The results suggest that electron addition is followed by protonation and hence that the anion of HCCSiMe3 is strongly basic. The relative concentration of the latter increased markedly on annealing, presumably via proton transfer. Spectral data have been obtained for various radicals (16), (19), (23), (24) formed by intermolecular addition.Įxposure of dilute solutions of Me3SiCCH in CD3OD to 60Co γ-rays at 77 K gave mainly H(D)CĊSiMe3 radicals together with traces of H2CCSiMe3. The radical (22) formed from allyl bromoacetate does not undergo ring closure. Similarly, 1,5-ring closure of the ester derived radical (13) is slow because of unfavourable conformational effects arising from restricted rotation about the CO-O bond. Spectral evidence has been obtained for restricted rotation about the O-CH2* bond in the 2-oxa radical (9) as a consequence of which its ring closure is relatively slow. The 3-oxa species (6) undergo 1,5-ring closure more rapidly than does hex-5-enyl radical (1) because of favourable stereoelectronic factors. The hex-5-enyl (1), 3-oxahex-5-enyl (6), 2-oxahex-5-enyl (9) and 2,2-dimethylbut-3- enoyloxymethyl (13) radicals have been generated by interaction of the corresponding bromides with trialkyltin or trialkylgermanium radicals, and their rate constants and activation parameters for cyclization have been determined by kinetic e.s.r. All of these data indicate that relief of ring strain is a major contributing factor to the rapid rearrangement of these radicals. UHF/3-21 G level of theory predicts a barrier of 7.8 kcal mol–1, in excellent agreement with that obtained experimentally, while a somewhat higher value (11.2 kcal mol–1) is predicted at the UHF/6-31 G* level. Rearrangement of 9 has an unusually low activation barrier this has also been investigated by ab initio molecular-orbital calculations. Activation energies of 7.1 ± 1.2 kcal mol–1 and 9.3 ± 0.5 kcal mol–1 have been determined for 9 and 10, respectively, while the corresponding pre-exponential factors have been determined to be 12.3 ± 1.5 and 12.0 ± 1.0, respectively. The rearrangement of the bicyclopentan-1-ylmethyl radical (9) and the bicyclohexan-1-yl-methyl radical (10) to the corresponding methylenecycloalkylmethyl radical (11, 12) has been studied kinetically by EPR spectroscopy and, in the case of 10, by tributyltin hydride reduction. This document is part of Subvolume A1 ‘Inorganic Radicals, Metal Complexes and Nonconjugated Carbon Centered Radicals’ of Volume 26 ‘Magnetic Properties of Free Radicals’ of Landolt-Börnstein Group II ‘Molecules and Radicals’.
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