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 rela tionships between allylindation reactivity and alkyne structure were system atically investigated. While terminal alkynes with protected hydroxyl group s gave the corresponding allylated branched 1,4-dienes (Markovnikov product s) within 5-6 h, the unprotected alkynols reacted markedly faster, requirin g only 2-4 h of ultrasonication in THF to produce the 1,4-dienols in good y ields. In the latter reactions, the regioisomeric outcome was found to depe nd on the distance between the hydroxyl group and the alkyne moiety: propar gylic substrates gave linear 1,4-dienes (anti-Markovnikov products), sugges ting the involvement of bicyclic "chelation-controlled" transition state, w hile 4-pentynol and higher homologues exclusively afforded the branched 1,4 -dienes. Moreover, the proposed vinylic alpha,alpha-bis-indium intermediate s 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 il lustrate the remarkable stability of the allylindium and vinylindium interm ediates despite the presence of protic functionalities in the reaction mixt ure.