Generation of Cycloalkylidene Carbenes via Exo-Type Cyclization of Alkynyllithiums Bearing Remote Leaving Group

Abstract

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-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.