Organic Photochemistry with 6.7-eV Photons: 1,4-Cyclohexadiene and 1,4-Cyclohexadiene-3,3,6,6-d4

Article abstract of DOI:10.1021/ja00414a043

The photolysis of 1,4-cyclohexadiene has been investigated in solution at 185 and 214 nm, as well as by sensitization by benzene in solution at 254 nm or by mercury (³P₁) atoms in the vapor phase.The reaction mechanisms under these conditions have been probed by the use of 1,4-cyclohexadiene-3,3,6,6-d₄ (4).Both at 185 and 214 nm, the principal products are bicyclo<3.1.0>hex-2-ene (3), 1,3-cyclohexadiene, 1,3,5-hexatriene, and benzene.NMR analysis of the products formed in the photolysis of 4 at 185 nm shows that the formation of 3 is exclusively by a 1,2-hydrogen migration.The other two decomposition pathways at this wavelength correspond to 1,3- and 1,4-H migration.The relative importance of these reactions is sensitive to the wavelength in the range 185-214 nm.Triplet sensitization of 1,4-cyclohexadiene gives benzene (+H2) as a major product and 3 as a minor product.Deuterium labeling shows that in this instance 3 is formed by a di-?-methane rearrangement.Ab initio calculations on the ground state of 1,4-cyclohexadiene employing a STO-3G basis yield a dihedral angle of about 140<*> with a relatively flat minimum.The intermediate neglect of differential overlap (INDO-SCF-CI) method was used to calculate the energies of the low-lying singlet and triplet states in 1,4-cyclohexadiene for several values of the dihedral angle.At the experimental dihedral angle of 160<*>, it was computed that there were two triplet states, ³A₂ (3.7 eV) and ³B₂ (3.8 eV), and three closely spaced singlet states, ¹A₂ (5.9 eV), ¹B₂ (6.3 eV), and ¹B₂ (6.4 eV).This is in reasonably good agreement with recent electron-impact data.The ³B₂ state is deduced to undergo the di-?-methane rearrangement under triplet sensitization while all of the hydrogen migration reactions that are observed on direct irradiation are attributed to the higher ¹B₂ state.This state is derived from the excitation of an electron to the ?₁^* + ?₂^* orbital and contains significant C-H antibonding character.