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

The absolute rate constants for reactions of the 1-adamantyl radical w ith electron-deficient substrates have been measured by competition ki netics. Addition to electron-poor alkenes or to protonated heteroaroma tic bases, as well as chlorine abstraction from CCl4 and CHCl3, are tw o 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 adamanta ne. These results contribute to the understanding of the Gif reaction mechanism; in this debate, adamantane oxidation was one of the most co ntroversial aspects because the 1-adamantyl radical is trapped by pyri dinium ions, while the 2-adamantyl radical is not. This behaviour is e xplained by the large difference in the reactivity of the two isomeric adamantyl radicals towards protonated pyridine.