| Research Abstract |
Bimolecular nucleophilic substitution (SN2) reaction at saturated carbon such as hydrolysis of methyl halides is one of the most important and the most popular reactions in organic chemistry. The mechanism of the reaction invoking inversion of configuration of the central carbon is one of the fundamental ideas of organic reactions and is described commonly in textbooks for undergraduate students. The structure of the transition state (TS) of SN2 should be trigonal bipyramid (TBP) around the central carbon. Hence the bonding about the carbon involves, at least formally, expansion of the valence shell and is called hypervalent. 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. In this research, we succeeded in synthesis and the crystal structure of 1,8-dimethoxy-9-dimethoxymethylanthracene monocation as the fi
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rst fully characterized hypervalent 10-C-5 compound by use of newly prepared anthracene ligands.
1. Preparation of anthracence tridentate ligands having donor atoms (O, N, and P) at 1,8-positions.
2 Synthesis and structure of first hypervalent pentacoordinate carbon compounds. The newly prepared ligand was used for introduction of a carbon atom at 9 position mediated by Pd(0) with CO gas. The obtained ester was treated with trimethyloxonium tetrafluoroborate (Me3O+BF4-) to give 1,8-dimethoxy-9-dimethoxymethylanthracene monocation. The crystal structure of the cation, in the form of heptafluorodiborate salt, shows the symmetrical nature of the compound with the almost identical two O--C distances (2.43(1) and 2.45(1) Å). The geometry around the central carbon atom is trigonal bipyramid (TBP). Comparison of the crystal structure of the cation with those of several newly prepared anthracene compounds provides evidence for the attractive interaction between the central carbon atom and the two methoxy oxygens at 1 and 8 positions. The density functional calculation on the cation indicates that the symmetrical Cs structure is the energy minimum. It should be noted that the bond paths are found between the central carbon atom and the two oxygen atoms, clearly showing that these atoms are bonded even if the bond is weak and ionic. In conclusion, the monocation should be regarded as a hypervalent carbon compound for a model of SN2 transition state.
3. Synthesis and structure of hypervalent pentacoordinate boron compounds. The reaction of 1,8-dimethoxy-9-lithioanthracene with B-chlorocatecholateborane gave 1,8-dimethoxy-9-B-catecholateborylanthracene and the crystal structure shows the symmetrical nature of the compound with the almost identical two O--B distances. Several derivatives could be prepared and the fluorine substituted boron compounds showed a tight 5-coordinate structure. Less
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