[1.1]Paracyclophane. Photochemical Generation from the Corresponding Bis(Dewar benzene) Derivative and Theoretical Study of Its Structure and Strain Energy

The first generation of [1.1]paracyclophane (1a) and its bis(methoxycarbonyl) derivative (1b) from the corresponding bis(Dewar benzene) precursors, 3a and 3b, has been investigated. Irradiation of 3a in a glassy mixture of ether−isopentane−ethanol at 77 K leads to the formation of species exhibiting absorption extending to 450 nm, which readily undergoes secondary photolysis to give an isomer showing λmax at 244 nm. On the basis of these UV/vis spectral observations and the accompanying 1H NMR measurement, the structures of 1a and the corresponding transannular [4 + 4] adduct (21a) are assigned to the initial and the secondary products, respectively. Compound 3b undergoes similar successive phototransformation into 1b and 21b. [1.1]Paracyclophanes, 1a and 1b, and their photoisomers, 21a and 21b, are sufficiently stable to permit the measurement of 1H NMR spectra at low temperature, but are consumed fairly rapidly in solution at ambient temperature, defying their isolation. The results of geometrical optimization of 1a undertaken at the RHF-SCF, MP2, and B3LYP levels employing the 6-31G* basis set are also presented. Calculations indicate that 1a is a highly strained molecule, but more stable than the related isomers, 3a and 21a: the strain energy calculated for 1a is 128.1, 106.5, and 93.6 kcal/mol at the RHF/6-31G*, B3LYP/6-31G*, and MP2/6-31G* levels, respectively. The closest nonbonding interatomic distance between the aromatic rings in 1a is in a range of 2.36−2.40 Å, and the degree of bending of the benzene rings is comparable to that in [5]paracyclophane, much less as compared to that in [4]paracyclophane. Calculations also support strong transannular electronic interactions between the π-bonds of the aromatic moieties of 1a, which lead to a significantly diminished HOMO-LUMO gap as compared to that in p-xylene. The preparation of 3a and 3b from diethyl 3,6-dihydroterephthalate in 15 and 11 steps, respectively, is described.