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