5-Hexynyl radical

Vinylidenecyclopentanes 45 have been synthesized by a sequential radical reaction. A careful evaluation of the suitable starting materials led to the choice of thiohydroxamic esters like 43 for starting the radical chain shown in Scheme 15. Photolytical decomposition of 43 in the presence of 5-fold molar excess of a radicophilic olefin generated the 5-hexynyl radical 44 which cyclized onto the triple bond. jS-Elimination of a phenylthiyl radical afforded the vinylidenecyclopentane 45 in 60-70% yield. Other insights in the mechanism of these radical chain reactions are given in the paper. 

S-Allyloxy tellurides also underwent similar radical 5-exo cyclizations catalyzed by 7 mol% of Ni(acac)2 and 2 equiv. of Et2Zn [113]. The reaction proceeded with high r/.v-selectivity in 56% yield. In contrast to the reactions of 5-hexenyl iodides shown above, 5-exo cyclization reactions of 5-hexynyl iodides were proposed to proceed by a two-electron pathway consisting of alkyne coordination/oxidative addition/intramolecular carbonickelation and reductive elimination, resulting in alkylidenecyclopentanes [114]. 

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