Round-Trip Radical Probes: Ring Cleavage of the Bicyclo[1.1.1 ]pentylcarbinyl Radical

The novel concept of a round-trip probe for radical intermediates in reaction mechanisms is proposed and defined in this paper. A round-trip radical probe undergoes skeletal rearrangements such that the radical is returned to its site of origin. These probes should be especially useful for the study of enzyme mechanisms, since the special requirements of the active site may lead to ambiguous results using standard nonround-trip radical probes. The ring cleavages and rearrangements of the bicyclo[1.1.1]pentylcarbinyl radical are described as the prototypical round-trip radical probe. We have measured the rate constant for the ring opening of the bicyclo[1.1.1]pentylcarbinyl radical over the temperature range −42 to 60 °C and have determined a temperature-dependent function for the ring opening of log (k₃/s⁻¹) = 12.78 (±0.26) – 7.79 (±0.35)/θ and a rate constant for ring opening of 1.15 × 10⁷ s⁻¹ at 25 °C.