掌性硼陽離子: 合成、鑑定與不對稱催化反應

Abstract

不對稱催化對於天然產物的合成非常重要。至今為止，已有大量過渡金屬為基底的催化劑已被成功開發用來做各種不對稱催化。在近幾年中，也有以主族元素為基底作為不對稱催化劑的文獻被刊登，其中又以Corey-Bakshi-Shibata的硼雜噁唑烷催化劑最為出色，其不對稱催化劑在還原醛類及酮類的反應中具有極高的轉換效率及鏡像選擇性。受其啟發後，我們也製備了一系列的掌性二聚體硼陽離子催化劑，其催化劑的特點在於它包含了立障大的五甲基茂取代基和掌性噁唑烷酮配體，而五甲基茂取代基的功能在於可以有效避免硼中心被過度配位，而我們選用掌性噁唑烷酮的原因在於它容易取得且易去質子化，這對我們的合成路徑非常有幫助。我們以一系列的中性掌性二聚體硼化合物透過in-situ產生催化劑的方式催化Diels-Alder反應來測試其催化效力，且發現這類催化劑在反應中確實能導入立體選擇性，也發現了催化劑取代基和親鹵試劑的不同會影響產物的光學純度。最後，我們也做了低溫1H NMR實驗來驗證在反應中確實生成了掌性二聚體硼陽離子，且此掌性硼陽離子能催化不對稱Diels-Alder反應。

Asymmetric catalysis is crucial for natural product synthesis. Thus, a considerable amount of synthetic efforts have been devoted into the design of transition metal-based catalysts for enantioselective synthesis. In the past few years, chiral catalysts focusing on the s- and p-block elements have also emerged. Inspired by the highly efficient Corey-Bakshi-Shibata catalyst in various asymmetric transformations, we have prepared a series of chiral boron-based dimers consisting of bulky Cp* (1,2,3,4,5-Pentamethylcyclopentadiene) and chiral auxiliary oxazolidinone ligands. While the commercially available chiral oxazolidinone can be easily deprotonated and added to the boron center, the incorporation of electron-rich and steric protective Cp* group was found to be critical in preventing over-substitution at the boron. We found that the chirality can be transferred to the product of Diels-Alder reaction via an in-situ generated cationic diboron catalyst. Furthermore, the enantioselectivity is sensitive towards the substituents of oxazolidinone and halophiles. Finally, the observed asymmetric Diels-Alder reaction was proposed to be associated with a cyclic diboron mono-cation, whose generation under reaction condition was also confirmed by low-temperature 1H NMR studies.