Activation of an anilido ligand for nucleophilic aromatic substitution by an oxidizing Os(IV) center
Activation of an anilido ligand for nucleophilic aromatic substitution by an oxidizing Os(IV) center
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DOI：
10.1021/ja0159214
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发表时间：
2001-06-13
影响因子：
15
通讯作者：
Mayer, JM
中科院分区：
文献类型：
--
作者：
Soper, JD;Kaminsky, W;Mayer, JM
配体与过渡金属的配位可以极大地改变其反应性。例如，简单的烯烃受到亲电试剂的攻击，而烯烃配合物则容易受到亲核试剂的攻击。本文报道了金属配合物中苯胺配体（NHPh-）在TpOs （NHPh) Cl2 (1)[Tp）水合（1-吡唑基）硼酸盐]中的亲核芳香取代反应的第一个例子。亲核芳香取代（SNAr）发生在缺乏电子的芳香环上，通常涉及卤素或其他核基的位移。金属苯胺配合物，如苯胺和苯胺阴离子NHPh-，被认为是富电子的芳香化合物，不容易受到亲核攻击，它们通常很容易质子化和水解。这里描述的反应也不寻常，因为它们涉及氢化物的形式亲核取代。这些过程通常需要氧化剂，如O2或KMnO4，或一个氧化步骤。在这里描述的反应中，Os （IV）中心既起到激活苯胺配体芳基环的作用，又起到氧化剂的作用。将哌啶加入到TpOs (NHPh)-Cl2(1) 5的乙腈溶液中，在室温下，数天内颜色由红色变为深蓝色。这个反应被尝试作为研究1,6不寻常的酸碱性质的一部分，但没有形成去质子化的配合物。完成反应的1H NMR谱显示，一种新的锇种的产率为~ 30%(2)，经柱层析和重结晶分离为蓝色固体。类似产物(3)在1与吡咯烷的反应中形成。单晶x射线衍射表明，这些材料为TpOs [NH-p-C6H4N (c-C5H10)] Cl2(2；图1)7和TpOs [NH-p-C6H4N (c-C4H8)] Cl2(3；图S18)，其中胡椒基或吡啶基取代基取代了苯胺基芳基环的对氢。光谱和分析数据支持了这一特性。8,9 2和3的固态结构与1,5的赝八面体配位、键长和锇中心角都非常相似。例如，1.945(7)(2)和1.922 (5)Å(3)的Os-N（酰胺）键长与1 [1.919 (6)Å]及其他相关配合物的键长接近。因此，2和3可以描述为Os （IV）苯胺配合物。然而，结构数据也表明了Os （II）醌二亚胺共振形式的贡献（如图1中所示，插入），如Joss等人对相关化合物的建议。10胺环的r -碳与芳环共面（2中的CCNC扭转角为0.6（14）和4.3(15)），这在空间上不太优选，但在类醌形式中是必需的。此外，“芳香”CC距离显示醌类模式：1.417(11),1.355(12),1.413(11),1.396(12),1.367（11）和1.383 （10）Å。第二个主要的锇产物，Os （III）苯胺配合物TpOs (NH2Ph) Cl2(4)，在反应混合物的1H NMR中几乎不明显，在δ 64 -51 ppm （fwhm 68-440 Hz）处有宽峰。这个指派被两个独立的合成证实：用钴二烯还原1，然后是三酸，以及Cp2Fe+[TpOs (OTf) Cl2-](5)与苯胺的反应。从5取代三氟酸酯已被证明是一个有价值的途径Os （III）配合物。11 CD3CN9中分离的4的1H NMR谱与1与哌啶和吡咯烷反应的第二个产物相同。在这两种反应中，虽然由于其共振的宽度难以量化，但4的产率大约为60%。
Coordination of a ligand to a transition metal can dramatically change its reactivity. Simple alkenes, for instance, are attacked by electrophiles, while alkene complexes are susceptible to nucleophilic attack. We report here the first example of nucleophilic aromatic substitution reactions of an anilido ligand (NHPh-) in a metal complex, in TpOs (NHPh) Cl2 (1)[Tp) hydrotris (1-pyrazolyl) borate]. Nucleophilic aromatic substitution (SNAr) occurs at electron deficient aromatic rings and typically involves displacement of halogens or other nucleofugal groups. 1 Metal anilido complexes, like aniline and the anilido anion NHPh-, are considered electron-rich aromatic compounds and are not susceptible to nucleophilic attacksthey are usually easily protonated and hydrolyzed. 2 The reactions described here are also unusual in that they involve formal nucleophilic substitution of hydride. 3 Such processes usually require an oxidizing agent such as O2 or KMnO4, or an autoxidation step. 1c, 3c, 4 In the reactions described here, the Os (IV) center serves both to activate the aryl ring of the anilido ligand and as the oxidant. Addition of piperidine to acetonitrile solutions of TpOs (NHPh)-Cl2 (1) 5 causes a color change from red to deep blue over several days at room temperature. This reaction was attempted as part of a study of the unusual acid/base properties of 1, 6 but the deprotonated complex was not formed. 1H NMR spectra of completed reactions show a∼ 30% yield of a new osmium species (2), which was isolated as a blue solid after column chromatography and recrystallization. An analogous product (3) is formed on reaction of 1 with pyrrolidine. Single-crystal X-ray diffraction showed these materials to be TpOs [NH-p-C6H4N (c-C5H10)] Cl2 (2; Figure 1) 7 and TpOs [NH-p-C6H4N (c-C4H8)] Cl2 (3; Figure S18), in which a piperidyl or pyrrolidyl substituent has replaced the para hydrogen of the anilido aryl ring. This characterization is supported by spectroscopic and analytical data. 8, 9 The solid-state structures of 2 and 3 are quite similar to that of 1, 5 in both their pseudooctahedral coordination and their bond lengths and angles about the osmium center. For instance, the Os-N (amide) bond lengths of 1.945 (7)(2) and 1.922 (5) Å (3) are close to that in 1 [1.919 (6) Å] and other related complexes. 5, 6 Thus 2 and 3 can be described as Os (IV) anilido complexes. However, the structural data also indicate a contribution from an Os (II) quinone diimine resonance form (illustrated for 2 in Figure 1, inset), as suggested by Joss et al. for a related compound. 10 The R-carbons of the amine rings are coplanar with the aromatic ring (CCNC torsion angles of-0.6 (14) and-4.3 (15) in 2), which is sterically less preferred but required in the quinonoid form. In addition, the “aromatic” CC distances show a quinonoid pattern: 1.417 (11), 1.355 (12), 1.413 (11), 1.396 (12), 1.367 (11), and 1.383 (10) Å.A second major osmium product, the Os (III) aniline complex TpOs (NH2Ph) Cl2 (4), is barely evident in the 1H NMR of reaction mixtures as broad peaks at δ 64 to-51 ppm (fwhm 68-440 Hz). This assignment was confirmed by two independent syntheses: by reduction of 1 with cobaltocene followed by triflic acid, and by reaction of Cp2Fe+[TpOs (OTf) Cl2-](5) with aniline. Substitution of triflate from 5 has proven to be a valuable route to Os (III) complexes. 11 The 1H NMR spectrum of isolated 4 in CD3CN9 is identical to the second product in the reactions of 1 with piperidine and pyrrolidine. The yield of 4 in both these reactions, though difficult to quantitate because of the breadth of its resonances, is roughly 60%.