Preparation and ESR-Spectroscopical Investigation of Remarkably Persistent Oxoisobenzofuranyl Radicals.

The synthesis of the diastereoisomeric 1,1′-diaryl-1,1′-bi(isobenzofuran)-3,3′(1H,1′H)-diones 3a–d starting from the readily available 2-aroylbenzoic acids 1a–d is described (Scheme 1). Of the colourless dimers 3a–d, only the sterically congested 3a and 3b dissociate at ambient temperature in solution to the deep red free 3-oxoisobenzofuran-1-yl radicals 4a and 4b, respectively. The radicals 4a, b are extremely persistent in the absence of O2. The structures of these radicals are confirmed and the coupling constants assigned by ESR and ENDOR spectroscopy and computer simulation of their ESR spectra. The dissociation equilibrium constant at 20° in toluene for 3a is determined to be 1.18 · 10−5M. By studying the steady-state radical concentration as a function of temperature, the enthalpy and entropy changes for the homolytic dissociation of 3a are determined.