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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

Theoretical study on chlorine and hydrogen shift in cycloheptatriene and cyclopentadiene derivatives.

The transition structures (TSs) for chlorine 1,7-shift and 1,5-shift in 1,7,7-trichlorocycloheptatriene (1) and those of chlorine 1,5-shifts in 1,5,5-trichlorocyclopentadiene (3) and 1,2,5-trichloro-1,3-pentadiene (5) derivatives have been located with density functional theory (DFT) at the Becke3LYP/6-311G [and Becke3LYP/6-311+G] level. The calculational results were compared with those for corresponding hydrogen shifts in nonsubstituted molecules (cycloheptatriene (2), cyclopentadiene (4), and 1,3-pentadiene (6)). The following points were clarified: (1) The activation energy (Delta E(++)) for chlorine 1,7-shift in 1 was evaluated to be only +50.1 [+49.2] kJ/ mol, which is smaller than that (+69.9 [+68.3]) for a 1,5-shift, supporting the theory that the conversion between two equivalent A and A' proceeds through a TS for direct chlorine 1,7-shift (Figure 1), rather than through a TS for a 1,5-shift (Figure 2). (2) The considerable amount of charge separation between a migrating chlorine atom (Cl(m)) and a seven-membered ring (-0.53 and +0.47 for Merz-Singh-Kollman scheme) occurs in a chlorine 1,7-shift, which is in good contrast to the result that the migrating hydrogen atom (H(m)) for a 1,7-shift in cycloheptatriene (2) carries almost no charge (Figure 3). This large charge separation can stabilize the TS for the chlorine 1,7-shift pathway. (3) The Delta E(++) values for suprafacial hydrogen 1,7-shift in 2 are quite large (+288.0 [+284.8] kJ/ mol), much larger than that (+166.8 [+167.0]) for a 1,5-shift in 4 which is orbital symmetrically allowed (Figure 3). The calculation suggests that the chlorine 1,7-shift in 1 occurs easily at room temperature (actually observed experimentally) by proceeding via concerted suprafacial 1,7-shift through the zwitterionic TS with the significant assistance of Coulomb interaction between charged fragments (negatively charged chlorine atom and positively charged tropylium ring), rather than via a suprafacial 1,5-sigmatropic pathway. Other cases studied in this paper showed usual results predicted by orbital symmetrical consideration.[1]


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