Kinetics of Cyclopropyl Radical Reactions. 3. Study of Some 1-Substituted Cyclopropyl Radicals by EPR Spectroscopy. The Inversion Barrier for 1-Methylcyclopropyl

Article abstract of DOI:10.1021/ja00250a032

The 1-methyl-, 1-ethoxy-, and 1-chlorocyclopropyl radicals have been observed by low-temperature EPR spectroscopy in "frozen" configurations in which the ring hydrogens that are syn and anti to the unpaired elecron's orbital have different hyperfine splittings.The a^H(syn)/a^H(anti) ratios are 1.5 (CH3), 1.8 (EtO), and 1.9 (Cl), all considerably lower than the ratio of ca. 3.3 found by Kawamura et al. for methyl-substituted 1-fluorocyclopropyl radicals.The out-of-plane angles of the 1-substituent have been calculated from measured a^13Cα values to be 22.5 deg (cyclopropyl), 22.9 deg (CH3), 29.1 deg (EtO), and 5.8 deg (MeSi).These angles are considerably smaller than those that have been calculated for some of these radicals by ab initio and other methods.Variable-temperature EPR spectroscopy on 1-methylcyclopropyl yields the following Arrhenius equation for its inversion: log (k_inv/s^-1) = (13.1 +/- 0.3) - (3.1 +/- 0.2)/2.3RT kcal/mol.For 1-ethoxycyclopropyl the rate constant for rotation about the ^.C-OEt bond can be represented by log (k_rot/s^-1) = (12.5 +/- 0.2) - (5.8 +/- 0.2)/2.3RT.The barrier to inversion of this radical is >/= 9 kcal/mol.The 1-chlorocyclopropyl radical could only be observed at very low temperatures.