Creation of Hypervalent Carbon Species : Study on Bond Formation and Interaction

五配位高价碳是不稳定的，被认为是SN2反应的过渡态。由于SN2的重要性，人们一直在努力稳定TS，甚至制备TS的模型化合物。然而，五配位碳化合物的x射线结构从未被报道过，直到我们最近报道了1,8-二甲氧基-9-二甲氧基甲基蒽单位与新制备的蒽配体。然而，中心碳原子和供体氧原子之间的相互作用相当小。因此，在本研究中，我们设计了一种新型的三叉戟配体，该配体带有一个苯环，由两个七元环缩合而成，希望中心原子与配位硫原子之间的相互作用更强。经过多次试验，2-溴-间二甲苯经8步合成了配体前驱体，总收率为2.4%，并可与n-BuLi锂化。锂衍生物与各种亲电试剂反应生成碳和硼化合物。利用三甲基阳离子可以合成五配位碳化合物。虽然碳阳离子的结构取决于苯取代基对取代基(X)的吸电子能力，但五配位碳可以通过X射线分析确定。中心碳原子的13C-NMR化学位移显示出高场位移（约20ppm）。结果表明，中心碳与供体硫原子之间存在较强的相互作用。

Pentacoordinate hypervalent carbon species are unstable and regarded as a transition state (TS) of SN2 reaction. Due to the fundamental importance of SN2, there have been a variety of efforts to stabilize the TS and even to prepare model compounds of TS. However, X-ray structures of pentacoordinated carbon compounds have never been reported until our recent reports on 1,8-dimethoxy-9-dimethoxymethylanthracene monocation bearing newly prepared anthracene ligands. However, the interaction between the central carbon and the donating oxygen atom is rathex small. Therefore, in this research we designed a novel tridentate ligand bearing a benzene ring condensed with two seven membered rings in the hope of stronger interaction between the central atom and the coordinating sulfur atoms. After several trials, the ligand precursor could be prepared from 2-bromo-m-xylene via 8 steps (total yield 2.4%), and could be lithiated with n-BuLi. The lithium derivative reacted with various electrophiles to give carbon and boron compounds. Synthesis of pentacoodinated carbon compounds could be carried out by use of trityl cation. Although the structure of the carbocation was dependent on the electron-withdrawing ability of the para-substituent (X) of the benzene substituents, the pentacoordinated carbon species could be confirmed by X-ray analysis. The 13C-NMR chemical shift of the central carbon atom showed high-field shift (ca. 20ppm). The results indicated the stronger interaction between the central carbon and the donating sulfur atom.