Abstract
A lithium-halogen exchange reaction occurs when the chloro[bis(diisopropylamino)]cyclopropenium tetrafluoroborate salt 1 (X = BF(4)) is treated with n-butyllithium. The resulting cyclopropenylidene-lithium adduct 3 has been isolated in 45% yield. In the solid state, this compound exists as a polymeric chain with an overall stoichiometry of two LiBF(4) per carbene ligand. Addition of 12-crown-4-ether does not liberate the carbene from the lithium cation, but affords a monomeric tertiary complex (60% yield) that includes the crown ether. Moreover, complex 3 can also be synthesized by depro tonation of the bis(diisopropylamino)cyclopropenium tetrafluoroborate salt 2 (X = BF(4)) with n-butyllithium, whereas using potassium bis(trimethylsilyl)amide the free cyclopropenylidene was isolated in 53% yield. These results as whole seem to demonstrate that only certain counteranions allow for the isolation of cyclopropenylidene-lithium adducts, and only bases not containing lithium allow for the isolation of the free cyclopropenylidene. The former and the latter presumably prevented Weiss and Yoshida from isolating what would have been the first example of a stable carbene-lithium adduct and a free carbene, respectively.