A Product Study of 1-Adamantyl and 1-Bicyclooctyl Radicals in Hydrocarbon Solvents. An Unusually Large Hydrogen Isotope Effect

Article abstract of DOI:10.1021/ja00353a029

1-Adamantyl (ada^.) and 1-bicyclo<2.2.2>octyl (bo^.) radicals have been generated by photolysis of the corresponding azoalkanes in various hydrocarbon solvents.Both radicals abstract hydrogen readily from hydrocarbons and they add to aromatic rings much faster than tert-butyl^. does.Despite its reactivity, ada^. is remarkably selective in hydrogen atom abstraction, preferring a benzylic hydrogen 25:1 over a cyclohexane hydrogen.The effect of solvent viscosity indicates that formation of the radical dimers biada and bibo occurs in the solvent cage.The most striking result of this work is a deuterium isotope effect of 25 for hydrogen transfer from cyclohexane to ada^. at 65 deg C.Steric compression in the transition state is postulated to cause an unusually large tunnel correction and hence a large k_H/k_D.