Kinetics of nitroxide radical trapping. 2. Structural effects

Article abstract of DOI:10.1021/ja00039a006

Laser flash photolysis and kinetic competition product demonstrated that in isooctane at ambient temperature the rate constant for coupling of carbon-centered with presistent nitroxides, k_T, upon the of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of k_T are observed for changes in both the structure of the nitroxide the structure of the carbon radical. Thus, for any particular carbon k_T is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the "usual" di-tert-alkyl nitroxides k_T decreases along the series, 1,1,3,3-tetra-methylisoindoline-2-oxyl ≥ 2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo) > di-tert-butyl nitroxide, i.e., k_T decreased on going from a five-membered ring to a six-membered ring to a noncyclic structure. Cyclopropyl triphenylmethyl are trapped at the fastest and slowest rates, respectively, the corresponding k_T value 3.0 × 10⁹ and 1.2 × 10⁸ M^-1 s^-1 for ABNO and 2.1 × 10⁹ <1 × 10⁶ M^-1 s^-1 for Tempo. Steric effects in the carbon radicals are for Tempo than for ABNO. For example, the ratio of k_T's for the trapping of nonyl tert-butyl is 1.7 for Tempo but 1.3 for ABNO, while for the trapping of benzyl cumyl the ratio of k_T's is 4.1 for Tempo 0.9 for ABNO. The effect of resonance stabilization can be illustrated by the k_T values for three sterically unhindered primary radicals, n-nonyl, benzyl, 2-naphthyhmethyl, viz., 1.2 × 10⁹, 4.8 × 10⁸, and 5.7 × 10⁷ M^-1 s^-1, respectively, for Tempo and 2.2 × 10⁹, 1.2 × 10⁹, and 8.1 × 10⁸ M^-1 s^-1, respectively, for ABNO.