Transition structures, energetics, and nucleus-independent chemical shifts for 6 pi electrocyclizations of dienylketenes to cyclohexadienones: A DFT study


Zora M.

JOURNAL OF ORGANIC CHEMISTRY, vol.69, no.6, pp.1940-1947, 2004 (Journal Indexed in SCI) identifier identifier identifier

  • Publication Type: Article / Review
  • Volume: 69 Issue: 6
  • Publication Date: 2004
  • Doi Number: 10.1021/jo035548d
  • Title of Journal : JOURNAL OF ORGANIC CHEMISTRY
  • Page Numbers: pp.1940-1947

Abstract

6pi electrocyclizations of dienylketenes to 2,4-cyclohexadienones have been investigated at the (U)B3LYP/6-31G* level and found to be a favored and exothermic process for most dienylketenes. As evidenced by calculations, dienylketene cyclizations proceed via a pseudopericyclic process. If the terminal double bond of dienylketenes is embedded into a benzenoid-type aryl moiety, the partial or complete loss of aromaticity, as indicated by NICS values, increases the activation barrier and makes the reaction less exothermic or even endothermic. The effect of aromaticity is slightly less pronounced for dienylketenes carrying five-membered heterocyclic aromatic substituents. Slightly distorted planar transition structures have been located for these types of cyclizations. Forming bond lengths in transition structures range from 1.950 to 2.339 Angstrom.