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 C_s 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.