Organic Photochemistry with 6.7-eV Photons: 1,4-Cyclohexadiene and 1,4-Cyclohexadiene-3,3,6,6-d4
Abstract
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 l,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-π-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-π-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 σ1* + σ2* orbital and contains significant C-H antibonding character. © 1981, American Chemical Society. All rights reserved.