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

Abstract

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 optim...