Carboindation of Carbon−Carbon Triple Bonds: Regioselective Indium-Mediated Allylation of Functionalized Alkynes and Transformation into Halogen-Substituted 1,4-Dienes

By examining the allylindation of a series of substituted alkynes, the relationships between allylindation reactivity and alkyne structure were systematically investigated. While terminal alkynes with protected hydroxyl groups gave the corresponding allylated branched 1,4-dienes (Markovnikov products) within 5−6 h, the unprotected alkynols reacted markedly faster, requiring only 2−4 h of ultrasonication in THF to produce the 1,4-dienols in good yields. In the latter reactions, the regioisomeric outcome was found to depend on the distance between the hydroxyl group and the alkyne moiety: propargylic substrates gave linear 1,4-dienes (anti-Markovnikov products), suggesting the involvement of bicyclic “chelation-controlled” transition state, while 4-pentynol and higher homologues exclusively afforded the branched 1,4-dienes. Moreover, the proposed vinylic α,α-bis-indium intermediates from both protected and unprotected substrates were successfully quenched with N -bromosuccinimide (NBS) and N -iodosuccinimide (NIS) to form tri- and tetrasubstituted dienes in good yields. Taken as a whole, these results illustrate the remarkable stability of the allylindium and vinylindium intermediates despite the presence of protic functionalities in the reaction mixture.