Iridium-mediated borylation of benzylic C-H bonds by borohydride.
Iridium-mediated borylation of benzylic C-H bonds by borohydride.
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DOI：
10.1002/anie.201006320
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发表时间：
2011-02
影响因子：
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通讯作者：
Christina Tang;William Smith;A. Thompson;D. Vidović;S. Aldridge
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文献类型：
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作者：
Christina Tang;William Smith;A. Thompson;D. Vidović;S. Aldridge
过渡金属介导的CÀH键到CÀB键的转化是最近在饱和和不饱和烃功能化方面的一个令人兴奋的发展这在一定程度上反映了这样一个事实，即所得的硼化化合物（硼酯或硼酸）是通过一系列已建立的方案进行进一步化学反应的有吸引力的底物[{Ir (cod) X} 2]/ 4,4 ' -二叔丁基联吡啶体系（X= Cl, OMe, indenyl; cod= cycloctadiene）催化芳烃/杂芳烃的转化（CÀH到CÀB）已经得到了特别好的发展，[1,3]在某些情况下实现了替代模式的选择性，这是用经典合成方法难以获得的这些硼化方案通常使用HBpin或B2pin2作为硼试剂（pin= pinacolato, OCMe2CMe2O）， IrIII-tris （Bpin）配合物被认为是关键的催化中间体CÀB键的形成通过MÀB/CÀH σ键的复分解或IrIII/IrV循环中不同的CÀH氧化加成/BÀC还原消除步骤进行。[1,5,6]有人提出了一种利用HBpin进行苯硼化反应的机制，该机制涉及单（Bpin）配合物和RhI/RhIII催化循环在最近的工作中，我们研究了含有双（n -杂环碳）（NHC）配体的铑和铱配合物与硼烷的相互作用。[8,9]在此过程中，我们发现了一个由[{Ir (coe) 2Cl} 2]（coe=环烯）介导的不寻常的分子内CÀH硼化过程，该过程导致BH3片段从LiBH4转移到苯基碳中心。[7,10,11]在这里，我们研究导致这种化学反应的基本机制步骤。N, N ' -双（2,4,6 -三甲基苯基）-咪唑-2-酰基的反应[1]与过量的LiBH4在乙醚中产生已知的化合物IMes。BH3(2) 75%产率相比之下，1与[{Ir (coe) 2Cl} 2]（0.25当量二聚体）/过量LiBH4反应生成锂盐3，其中一个邻甲基取代基经历了额外的CÀH活化，从而生成[ArCH2BH3] À函数（方案1）。11B核磁共振光谱显示了3的形成，显示了两个四重奏共振（dB À35）。1,1jbh = 81 Hz和À27。4 ppm, 1JBH= 78 Hz)，前者与报告的2 （dB À36）相似。8 ppm, 1JBH= 88 Hz)，[12b]后者与[RBH3] À物种的其他例子一致[例如dB À26]。8 ppm, 1JBH= 79 Hz（2-萘基）BH3À]这些光谱推断随后被晶体学研究证实，3在固态中以中心对称二聚体的形式存在（图1）。每个锂中心与六个BH氢原子相互作用（距离在1.86-2.24之间），其中两个来自每个碳烯。一个[(times ' BH3) BH3] À单元的BH3和[ArCH2BH3] À，另一个[ArCH2BH3] À单元的BH3和[ArCH2BH3] À。两种不同碳供体的CÀB距离随着键的缩短而略有不同[1.587(3)和1.634 (3)]
The transition-metal-mediated conversion of CÀH to CÀB bonds is an exciting recent development in the functionalization of both saturated and unsaturated hydrocarbons.[1] In part this reflects the fact that the resulting borylated compounds (boronic esters or acids) are attractive substrates for further chemistry through a range of established protocols.[2] CÀH to CÀB conversion in arenes/heteroarenes catalyzed by [{Ir (cod) X} 2]/4, 4’-di-tert-butylbipyridine systems (X= Cl, OMe, indenyl; cod= cyclooctadiene) has been particularly well developed,[1, 3] in some cases achieving selectivity for substitution patterns which have proved difficult to access using classical synthetic methods.[4] Typically these borylation protocols utilize HBpin or B2pin2 as the boron reagent of choice (pin= pinacolato, OCMe2CMe2O), with IrIII–tris (Bpin) complexes thought to be key catalytic intermediates.[5] CÀB bond formation proceeds through either MÀB/CÀH σ bond metathesis or through distinct CÀ H oxidative addition/BÀC reductive elimination steps in an IrIII/IrV cycle.[1, 5, 6] An alternative mechanism implicating a mono (Bpin) complex and a RhI/RhIII catalytic cycle has been proposed for benzylic borylation using HBpin.[7] In recent work we have examined the interaction of rhodium and iridium complexes containing bis (N-heterocyclic carbene)(NHC) ligand sets with boranes.[8, 9] In doing so we have discovered an unusual intramolecular CÀH borylation process mediated by [{Ir (coe) 2Cl} 2](coe= cyclooctene) which leads to the transfer of a BH3 fragment from LiBH4 to a benzylic carbon center.[7, 10, 11] Here, we investigate the fundamental mechanistic steps which lead to this chemistry. The reaction of IMes [N, N’-bis (2, 4, 6-trimethylphenyl)-imidazol-2-ylidene; 1] with excess LiBH4 in diethyl ether generates the known compound IMes. BH3(2) in 75% yield.[12] By contrast, the reaction of 1 with [{Ir (coe) 2Cl} 2](0.25 equiv of dimer)/excess LiBH4, leads to the formation of the lithium salt 3, in which one of the ortho-methyl substituents has undergone additional CÀH activation, thereby generating an [ArCH2BH3] À function (Scheme 1).The formation of 3 is suggested by 11B NMR spectroscopy which reveals two quartet resonances (at dB À35. 1, 1JBH= 81 Hz and À27. 4 ppm, 1JBH= 78 Hz), the former being similar to that reported for 2 (dB À36. 8 ppm, 1JBH= 88 Hz),[12b] the latter consistent with other examples of [RBH3] À species [eg dB À26. 8 ppm, 1JBH= 79 Hz for (2-naphthyl) BH3À].[13] These spectroscopic inferences were subsequently confirmed by crystallographic studies, with 3 being shown to exist as a centrosymmetric dimer in the solid state (Figure 1). Each lithium center interacts with six BH hydrogen atoms (with distances in the range 1.86–2.24), two of which originate from each of the carbene. BH3 and [ArCH2BH3] À units of one [(IMes’ BH3) BH3] À moiety, and the other two in the [ArCH2BH3] À unit of the second. The CÀB distances associated with the two different carbon donors are marginally different [1.587 (3) and 1.634 (3)] with the shorter bond