Transition Structures of the Electrocyclic Reactions of cis,cis,cis‐1,3,5‐Cyclooctatriene

The electrocyclic reactions of cis,cis,cis‐1,3,5‐cyclooctatriene have been studied using ab initio molecular orbital theory. cis,cis,cis‐1,3,5‐cyclooctatriene can undergo an electrocyclic ring opening in a conrotatory fashion to form cis,cis‐1,3,5,7‐octatetraene and a disrotatory electrocyclization to form bicyclo[4.2.0]octa‐2,4‐diene. The transition structures for these electrocyclic reactions have been located. Geometry optimizations employed restricted Hartree‐Fock calculations and the 3–21G and 6–31G* basis sets. Electron correlation energies were calculated using second‐order, and in some cases fourth‐order, Møller‐Plesset theory. Scaled RHF/6–31G* force constants were employed in the prediction of secondary deuterium isotope effects for the conrotatory ring opening. The ground state of cis,cis,cis‐1,3,5‐cyclooctatriene exists in a twist‐boat conformation with staggering at the saturated linkage. The transition structure for the conrotatory electrocyclic ring opening to form cis,cis‐1,3,5,7‐octatetraene has a helical structure, which has implications for the stereoselectivities of ring closure of 1‐substituted‐cis,cis‐1,3,5,7‐octatetraenes. The disrotatory transition structure for the electrocyclization to form bicyclo[4.2.0]octa‐2,4‐diene is strongly distorted from Cs symmetry, in contrast to the transition structure for the disrotatory electrocyclization of cis‐1,3,5‐hexatriene. This distortion is caused by staggering about the saturated linkage.