末端烯烃和仲胺电化学合成烯丙胺

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脂肪胺普遍存在于药物、天然产物和其他生物活性分子中。因此,开发快速构建C-N键的策略极具重要性。其中烯丙胺是特别有价值的结构单元,其具有合成多功能性。

一种简单的制备烷基烯丙胺的策略是烯烃和脂肪胺的氧化偶联。这种反应将是对之前依赖预官能化亲电试剂的经典胺烯丙基化策略(例如SN2过程和π-烯丙基取代反应)很好的补充。成功实现这种转化需要一种策略:促进所需的氧化C-N键形成过程,同时抑制脂肪胺原料或脂肪族烯丙胺产物的自身氧化(0.8-1.1 Vvs SCE),因此发展起来也有较大的难度(图1-上)。

还有一种C-H官能化策略构建烯丙基C-N键,利用氧化稳定的氮源实现分子间胺化反应。这些反应主要利用电子活性低(吸电基)的氮亲核试剂和氮烯前体,因此只能有限地得到具有缺电子氮原子的产物,例如烯丙基磺酰胺和氨基甲酸酯(图1-下)。

图1

尽管在其他氧化胺化反应中取得了重大进展,但迄今为止,只有一例报道分子间烯丙基C-H胺化反应可以直接得到脂肪族烯丙基胺产物——2020年,Ritter教授课题组报道了光活化硫亚胺试剂作为氮源(J. Am. Chem.Soc2020142, 1728717293.)。虽然这是烯丙基C-H胺化的里程碑式报告,但需要制备不同的硫亚胺亲电试剂才能合成带有的不同N取代基的产物。此外,这种试剂设计策略不适用于叔胺合成。因此,通过未活化的胺亲核试剂和烯烃的氧化偶联产生烯丙胺将会是合成胺领域的重要策略。

作者课题组之前实现了伯胺亲核试剂与烯烃偶联,通过一锅两步操作的过程获得环氮乙烷类产物。反应过程使用噻蒽(TT)在电催化条件下与烯烃形成双阳离子中间体,再与亲核试剂反应(Nature2021596, 7479)(图2)。

图2

在之前工作的基础上,本篇文献同样利用电化学策略,通过将双阳离子烯烃与噻蒽加合物与二级胺进行取代,消除过程得到直链的Z式三级烯丙胺产物(图3)。

图3

烯丙胺这一产物是在之前的工作中加入过量的大位阻一级胺发现的,在亲核胺过量的条件下并不能得到环丙胺产物(图4)。对此,作者认为这与N亲核试剂的位阻有关,当加入二级胺时,则可以很好地得到烯丙胺产物。之后确定加入合适的化学计量碱以降低胺的当量(图5),作者还考察了其他的碱,最后确定了最佳条件为加入二异丙基乙基胺。

图4

图5

接着作者进行了底物普适性考察(图6,7,8),不同取代基的烯烃,二级胺以及应用于一些生物活性分子的衍生化。

图6

图7

图8

为了验证机理,作者使用中间体乙烯基盐36投入标准条件,得到目标产物基于此,作者认为其中乙烯基盐经历碱诱导异构化成烯丙基中间体,然后该物质被胺亲核试剂迅速捕获。

图9

总结:作者开发了一种从末端烯烃和仲胺出发电化学合成烯丙胺的方法。这代表了通过C-H官能化得到三级脂肪族烯丙基胺的第一个例子,并且是获得Z式的氧化烯丙基策略的第一个例子。在这个过程表现出良好的官能团耐受性,且对水和空气不敏感,在合成上有潜在应用。

文章链接:doi.org/10.1021/jacs.1c11763


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