Generation of cycloalkylidene carbenes via exo-type cyclization of alkynyllithiums bearing remote leaving group

Article abstract of DOI:10.1021/jo981411p

The reaction of 5-hexynyl rosylate (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-lithio-5-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 (and 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.