A case of anti carbolithiation of alkynes resulting from intramolecular lithium coordination.

The mechanism of the intramolecular carbolithiation of lithiated propargylic ether 2 has been investigated both experimentally and theoretically. The results show that the action of one equivalent of n-butyllithium on 1 is sufficient to trigger halogen-lithium exchange and the subsequent heterocyclization step. Interestingly, the reaction stops at the stage of dihydrobenzofuran 6; no spontaneous elimination of lithium ethylate was observed. The fact that the E configuration of this adduct was exclusively produced suggests that the reaction proceeds by following an unprecedented anti addition on the alkyne. According to DFT calculations, this unexpected outcome is related to the intramolecular coordination of the lithium by one oxygen atom of the terminal acetal appendage: the O-Li interaction, which persists all along the ring-closure process, drives the cation to the E site of the final olefin. The calculations also show that in the absence of this coordination (as in conformers B and C of acetal 2), the Z olefin that results from a classical syn addition of the aryllithium should be obtained. The experiments were repeated with allene 1d. In this case, one equivalent of n-butyllithium suffices to trigger not only the exchange and the cyclization, but also the final elimination of lithium ethoxide. The DFT results indicate that the intramolecular addition of the original aryllithium on the central carbon atom of the allene 2b yields the expected benzofuran skeleton 3b, which bears a lithiated lateral chain at the 3-position. Both cyclizations go through low-lying transition states, as is expected for rapid reactions at low temperature.