Cyclophanes, LX. A cyclobutadiene intermediate in the intramolecular cycloaddition of 4,15-bis(phenylethynyl)[2.2]paracyclophane

Abstract

The pseudo-geminally substituted paracyclophane 4,5-bis(phenylethynyl)[2.2]paracyclophane (13) was prepared and its photochemical behaviour in solution was studied. Irradiation with a medium-pressure mercury lamp converts 13 into two dimers: cyclooctatetraene 14 and bicyclo[4.2.0]octatriene derivative 15. The structures of both the substrate and the photolysis products were determined by the usual spectroscopic methods and also by X-ray structural analysis; an unusually long C-C bond of 1.659 angstrom opposite the double bond in the cyclobutene ring of 15 was found. It is suggested that product formation involves cyclobutadiene 16 as an intermediate. Photolysis of 13 in the presence of pentacarbonyliron furnishes cyclopentadienone complex 19 in quantitative yield. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).