The synthesis of the diastereoisomeric l,l'-diaryl-l,l'-bi(isobenzofuran)-3,3'( IH,l'H)-diones 3a-d starting from the readily available 2-aroylbenzoic acids l a 4 is described (Scheme I ) . Of the colourless dimers 3 a 4 , 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 0,. 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' M. 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.
Introduction. -Diastereoisomeric meso-and rac-1, 1'-diaryl-1, 1'-bi(isobenz0furan)-3,3'(1H,l'H)-diones of type 3 can be prepared by the action of a variety of reducing agents [I] on 2-aroylbenzoic acids. Another method for their synthesis is the action of alkali iodide on 3-aryl-3-chloroisobenzofuran-1(3H)-ones as shown by Bhatt et al. [2] in 1971. A mechanism for this dimerisation which involves the formation of the intermediate oxoisobenzofuranyl radicals was postulated by the same authors, based on a subsequent kinetics study [3]; the presence of free radicals in this reaction was proven by trapping them as adducts with acrylonitrile. Free oxoisobenzofuranyl radicals were also proposed as intermediates in the photolysis of tertiary 2-benzoylbenzamides [4] which afforded 1 ,l'-diaryl-l,l'-bi(isobenzofuran)-3,3'( lH, 1'H)-diones 3. The reversible thermal homolytic cleavage of 1,l '-diaryl-1,l'-bi(isobenzofuran)-3,3'( lH, 1'H)-diones into oxoisobenzofuranyl radicals would, on the other hand, explain the well known [5] thermal interconversion of their pure diastereoisomers during their recrystallisation from higher-boiling solvents.
We now report the preparation of the sterically highly strained 1,l'-diaryl-1,l'bi(isobenzofuran)-3,3'( 1 H,l'H)-diones 3a and 3b and the ESR spectroscopic investigation of remarkably persistent oxoisobenzofuranyl radicals 4a, b which in solution are in equilibrium with the former, even at room temperature.
Results. -1. Synthesis and Properties of 3 a 4 . The 1,l'-diaryl-1,l'-bi(isobenz0furan)-3,3'( 1H, 1'H)-diones 3a-d were prepared from the readily available 2-aroylbenzoic acids