Effect of Ring Size and Migratory Groups on 1,n Suprafacial Shift Reactions. Confirmation of Aromatic and Antiaromatic Transition-State Character by Ring-Current Analysis

摘要

Suprafacial sigmatropic shift reactions of 5-substituted cyclopentadienes,ud3-substituted cyclopropenes, and 7-substituted cycloheptatrienes have been studiedudcomputationally at the MP2/6-31+G* level for structures and energetics, and using theudipsocentric method at the CHF/6-31G** level to calculate current-density maps. Theudhydrogen shifts in cyclopentadienes have a diatropic ring currents indicating aromatic,udcyclopentadienide anion character. This result stands in contrast to the fluorine shift in 5-udfluorocyclopentadiene which requires much more energy, and has a paratropic ringudcurrent in the TS pointing to antiaromatic, cyclopentadienyl cation character. [1,3]udhydrogen shifts in cyclopropenes are very difficult, passing through transition states thatudhave an extended C-C bond. For 3-fluorocyclopropene the [1,3] fluorine shift is muchudeasier than the hydrogen shift. For 7-fluorocycloheptatriene the [1,7] hydrogen shift isudpredicted, but requires very high energy and has a paratropic ring current andudantiaromatic character. The [1,7] suprafacial fluorine shift is relatively easy, having a TSudwith cycloheptatrienyl cation character. Patterns of currents, and the reversal for H and Fudmigration, are rationalized by orbital analysis based on the ipsocentric method.udCalculated charges and structural features for reactants and transition states support theseudconclusions.