[1.1]PARACYCLOPHANE - PHOTOCHEMICAL GENERATION FROM THE CORRESPONDINGBIS(DEWAR BENZENE) DERIVATIVE AND THEORETICAL-STUDY OF ITS STRUCTURE AND STRAIN-ENERGY

The first generation of [1.1]paracyclophane (1a) and its bis(methoxyca rbonyl) derivative (1b) from the corresponding bis(Dewar benzene) prec ursors, 3a and 3b, has been investigated. Irradiation of 3a in a glass y mixture of ether-isopentane-ethanol at 77 K leads to the formation o f species exhibiting absorption extending to 450 nm, which readily und ergoes secondary photolysis to give an isomer showing lambda(max) at 2 44 nm. On the basis of these UV/vis spectral observations and the acco mpanying H-1 NMR measurement, the structures of la and the correspondi ng transannular [4 + 4] adduct (21a) are assigned to the initial and t he secondary products, respectively. Compound 3b undergoes similar suc cessive phototransformation into 1b and 21b. [1.1]Paracyclophanes, 1a and 1b, and their photoisomers, 21a and 21b, are sufficiently stable t o permit the measurement of H-1 NMR spectra at low temperature, but ar e consumed fairly rapidly in solution at ambient temperature, defying their isolation. The results of geometrical optimization of 1a underta ken at the RHF-SCF, MP2, and B3LYP levels employing the 6-31G basis s et are also presented. Calculations indicate that 1a is a highly strai ned molecule, but more stable than the related isomers, 3a and 21a: th e strain energy calculated for 1a is 128.1, 106.5, and 93.6 kcal/mol a t the RHF/6-31G, B3LYP/6-31G*, and MP2/6-31G* levels, respectively. T he closest nonbonding interatomic distance between the aromatic rings in 1a is in a range of 2.36-2.40 Angstrom, and the degree of bending o f the benzene rings is comparable to that in [5]paracyclophane, much l ess as compared to that in [4]paracyclophane. Calculations also suppor t strong transannular electronic interactions between the pi-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 a nd 3b from diethyl 3,6-dihydroterephthalate in 15 and 11 steps, respec tively, is described.