維基百科

雙噁唑啉配體

雙噁唑啉配體(英語：bisoxazoline ligands)是一類含有雙噁唑啉環的C2對稱性手性配體。最簡單的雙噁唑啉配體以一個亞甲基（-CH2-）橋連雙噁唑啉環，簡稱為BOX；通過吡啶環連接噁唑啉的配體十分常用，簡稱為PyBOX。含雙噁唑啉配體的配合物主要被用於不對稱催化中。

歷史

 
雙噁唑啉配體的發展

1984年Brunner等人首次將噁唑啉配體用於不對稱催化，連同許多席夫鹼，進行對映選擇性的類卡賓環丙烷化反應[1]。席夫鹼是當時的主要配體，如野依良治在1968年發現不對稱催化的過程中就使用了席夫鹼[2]，其同事Tadatoshi Aratani[3]在1970年代發表的多篇使用席夫鹼的對映選擇性環丙烷化的論文在隨後深刻地影響了Brunner的工作[4][5][6]。 最初噁唑啉配體的ee僅為4.9%，相比於席夫鹼配體的65.6%表現不佳。隨後Brunner在研究二醇的單苯基化過程中重新研究了噁唑啉配體，並發展出了手性吡啶噁唑啉配體，其ee在1986年報道時為30.2% [7]，在1989年時達到了45% [8]。同年Pfaltz等人報道了使用C2對稱的半咕啉配體進行的對映選擇性卡賓環丙烷化反應，ee值達到了92-97% [9]，但此類配體的缺點是需要多步進行合成，且產率低，約為30% [10]。

 
C2對稱軸向手性的雙噁唑啉配體

到了1989年Nishiyama等人在Brunner工作的基礎上發展出了第一個雙噁唑啉配體，合成了PyBOX配體用於酮的硅氫化反應，ee高達93% [11]。一年後，Masamune等人報道了第一個BOX配體[12]並首次用於銅催化的類卡賓環丙烷化反應，在1%摩爾負載量下實現高達99%的ee，該項工作引起了人們對BOX家族的極大興趣。至彼時由於2-噁唑啉環的合成已經十分成熟[13] [14]，相關研究進展迅速[15][16] [17]。這些研究主要基於經典的BOX和PyBOX配體，並出現了許多替代結構[18][19]。


合成

噁唑啉環的合成已經十分完備，一般通過2-氨基醇與合適的官能團關環來進行。合成雙噁唑啉配體時，最好使用含雙官能團的起始原料，以期在同一步構建雙環。二腈與二羧酸類化合物是最常使用的原料，被用於生產絕大多數的雙噁唑啉配體。

 

BOX和PyBOX的廣泛應用，可能有它們能夠從丙二腈和吡啶二甲酸開始非常方便地構建雙噁唑啉環的原因，分子的手性一般在此步由氨基醇引入。

應用

催化

一般而言，對於BOX配體，立體化學結構與預期的扭曲平面正方形分子構型中間體一致[20][21]。噁唑啉4號位上的取代基阻擋了底物一側的對映面，使產物有對映選擇性。該方法可以用於羥醛反應 [22]，並能推廣到多種反應，包括但不限於曼尼希反應 [23]、烯反應 [24]、邁克爾加成 [25]、納扎羅夫環化反應 [26]和雜原子狄爾斯-阿爾德反應 [27]等。

 
BOX對映選擇性範例

另一方面，攜有徑向的三齒PyBOX配體與底物路易斯酸的兩點鍵合形成了四方錐形的結構，以苄氧基乙醛作親電試劑為例，立體化學結果顯示羰基氧在赤道面結合，醚氧在四方錐軸向結合[28]。

 
PyBox Stereochemical model

含雙噁唑啉的配合物可以用於各種不對稱催化之中，其電中性的特性使其適合與貴金屬一起使用[29][30][31]，但最常見的是與銅的絡合物[30]。

構建碳-碳鍵

雙噁唑啉配體的一個重要應用是構建碳-碳鍵，對一系列的不對稱環加成反應有效，最初在類卡賓的構建環丙烷體系的反應中被引入，例如西蒙斯–史密斯反應 [12]，並拓展到1,3-偶極環加成反應和狄爾斯-阿爾德反應之中。

 
BOX參與的羥醛反應[32]
 
BOX參與的D-A反應[33]:合成馬鞭草烯酮，最後含DPPA的轉化過程包括改性的庫爾提斯重排反應。

其他

與環丙烷化類似，雙噁唑啉可以用於氮丙啶化；另一個常見的反應是硅氫化（英語：HydrosilylationHydrosilylation）反應，最初在PyBOX的使用中被引入[11]。其它的利基應用有作氟化催化劑[34]和瓦克爾法環化等[35]。

 
對映選擇性硅氫化[11]

參見

參考文獻

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