RADICAL-ION PROBES .3. THE IMPORTANCE OF RESONANCE VS STRAIN-ENERGY IN THE DESIGN OF SET PROBES BASED UPON THE CYCLOPROPYLCARBINYL -] HOMOALLYL RADICAL REARRANGEMENT

Aryl cyclopropyl ketones have frequently been used as probes for singl e-electron transfer in organic reactions. The implicit assumption in t hese studies is that upon one-electron reduction, the corresponding ar ylcyclopropylketyl anions will undergo ring opening in analogy to the cyclopropylcarbinyl free radical. Earlier work in our laboratory has s hown that ring opening of arylcyclopropylketyl anions is kinetically a nd thermodynamically unfavorable when the only substituents on the cyc lopropane ring are alkyl or H. This sluggishness is attributable to a loss of resonance energy upon ring opening which is not fully compensa ted by the relief of cyclopropane ring strain. Consequently, aryl cycl opropyl ketones are unsuitable probes for single-electron transfer. In this paper, we examine the effect of radical-stabilizing substituents (phenyl and vinyl) on the ring opening of arylcyclopropylketyl anions . We have studied the mechanism, kinetics, and products of decay of ra dical anions generated electrochemically from trans-1-benzoyl-2-phenyl cyclopropane and 1-benzoyl-2-vinylcyclopropane. The radical anions gen erated from these precursors each undergo ring opening(k approximate t o 10(6-7) s(-1)) which is estimated to be exothermic by ca. 2 kcal/mol .