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

Abstract

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 using the ipsocentric method at the CHF/6-31G** level to calculate current-density maps. The hydrogen shifts in cyclopentadienes have a diatropic ring currents 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.

Can aromaticity trigger thermal [1,5]‐halogen shift towards the forbidden antarafacial mode?: A density functional case study

“…Further, the [1,5]-shifts in cyclic alkenes like cyclopentadiene, cyclopropene, and cycloheptatriene have also been experimentally and theoretically explored. [21][22][23][24] Not only hydrogen but also halogens are good migrating groups and are known to undergo [1,5]-sigmatropic shifts. 7b, 10,19,[25][26][27][28][29][30][31] To the best of our knowledge, there are no reports that compare the rate of migratory aptitude and stereochemical preference (supra/antara) of halogens during [1,5]-shift in 1,3-pentadiene.…”

“…The [1,5]‐hydrogen shift in 1,3‐pentadiene is found to occur via concerted suprafacial fashion. Further, the [1,5]‐shifts in cyclic alkenes like cyclopentadiene, cyclopropene, and cycloheptatriene have also been experimentally and theoretically explored . Not only hydrogen but also halogens are good migrating groups and are known to undergo [1,5]‐sigmatropic shifts 7b, 10, 19, 25–31.…”

Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes

“…[11] Note that these studies focused on the geminally disubstituted compounds, although for monosubstituted cyclopentadienes, the relative stability, suprafacial shift reactions, and intrinsic acidity have been investigated. [12] However, the influence of substituents on hyperconjugative aromaticity in monosubstituted compounds remains uncertain. In addition, when one electronegative and one electropositive substituent are simultaneously considered, which one will dominate?…”

Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier

“…[9] Recently,F owler and co-workerss howed that electropositive migratory groups( e.g., CH 3 and SiMe 3 )c ould reduce the [1,5]-shift activation energies, whereas the electronegative migratory groups (e.g.,F )l ed to larger activation energies as ar esult of the aromatic and antiaromatic character of the TSs. [10] Owing to many fascinating and ever-increasing manifestations, hyperconjugative aromaticity [11] and [1,5]-shifts [12] in cyclopentadienes have received much attention.I n2 016, the first hyperconjugative aromatic heterocycles containing transition-metal substituents werer eportede xperimentally and computationally. [13] Recently,w e also predicted the most aromatic pyrrolium and cyclopentadiene rings through hyperconjugation [14] caused by transitionmetal substituents.…”

“…In 2010, Yáñez and co‐workers proposed that the activation barriers of the [1,5]‐shift could be regulated by migratory groups (e.g., CH 3 , SiH 3 , and GeH 3 ), owing to hyperconjugative aromaticity . Recently, Fowler and co‐workers showed that electropositive migratory groups (e.g., CH 3 and SiMe 3 ) could reduce the [1,5]‐shift activation energies, whereas the electronegative migratory groups (e.g., F) led to larger activation energies as a result of the aromatic and antiaromatic character of the TSs . Owing to many fascinating and ever‐increasing manifestations, hyperconjugative aromaticity and [1,5]‐shifts in cyclopentadienes have received much attention.…”