Substituent effects upon the kinetics of hydrogen transfer from triorganotin hydrides to the 5-hexen-1-yl radical

Article abstract of DOI:10.1021/om00100a021

Steric and electronic substituent effects were probed for the hydrogen atom transfer reaction of a series of triorganotin hydrides with the 5-hexen-1-yl radical. Rate data were obtained for tributyltin hydride (1), triisopropyltin hydride (2), tri-tert-butyltin hydride (3), trineopentyltin hydride (4), trimesityltin hydride (5), and dibutylethoxytin hydride (6). Arrhenius parameters are reported for the reactions of 1-4 and 6; the reaction of 5 was studied only at 50°C. For compounds 1-4, the activation energy, E_a, decreased monotonically with increasing alkyl size from 3.9 ± 0.1 to 3.1 ± 0.1 kcal mol^-1. The preexponential factor, expressed as log A, decreased monotonically from 9.4 ± 0.1 to 8.8 ± 0.1. The effect of the ethoxy substituent, 6, relative to an n-butyl substituent, 1, upon the activation energy was negligible (4.2 ± 0.2 versus 3.9 ± 0.1 kcal mol^-1, respectively), but the preexponential factor, log A, increased from 9.4 ± 0.1 to 10.0 ± 0.1. The activation data for 1-4 are interpreted in terms of the steric requirements of the alkyl groups appended to tin. The lack of a significant substituent effect upon E_a by the ethoxy group is reflective of a nonpolarized transition state for the hydrogen atom transfer reaction.