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

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 A.