Vertikale oder nicht‐vertikale Stabilisierung im Pentacyclo[4.3.0.0^2,4.0^3,8.0^5,7]non‐9‐yl‐Kation? Das 9‐Methylenpentacyclononan‐Radikalkation

Abstract

Durch eine photoelektronenspektroskopische Untersuchung des Radikalkations des Methylenhomocuneans 8 in Verbindung mit den sterisch fixierten Bicyclopropylen 5, 6, 7 und 9 wird gezeigt, daB die vertikale Stabilisierung in den Radikalionen von 5 und 8 von vergleichbarer GroBenordnung ist (p z -0.3 eV). Der Unterschied in den Solvolysebeschleunigungen 17/18 (Faktor 20) und 3/1 (Faktor 10") kann deshalb nicht auf vertikale Stabilisierungseffekte zuriickgefiihrt werden, sondern demonstriert eindringlich die homoaromatische Stabilisierung in einem verzerrten, iiberbriickten ubergangszustand auf der Reaktionskoordinate zwischen 3 und 4.Vertical or Nonvertical Stabilization in the 9-Pentacycl0~4.3.0.0~ *'.@*8.@.']nonyl Cation?The 9-Methylenepentacyclononane Radical Cation') A photoelectron spectroscopic investigation of the radical cation of methylenehomocuneane 8, together with the sterically rigid bicyclopropyls 5, 6, 7 and 9, reveals a vertical stabilization of comparable order (B % -0.3 eV) in the radical cations of 5 and 8. The differences in solvolysis rate enhancements of 17/18 (a factor of 20) and 3/1 (a factor of lo'*) therefore cannot he accounted for by vertical Stabilization effects but demonstrate impressively the homoaromatic stabilization in a distorted bridged transition state on the reaction coordinate between 3 and 4.Die anchimere Beschleunigung der Solvolyse von 7-Norbornylderivaten durch endoanti-standige Dreiringe ist gut belegt. Dabei wurde die Beteiligung des Dreirings mit der Ausbildung stabiler Trishomocyclopropenylionen 4 (oder rasch aquilibrierender Aquivalente) erklart ').

Photoelectron Spectra of Cyclopropane and Cyclopropene Compounds

“…Anchimeric assistance of solvoltytic reactions of 7-norbornyl derivatives by three-membered rings in endo−anti positions was analyzed by Martin et al The function of the three-membered rings is explained by formation of trishomocyclopropyl carbocations. The authors studied the PE spectrum of methylenehomocuneane ( 173 ; Table , entry 43; see also ref ) and compared its first IP with those of related compounds.…”

Orbital-Controlled Stereoselections in Sterically Unbiased Cyclic Systems

“…A cyclopropyl group is known to act as a strong π donor due to a high-lying occupied Walsh orbital, which is frequently regarded as an equivalent of a double bond. ,− Photoelectron spectra of the olefins 40a and 41a were previously measured and the signals assigned. ,− Vertical ionization potentials of the olefin π orbitals of 40a and 41a were found to be higher than those of the unsubstituted parent bicyclo[2.2.2]octene 35a (R 1 = R 2 = H) and 7-methylenenorbornane 28a (R 1 = R 2 = H), respectively, indicating a sizable interaction of the π orbital of the double bond with the occupied Walsh orbital of the fused cyclopropane ring. The previous account of the observed anti facial preference of 41a was based on this interaction, in particular, out-of-phase interaction of the relevant orbitals ( 4.36 ). − However, the corresponding out-of-phase interaction of the olefinic π orbital with the Walsh orbital of the cyclopropyl group ( 4.35 ) does not seem to be relevant to 40a because of the observed reverse syn preference.…”

“…154,156 A cyclopropyl group is known to act as a strong π donor due to a high-lying occupied Walsh orbital, 158 which is frequently regarded as an equivalent of a double bond. 154,[159][160][161][162][163][164][165][166] Photoelectron spectra of the olefins 40a and 41a were previously measured and the signals assigned. 154,[163][164][165][166] Vertical ionization potentials of the olefin π orbitals of 40a and 41a were found to be higher than those of the unsubstituted parent bicyclo[2.2.2]octene 35a (R 1 ) R 2 ) H) and 7-methylenenorbornane 28a (R 1 ) R 2 ) H), respectively, indicating a sizable interaction of the π orbital of the double bond with the occupied Walsh orbital of the fused cyclopropane ring.…”

“…154,[159][160][161][162][163][164][165][166] Photoelectron spectra of the olefins 40a and 41a were previously measured and the signals assigned. 154,[163][164][165][166] Vertical ionization potentials of the olefin π orbitals of 40a and 41a were found to be higher than those of the unsubstituted parent bicyclo[2.2.2]octene 35a (R 1 ) R 2 ) H) and 7-methylenenorbornane 28a (R 1 ) R 2 ) H), respectively, indicating a sizable interaction of the π orbital of the double bond with the occupied Walsh orbital of the fused cyclopropane ring. The previous account of the observed anti facial preference of 41a was based on this interaction, in particular, out-of-phase interaction of the relevant orbitals (4.36).…”