Solvent-Dependent Lithium Bridging in Allenyl−Propargyllithium Reagents

The solution structures of two allenyl-propargyllithium reagents (6-Li and 7-Li) which give (13)C NMR chemical shifts intermediate between those expected for the allenyl and propargyl isomers have been studied by the Saunders isotope perturbation technique. Variable equilibrium isotope shifts were detected, showing that these reagents adopt structures ranging from equilibrating localized allenyl- and propargyllithium reagents (1A right harpoon over left harpoon 1B) to equilibrating unsymmetrically bridged structures (11) and symmetrically bridged structures (2), depending mostly on the details of solvation.