A Curtin–Hammett pentamethylene chain symmetrization process in the Bergman cyclization of an 11‐membered ring enediyne

Abstract

Pentamethylene chain conformational effects for the Bergman cyclization of the 11-membered ring enediyne, (3Z)-3cycloundecene-1,5-diyne, 2, are examined theoretically with unrestricted Becke, three-parameter, Lee-Yang-Parr/6-31 G(d,p) calculations. A C 1 symmetric enediyne conformation was found to be the global minimum, where its nonsymmetric pentamethylene chain prevented p-orbital alignment of the acetylene groups for C-C s bond product formation. The Bergman cyclization of 2 was found to be conformationally dependent. In a Curtin-Hammett type process, the C 1 symmetric 2 inverts to one of the C S or C 2 symmetric conformers required for the Bergman cyclization, which produced a C S or C 2 symmetric 1,4-diradical intermediate. The activation energy for the cyclization is slightly higher to reach the C 2 symmetry diradical compared with the C S symmetry diradical. a Enthalpies at 298 K: ΔH 298 = Δ(E 0 + H corr ), E 0 = the total electronic energy, H corr = E tot + k b T, E tot = internal thermal energy, k b = Boltzmann constant. b Energies at 298 K: Δ(E 0 + ZPE).

Butylated hydroxytoluene enediyne: access to diradical and electrophilic quinone methide intermediates

“…B3LYP calculations generally perform well in reproducing energetic values of Bergman cyclizations where experimental values are available. [4,[9][10][11][12] For example, the DFT energetics of the parent enediyne reaction reproduced the experimental value to within 1.5 kcal/mol. [4] Density functional theory has also been successful in calculating energetics of H-atom transfer reactions [13][14][15] such as polyphenols exhibiting antioxidant properties.…”

“…Geometries of diradicals were optimized with uB3LYP/6‐31G(d,p). B3LYP calculations generally perform well in reproducing energetic values of Bergman cyclizations where experimental values are available . For example, the DFT energetics of the parent enediyne reaction reproduced the experimental value to within 1.5 kcal/mol .…”

Concerted Reactions That Produce Diradicals and Zwitterions: Electronic, Steric, Conformational, and Kinetic Control of Cycloaromatization Processes

“…Greer et al described how a conformational change in a pentamethylene tether of 11-membered cycle enediyne is coupled to the Bergman cyclization. The global minimum C 1 geometry for this cyclic enediyne does not directly lead to the cyclization TS but should instead change into one of the higher energy C S or C 2 symmetric conformers before cycloaromatization is possible.…”