Clocking Tertiary Cyclopropylcarbinyl Radical Rearrangements

Three independent methods have been employed to estimate the rate constant, k(1), for ring-opening of the 2-cyclopropyl-2-propyl radical, 1, at room temperature. These three estimates are based on chemical trapping of 1 and the ring-opened 4-methylpent-3-ene-1-yl radical by thiophenol (k(1) = (1.6(5) +/- 0.4(1)) x 10(7) M(-1) s(-1)), 9-azabicyclo[3.3.1]nonane-N-oxyl (k(1) = (1.7(6) +/- 0.3(4)) x 10(7) M(-1) s(-1)) and 2,2,6,6-tetramethylpiperidine-N-oxyl (k(1) = (2.1 +/- 0.4) x 10(7) M(-1) s(-1)) and absolute rate constants for nonrearranging radicals structurally related to 1. The mean value for k(1) ((1.8(4) +/- 0.4) x 10(7) M(-1) s(-1)) should be used when 1 is employed as a tertiary alkyl free radical clock at ambient temperatures.