手性雙噁唑啉配體之鍺烯陽離子及錫烯陽離子催化劑

Abstract

矽、鍺、錫、鉛烯陽離子是泛稱帶一個正電荷的二價14族元素化合物，分子中的14族元素帶有一個孤電子對，同時也帶有一個空p軌域，因此可以做為一個路易士鹼，亦可做為一個路易士酸。這種兩性分子的特性，讓主族有機金屬化學家對於矽、鍺、錫、鉛烯陽離子的反應性產生興趣，並將其作為各式反應之催化劑。然而，將具有手性之矽、鍺、錫、鉛烯陽離子作為不對稱合成催化劑的研究就完全沒有報導。在這個研究中，我們使用手性雙噁唑啉配基去穩定鍺烯陽離子以及錫烯陽離子，並探討它們在不對稱催化反應中的應用。在催化劑的合成上，我們先將雙噁唑啉配體去質子化，再鑲入二價十四族金屬，最後拔除氯離子，便能合成出目標產物。我們使用核磁共振光譜以及晶體結構去證實我們成功合成出我們的手性鍺/錫烯陽離子。之後，我們將其作為手性催化劑去進行不對稱催化反應，如對酮類的硼氫化反應、矽腈化反應、矽氫化反應、麥可加成反應等。雖然大多反應都有很好的催化效果，可惜在鏡像選擇性的表現上較為遜色。

Tetrylium-ylidenes are cationic divalent Group 14 element complexes in which the central atom possesses a lone pair of electrons (Lewis basic) and a vacant p-orbital (Lewis acidic), resulting in Lewis ambiphilicity. This characteristic renders tetrylium-ylidenes versatile catalysts for various reactions. However, the use of chiral tetrylium-ylidenes in asymmetric catalysis remains largely unexplored. In this study, we utilize a series of chiral bis(oxazoline) ligands (abbreviated as BOX ligand) to stabilize tetrylium-ylidenes. The chiral tetrylium-ylidene complexes were synthesized through deprotonation of chiral BOX ligands, followed by coordination with divalent Group 14 halides and subsequent chloride abstraction. Although obtaining single crystals of the base-free tetrylium-ylidene proved challenging, we successfully isolated one DMAP-coordinated derivative, confirming the formation of the anticipated chiral tetrylium-ylidene. Employing these chiral tetrylium-ylidenes as catalysts, we investigated several reactions, including hydroboration, cyanosilylation, hydrosilylation of ketones, and the Michael addition of indoles to unsaturated ketones. Most reactions exhibited good catalytic efficiency and moderate enantioselectivity.