Transition structures, energetics, and nucleus-independent chemical shifts for divinylcyclobutene-to-cyclooctatriene rearrangement: a DFT study

J Org Chem. 2004 Feb 6;69(3):857-62. doi: 10.1021/jo0350209.


The minimum energy reaction paths and nucleus-independent chemical shifts (NICS) for the Cope rearrangement of cis-3,4-divinylcyclobutene, obtained by (U)B3LYP/6-31G calculations, are reported. Three transition structures (endo-boatlike, chairlike, and exo-boatlike) have been located, giving rise to formation of cis,cis,cis-, cis,cis,trans-, and trans,cis,trans-1,3,5-cyclooctatrienes, respectively. The minimum energy path proceeds through an endo-boatlike, aromatic transition structure. The reaction path of the rearrangement is intervened by enantiomerization saddle point of the product. NICS values calculated for transition structures agree qualitatively with their activation energy and reaction exothermicity orders. Cope rearrangement and electrocyclic ring-opening processes of cis-3,4-divinylcyclobutene are competitive, but the former is relatively more favored and exothermic than the latter.