1）机制进行的。接下来，进行实现缩醛取代反应的不对称催化的研究。经过多次实验，发现由五甲氧基铌[Nb（OMe）（5）]和四齿BINOL衍生物制备的手性铌配合物可以实现高的对映选择性。缩醛与硅烯醇醚的不对称羟醛型反应顺利地进行，以良好的产率和高的对映选择性得到相应的羟醛型加合物。
Substitution reactions of acetals with carbon nucleophiles are fundamental and conventional organic reactions. We succeeded in the preparation of an optically active acetal, which reacted with a silyl enol ether smoothly to afford the desired adducts in racemic forms. By comparison of the ees of the products with the ees of the recovered acetals, we concluded that the aldol-type reactions proceeded not via direct displacement (S(N)2) or contact ion pairs (intimate ion pair) (S(N)1) but by a free oxocarbenium ion (S(N)1) mechanism. Next, a study to achieve asymmetric catalysis of the acetal substitution reactions was conducted. After many trials, it was found that a chiral niobium complex prepared from pentamethoxyniobium [Nb(OMe)(5)] and a tetradentate BINOL derivative could achieve high enantioselectivities. Asymmetric aldol-type reactions of acetals with silyl enol ethers proceeded smoothly to afford the corresponding aldol-type adducts in good yields with high enantioselectivities.