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首页> 外文期刊>Chemistry: A European journal >Double Bonds in Motion: Bis(oxazolinylmethyl)pyrroles and Their Metal-Induced Planarization to a New Class of Rigid Chiral C_2-symmetric Complexes
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Double Bonds in Motion: Bis(oxazolinylmethyl)pyrroles and Their Metal-Induced Planarization to a New Class of Rigid Chiral C_2-symmetric Complexes

机译：运动中的双键：双（恶唑啉基甲基）吡咯及其金属诱导的平面化反应，生成一类新的刚性手性C_2-对称配合物

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摘要

The synthesis of a new class of chiral C_2-symmetric tridentate N-do-nor ligands, a series of 2,5-bis(2-oxazo-linylmethyl)pyrroles, was achieved in four steps starting from the known 2,5-bis(trimethylammoniomethyl)pyrrole diiodide (1). Reaction of 1 with NaCN in dimethyl sulfoxide gave 2,,5-bis(cyanomethyl)pyrrole (2) cleanly, which was then cyclized with amino alcohols to give the 2,5-bis(2-oxazolinylmethyl)pyr-roles 3a-c (3a: bis[2-(4,4'-dimethyl-5-hydrooxazolyl)methyl]pyrrole; 3b:(S,S)-bis[2-(4-isopropyl-4,5-dihydrooxazolyl)methyl]pyrrole; 3c: (S,S)-bis[2-(4-tertiobutyl-4,5-dihydrooxazolyl)methyl]-pyrrole). Metallation of 3q-c with one molar equivalent of tBuLi and their subsequent reaction with a stoichiometric amount of [PdCl_2(cod)] (cod=cyclooctadiene) gave the palladium(II) complexes 4a-c. Whereas the arrangement of the N-donor atoms in the crystallographically characterized complex 4a is almost idealy square planar, plex 4a is almost ideally square planar, all three heterocycles in the ligand are twisted out of the coordination plane, leading to a chiral conformation of the complex. Attempts to freeze out these two conformers in solution at 200 K (NMR) failed, and this suggests that the activation barrier for conformational racemization is significantly below 10 kcal mol~(-1). The palladium-induced shift of two double bonds as well as the porphyrinogen/porphyrin-type oxidation of the complexes 4a-c led to the planarization of the 2,5-bis(oxazolinyl-methyl)pyrrolide ligands in the palladium m(II) complexes 5a-c, 6b, and 6c, and to the formation of rigid chiral C_2-symmetric systems as shown by X-ray diffraction studies. The formation of the conjugated system of double bonds in this transformation is accompanied by the emergence of an intra-ligand chromophore. This is evident in the absorption spectrum of 6c which displays an intense band with a maximum at 485 nm attributable to an intra-ligand pi~*<-pi transition and a characteristic vibrational progression of v approx 1350 cm~(-1). Complexes 4b and 4c were tested in the catalytic asymmetric Michael addition of ethyl 2-cyanopropionate to methyl vinylketone (catalyst loading: 1 mol%) and were found to give maximum ee values of 43% (4b) and 21% (4c) at low conversions.
机译：从已知的2,5-双开始的四个步骤中，合成了新型的手性C_2-对称三齿N-do-nor配体，一系列2,5-双（2-恶唑-亚烷基甲基）吡咯（三甲基铵甲基）吡咯二碘化物（1）。 1与NaCN在二甲基亚砜中的反应干净地得到2，，5-双（氰基甲基）吡咯（2），然后将其与氨基醇环化，得到2,5-双（2-恶唑啉基甲基）吡咯-吡咯3a-c （3a：双[2-（4，4'-二甲基-5-氢恶唑基）甲基]吡咯; 3b：（S，S）-双[2-（4-异丙基-4，5-二氢恶唑基）甲基]吡咯; 3c：（S，S）-双[2-（4-叔丁基-4，5-二氢恶唑基）甲基]-吡咯）。用一摩尔当量的tBuLi进行3q-c金属化，然后与化学计量的[PdCl_2（cod）]（cod =环辛二烯）反应，得到钯（II）配合物4a-c。尽管晶体学特征的配合物4a中的N供体原子的排列几乎是理想的正方形平面，而络合物4a的排列几乎是理想的正方形平面，但配体中的所有三个杂环都扭曲出了配位平面，从而形成了手性构象。复杂。试图在溶液中以200 K（NMR）冻结这两个构象异构体的尝试失败了，这表明构象消旋作用的激活势垒明显低于10 kcal mol〜（-1）。钯诱导的两个双键转移以及配合物4a-c的卟啉原/卟啉型氧化导致钯m（II）中的2,5-双（恶唑啉基-甲基）吡咯化物配体平面化X射线衍射研究表明，这些化合物可以与5a-c，6b和6c配合，形成刚性手性C_2对称系统。在该转化中双键共轭体系的形成伴随有配体内发色团的出现。这在6c的吸收光谱中很明显，该光谱显示了一个强带，在485 nm处有最大值，这归因于配体内部pi〜* <-pi跃迁和v约1350 cm〜（-1）的特征振动级数。在将2-氰基丙酸乙酯催化不对称迈克尔加成到甲基乙烯基酮中（催化剂负载量：1 mol％）中测试了配合物4b和4c，发现在较低的温度下其最大ee值为43％（4b）和21％（4c）。转换。

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