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

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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.