Generation of Cycloalkylidene Carbenes via Exo-Type Cyclization of Alkynyllithiums Bearing Remote Leaving Group
摘要:
The reaction of 5-hexynyl tosylate (3a) with alkynyllithium (RC=CLi; R Ph, TMS) gives enynes 5 and 6. The reaction proceeds through a mechanism involving a novel exo-type cyclization of 6-lithiod-hexynyl tosylate to form cyclopentylidene carbene. Enyne 6 is produced by the addition of RC=CLi to the carbene, whereas rearrangement of the carbene to cyclohexyne followed by carbolithiation with RC=CLi gives enyne 5. The formation of cyclopentylidene carbene and cyclohexyne as intermediates is clearly demonstrated by trapping experiments with cyclohexene land triethylsilane) and with 1,3-diphenylisobenzofuran, respectively. Alkynyllithiums derived from 3-butynyl and 6-heptynyl p-fluorobenzenesulfonates (19a,b) undergo a similar exo-type cyclization to give cyclopropylidene and cyclohexylidene carbenes, respectively.
Cyclization of Gold Acetylides: Synthesis of Vinyl Sulfonates via Gold Vinylidene Complexes
作者:Janina Bucher、Thomas Wurm、Kumara Swamy Nalivela、Matthias Rudolph、Frank Rominger、A. Stephen K. Hashmi
DOI:10.1002/anie.201310280
日期:2014.4.7
Differently substituted terminal alkynes that bear sulfonate leaving groups at an appropriate distance were converted in the presence of a propynyl gold(I) precatalyst. After initial formation of a gold acetylide, a cyclization takes place at the β‐carbon atom of this species. Mechanistic studies support a mechanism that is related to that of dual gold‐catalyzed reactions, but for the new substrates
give (diiodomethylene)cycloalkanes. The diiodomethylene products are also obtained by iodine atom-transfer-type cyclization of ω-iodo-1-alkynes, using 1-iodo-1-hexyne as an external iodine atom source. Bromine atom-transfer and proton-transfer cyclization proceed as well by employing 1-bromo-1-octyne and 1-octyne, respectively. These reactions are proposed to proceed through a carbenoid-chain process