CoIII-Carbene Radical Approach to Substituted 1H-Indenes
CoIII-Carbene Radical Approach to Substituted 1H-Indenes
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DOI：
10.1021/jacs.6b05434
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发表时间：
2016-07-20
影响因子：
15
通讯作者：
de Bruin, Bas
中科院分区：
文献类型：
--
作者：
Das, Braja Gopal;Chirila, Andrei;de Bruin, Bas
利用Co-III羰基中间体的固有反应活性，提出了一种通过金属活化o-肉桂基n -甲酰腙催化合成取代1h -茚的新策略。该反应使用现成的起始原料，操作简单，因此代表了一种实用的方法来构建功能化1h -独立衍生物。廉价且易于制备的低自旋钴（II）配合物[Co-II(MeTAA)] （MeTAA =四甲基四氮杂环烯）被证明是所研究的催化剂中最活跃的催化剂，这证明了非卟啉钴（II）配合物的催化羰基自由基反应活性，并且首次证明了[Co-II(MeTAA)]的一般催化活性。该方法已成功地应用于广泛的衬底，以良好到优异的产量生产1h - indees。本文中金属自由基催化的茚合成代表了分子内净（正式）卡宾插入反应到乙烯基C(sp(2))-H键的独特例子，该反应是通过所涉及的卡宾自由基中间体的可控自由基环闭合过程实现的。对其机理进行了计算研究，并通过一系列配套的实验反应证实了结果。密度泛函理论计算揭示了重氮化合物的一个逐步活化过程，该过程包括导致Cons-carbene自由基的形成，然后是自由基环闭合以产生吲哚基/苄基自由基中间体。随后的独立产物消除涉及1,2-氢转移步骤再生催化剂。利用2,2,6,6-四甲基哌啶-1-氧（TEMPO）自由基或过氧化二苯甲酰（DBPO）进行的捕获实验证实了钴（III）羰基中间体的参与。利用苯基n-叔丁基硝基酮（PBN）进行的电子顺磁共振自旋俘获实验揭示了该反应的自由基性质。
A new strategy for the catalytic synthesis of substituted 1H-indenes via metalloradical activation of o-cinnamyl N-tosyl hydrazones is presented, taking advantage of the intrinsic reactivity of a Co-III carbene radical intermediate. The reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of functionalized 1H-indene derivatives. cheap and easy to prepare low spin cobalt(II) complex [Co-II(MeTAA)] (MeTAA = tetramethyltetraaza [14] annulene) proved to be the most active catalyst among those investigated, which demonstrates catalytic carbene radical reactivity for a nonporphyrin cobalt(II) complex, and for the first time catalytic activity of [Co-II(MeTAA)] in general. The methodology has been successfully applied to a broad range of substrates, producing 1H-indenes in good to excellent yields. The metallo-radical catalyzed indene synthesis in this paper represents a unique example of a net (formal) intramolecular carbene insertion reaction into a vinylic C(sp(2))-H bond, made possible by a controlled radical ring-closure process of the carbene radical intermediate involved. The mechanism was investigated computationally, and the results were confirmed by a series of supporting experimental reactions. Density functional theory calculations reveal a stepwise process involving activation of the diazo compound leading to formation of a Cons-carbene radical, followed by radical ring-closure to produce an indanyl/benzyl radical intermediate. Subsequent indene product elimination involving a 1,2-hydrogen transfer step regenerates the catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement of cobalt(III) carbene radical intermediates. Electron paramagnetic resonance spectroscopic spin-trapping experiments using phenyl N-tert-butylnitrone (PBN) reveal the radical nature of the reaction.