Carbon Skeletal Rearrangements Via Pyramidal Carbocations

Antiaromaticity in Open-Shell Cyclopropenyl to Cycloheptatrienyl Cations, Anions, Free Radicals, and Radical Ions

“…In 1925, Ziegler and Schnell 17a reacted pentaphenylcyclopentadienol ( 29 ) with concentrated H 2 SO 4 and upon hydrolysis obtained products derived from the cation 30 (eq 17). Later studies 17b,c showed the cation 30 is long-lived below −40 °C, and this was characterized by UV and ESR and shown to have a low-lying excited triplet state 17c. The pentachlorocyclopentadienyl cation 31 was shown to be a ground-state triplet 17c.…”

“…Later studies 17b,c showed the cation 30 is long-lived below −40 °C, and this was characterized by UV and ESR and shown to have a low-lying excited triplet state 17c. The pentachlorocyclopentadienyl cation 31 was shown to be a ground-state triplet 17c. The study of such ground-state triplets has been pursued because of possible applications as ferromagnets 17d…”

Computational Study of the Thermochemistry of C5H5+ Isomers: Which C5H5+ Isomer Is the Most Stable?

“…We also calculated the pyramidal isomer 4 , which is of interest as a consequence of its three-dimensional aromaticity . This structure, a number of derivatives of which were synthesized, has been suggested to be an intermediate in carbon atom scrambling in C 5 H 5 + prior to acetylene loss …”

Solvolytic Generation of Antiaromatic Cyclopentadienyl Cations

“…The cyclopentadienyl cation ( 1 ) is the prototypical carbocation destabilized by 4π-electron antiaromaticity, and has long been the subject of study. , These investigations include the electrochemical determination of the p K R + as −40 or lower, which is 20 units lower than those for representative conjugated cations,2c observation of C 5 H 5 + as a triplet with the D 5 h structure 1 in a matrix of SbF 5 at 78 K,2d and the failure of cyclopentadienyl iodide to form C 5 H 5 + solvolytically even when treated with silver perchlorate in propionic acid at −15 °C (eq 1) 2b2e …”

“…Carbon scrambling of C 5 H 5 + in the gas phase is suggested to involve pyramidal ions 3 ,2f although the singlet ion is calculated 3c to prefer C 2 v ethylene-type structures 4 which interconvert with an extemely low 0.09 kcal/mol barrier through C 2 v allylic-type structures 5 . The calculated 3d magnetic susceptibility exaltation and the homodesmotic destabilization of 4 of 56.7 kcal/mol for the reaction of eq 2 both indicate major antiaromaticity effects.…”