Thermal Isomerization of 12‐Oxa‐1,9‐dimetylhexacyclo[7.2.1.0^2,4.0^2,8.0^3,7.0^6,8]dodec‐10‐ene: A Computational Mechanistic Investigation

Abstract

Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday Flash-vacuum pyrolysis of the quadricyclane derivative 5a at 3508 afforded the oxabishomocubane 9a, whose structure was confirmed by an INADEQUATE-NMR experiment. A computational investigation of the mechanism of this unexpected reaction by DFT and CASPT2-SCF methods indicated that the reaction path of lowest energy involves a quadricyclane/oxaquadricyclane (5/12) isomerization, followed by a well-established cycloreversion of 12 to the carbonyl ylide 16, which subsequently undergoes an intramolecular 1,3-dipolar cycloadditon to 9. The lowest-energy path of the thermal isomerization of the syn-quadricyclane 6c is its conversion to the syn-sesquinorbornatriene 8c. The corresponding anti-isomer 5c, however, shows the capability of a degenerate quadricyclane/quadricyclane rearrangement.

Understanding the Electronic Reorganization along the Nonpolar [3 + 2] Cycloaddition Reactions of Carbonyl Ylides.

“…Note that Szeimies et al represented the CY 20 with the Lewis structures II and III (see Scheme 4). 14…”

“…On the other hand, π MO 18 can be related to a resonant structure between II and III , in which one of the lone pairs of the oxygen is delocalized over C2 and C2′ carbon atoms. Note that Szeimies et al represented the CY 20 with the Lewis structures II and III (see Scheme ) …”

“…14 The intramolecular 32CA reaction of the intermediate CY 20 presented also a very low activation energy (4.2 kcal/mol at UB3PW91/6-311G** and 3.6 kcal/mol at CASPT2(8,7)/6-31G*). 14 The CY 20, with a AEs 2 ae=0, was represented as a zwitterionic structure.…”

“…CYs derived from a thermal aperture of oxaquadricyclanes (OQC) may behave as the three-atom component in 32CA reactions. Recently, Szeimies et al have performed an experimental and computational study on the mechanism of the thermal rearrangement of the OQC 19 to give the cage compound 21 (see Scheme ) . The intramolecular 32CA reaction of the intermediate CY 20 presented also a very low activation energy (4.2 kcal/mol at UB3PW91/6-311G** and 3.6 kcal/mol at CASPT2(8,7)/6-31G*) .…”

“…Recently, Szeimies et al have performed an experimental and computational study on the mechanism of the thermal rearrangement of the OQC 19 to give the cage compound 21 (see Scheme ) . The intramolecular 32CA reaction of the intermediate CY 20 presented also a very low activation energy (4.2 kcal/mol at UB3PW91/6-311G** and 3.6 kcal/mol at CASPT2(8,7)/6-31G*) . The CY 20 , with a ⟨ s 2 ⟩ = 0, was represented as a zwitterionic structure.…”