Enhanced nucleophilic character of the 1-adamantyl radical in chlorine atom abstraction and in addition to electron-poor alkenes and to protonated heteroaromatic bases. Absolute rate constants and relationship with the Gif reaction

Abstract

The absolute rate constants for reactions of the 1-adamantyl radical with electron-deficient substrates have been measured by competition kinetics. Addition to electron-poor alkenes or to protonated heteroaromatic bases, as well as chlorine abstraction from CCl₄ and CHCl₃, are two to three orders of magnitude faster than the corresponding reactions of the tert-butyl radical. This behaviour is ascribed to the enhanced nucleophilic character of the 1-adamantyl radical compared to generic tertiary alkyl radicals; it is related to the particular stability of the 1-adamantyl cation and to the low oxidation potential of adamantane. These results contribute to the understanding of the Gif reaction mechanism; in this debate, adamantane oxidation was one of the most controversial aspects because the 1-adamantyl radical is trapped by pyridinium ions, while the 2-adamantyl radical is not. This behaviour is explained by the large difference in the reactivity of the two isomeric adamantyl radicals towards protonated pyridine.