PREPARATION AND ESR-SPECTROSCOPICAL INVESTIGATION OF REMARKABLY PERSISTENT OXOISOBENZOFURANYL RADICALS

The synthesis of the diastereoisomeric 1,1'-diaryl-1,1'-bi(isobenzofur an)-3,3'(1 H, 1'H)-diones 3a-d starting from the readily available 2-a roylbenzoic acids 1'' is described (Scheme 1). Of the colourless dimer s 3a-d, only the sterically congested 3a and 3b dissociate at ambient temperature in solution to the deep red free 3-oxoisobenzo-furan-1-yl radicals 4a and 4b, respectively. The radicals 4a, b are extremely per sistent in the absence of O2. The structures of these radicals are con firmed and the coupling constants assigned by ESR and ENDOR spectrosco py and computer simulation of their ESR spectra. The dissociation equi librium constant at 20-degrees in toluene for 3a is determined to be 1 .18 . 10(-5) m. By studying the steady-state radical concentration as a function of temperature, the enthalpy and entropy changes for the ho molytic dissociation of 3a are determined.