Efficient Cobalt Catalyst for Ambient-Temperature Nitrile Dihydroboration, the Elucidation of a Chelate-Assisted Borylation Mechanism, and a New Synthetic Route to Amides
Efficient Cobalt Catalyst for Ambient-Temperature Nitrile Dihydroboration, the Elucidation of a Chelate-Assisted Borylation Mechanism, and a New Synthetic Route to Amides
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DOI：
10.1021/jacs.9b07529
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发表时间：
2019-09-25
影响因子：
15
通讯作者：
Trovitch, Ryan J.
中科院分区：
文献类型：
--
作者：
Ghosh, Chandrani;Kim, Suyeon;Trovitch, Ryan J.
N，N-二硼胺在有机合成中已成为一种很有前途的试剂，然而，它们的有效制备和充分的合成用途尚未实现。为了解决这两个缺点，寻求用于腈二氢硼化的有效催化剂。在（PDI）-P-PyEt存在下加热CoCl 2得到六配位Co（II）盐[（（PDI）-P-PyEt）CoCl][Cl]。加入2当量NaEt 3BH后，观察到氢化物转移到一个螯合亚胺官能团，导致形成（kappa（4）-N，N，N，N-（IPNEtPy）-I-PyEt-N-CHMe）Co。单晶X射线衍射和密度泛函理论计算表明，该化合物具有低自旋Co（II）基态，其特征在于与单一还原的亚氨基（吡啶）部分反铁磁耦合。重要的是，发现（kappa（4）-N，N，N，N-（IPNEtPy）-I-PyEt-N-CHMe）Co催化使用HBPin的腈的二氢硼化，在环境温度下的转换频率高达380 h（-1）。化学计量加成实验表明，HBPin通过Co-N-酰胺键加成，生成可与其他HBPin或腈反应的氢化物中间体。计算评价的反应坐标显示，B-H加成和腈插入步骤发生在反铁磁耦合的三重态自旋流形。有趣的是，发现硼亚胺中间体的形成发生在BPin从硼基化螯合臂转移以再生（κ（4）-N，N，N，N-（IPNEtPy）-I-PyEt-N-CHMe）Co之后。硼亚胺还原又是容易的，并且遵循相同的配体辅助的硼基化途径。烷基和芳基亚胺的独立硼氢化反应也在25 ℃下进行。在一系列N，N-二硼胺的情况下，它们与羧酸的加成允许在120 ° C下直接合成酰胺，而不需要外源偶联试剂。
N,N-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl2 in the presence of (PDI)-P-PyEt afforded the six-coordinate Co(II) salt, [((PDI)-P-PyEt)CoCl][Cl]. Upon adding 2 equiv of NaEt3BH, hydride transfer to one chelate imine functionality was observed, resulting in the formation of (kappa(4)-N,N,N,N-(IPNEtPy)-I-PyEt-N-CHMe)Co. Single-crystal X-ray diffraction and density functional theory calculations revealed that this compound possesses a low-spin Co(II) ground state featuring antiferromagnetic coupling to a singly reduced imino(pyridine) moiety. Importantly, (kappa(4)-N,N,N,N-(IPNEtPy)-I-PyEt-N-CHMe)Co was found to catalyze the dihydroboration of nitriles using HBPin with turnover frequencies of up to 380 h(-1) at ambient temperature. Stoichiometric addition experiments revealed that HBPin adds across the Co-N-amide bond to generate a hydride intermediate that can react with additional HBPin or nitriles. Computational evaluation of the reaction coordinate revealed that the B-H addition and nitrile insertion steps occur on the antiferromagnetically coupled triplet spin manifold. Interestingly, formation of the borylimine intermediate was found to occur following BPin transfer from the borylated chelate arm to regenerate (kappa(4)-N,N,N,N-(IPNEtPy)-I-PyEt-N-CHMe)Co. Borylimine reduction is in turn facile and follows the same ligand-assisted borylation pathway. The independent hydroboration of alkyl and aryl imines was also demonstrated at 25 degrees C. With a series of N,N-diborylamines in hand, their addition to carboxylic acids allowed for the direct synthesis of amides at 120 degrees C, without the need for an exogenous coupling reagent.