| Popis: |
Suprafacial sigmatropic shift reactions of 5-substituted cyclopentadienes,\ud 3-substituted cyclopropenes, and 7-substituted cycloheptatrienes have been studied\ud computationally at the MP2/6-31+G* level for structures and energetics, and using the\ud ipsocentric method at the CHF/6-31G** level to calculate current-density maps. The\ud hydrogen shifts in cyclopentadienes have a diatropic ring currents indicating aromatic,\ud cyclopentadienide anion character. This result stands in contrast to the fluorine shift in 5-\ud fluorocyclopentadiene which requires much more energy, and has a paratropic ring\ud current in the TS pointing to antiaromatic, cyclopentadienyl cation character. [1,3]\ud hydrogen shifts in cyclopropenes are very difficult, passing through transition states that\ud have an extended C-C bond. For 3-fluorocyclopropene the [1,3] fluorine shift is much\ud easier than the hydrogen shift. For 7-fluorocycloheptatriene the [1,7] hydrogen shift is\ud predicted, but requires very high energy and has a paratropic ring current and\ud antiaromatic character. The [1,7] suprafacial fluorine shift is relatively easy, having a TS\ud with cycloheptatrienyl cation character. Patterns of currents, and the reversal for H and F\ud migration, are rationalized by orbital analysis based on the ipsocentric method.\ud Calculated charges and structural features for reactants and transition states support these\ud conclusions. |
