RADICAL-ION PROBES .3. THE IMPORTANCE OF RESONANCE VS STRAIN-ENERGY IN THE DESIGN OF SET PROBES BASED UPON THE CYCLOPROPYLCARBINYL -] HOMOALLYL RADICAL REARRANGEMENT
Jm. Tanko et al., RADICAL-ION PROBES .3. THE IMPORTANCE OF RESONANCE VS STRAIN-ENERGY IN THE DESIGN OF SET PROBES BASED UPON THE CYCLOPROPYLCARBINYL -] HOMOALLYL RADICAL REARRANGEMENT, Journal of the American Chemical Society, 116(5), 1994, pp. 1785-1791
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
.