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首页> 外文期刊>Zeitschrift fur Anorganische und Allgemeine Chemie >A Kinetic and Mechanistic Study of Nucleophilic Substitution at a Pentacoordinated Silicon Atom
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A Kinetic and Mechanistic Study of Nucleophilic Substitution at a Pentacoordinated Silicon Atom

机译：五配位硅原子上亲核取代的动力学和机理研究

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摘要

A model system for the exploration of nucleophilic substitution Lit a pentacoordinate silicon centre is reported. The degenerate nucleophilic exchange of free N-methylimidazole (NMI) with coordinated NMI in a chiral pentacoordinate silicon compound, 4, has been Studied by dynamic NMR spectroscopy. In 4 the axial ligands are A-methylimidazole and an intramolecularly-coordinated and chelating oxygen atom of an amido group. The equatorial ligands are N-CH2, Ph and Me. Two competing mechanisms for the nucleophilic substitution Lit silicon have been distinguished from the data. One mechanism takes place with retention of con-figuration at silicon and is zeroth order in added nucleophile. This is interpreted as involving rate limiting dissociation of the N-methylimidazole followed by rapid re-association of another NMI molecule. The other mechanism takes place with both inversion and retention of configuration at silicon but is also of zeroth order in added nucleophile. A mechanism for this reaction in which the O-Si coordination is lost in the rate-determining step is consistent with the data. In this particular example there is no evidence for a mechanism in which pentacoordinate silicon is attacked to give a hexacoordinate intermediate or transition state.
机译：报道了探索亲核取代的模型系统，例如一个五坐标的硅中心。通过动态NMR光谱研究了手性五配位硅化合物4中游离N-甲基咪唑（NMI）与配位NMI的简并亲核交换。在4个中，轴向配体是A-甲基咪唑和酰胺基的分子内配位且螯合的氧原子。赤道配体是N-CH2，Ph和Me。从数据中已经区分出两种亲核取代Lit硅的竞争机制。一种机制是保留了在硅处的构型，并且在添加的亲核试剂中为零级。这被解释为涉及到N-甲基咪唑的速率限制解离，然后是另一个NMI分子的快速重新缔合。另一机制发生在硅的构型的反转和保留中，但是在添加的亲核试剂中也是零级的。在该反应中，在速率确定步骤中失去O-Si配位的机制与数据一致。在这个特定的例子中，没有证据表明五配位硅被攻击以产生六配位的中间或过渡态的机理。

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