Tekla Strzalko

Formation of phosphonoacetate carbanions by lithiated bases or by the conjunction of a lithium salt and an amine: structural study of intermediate species

In THF, the anionic species (2) formed from methyl diethylphosphonoacetate (1b) by the action of Li bases coexist as aggregates (2A) and externally solvated monomeric ion pair (2M), while in CH3CN triple ions (2T) and small amounts of free ions (2–) are also characterized. When (1b) is in slight excess, relative to the lithiated base, these four species coexist in both solvents, together with other species (I). From n.m.r. and i.r. data, it appears that the intermediate species (I) include a neutral phosphonate (1b) moiety, as a Li+ cation bidentate ligand, the two other lithium solvation sites being occupied either by (2–), or by (1b) as a monodentate and a solvent molecule, or else by two solvent molecules. The structure of (1b) in this intermediate species is strongly perturbed as deduced from variations in 31P and 13C carbonyl shifts as well as that of νco. Furthermore, a fast proton exchange takes place between the anionic species (2) and free (1b) as shown by 1H n.m.r., but (I) does not participate in this exchange phenomenon. When (2) is formed by action of diazabicycloundecene (DBU) in the presence of LiCl, aggregates (2A), monomeric ion pari (2M), and small amounts of (2T) as well as two intermediate species (I) are characterized by i.r. or 31P n.m.r. spectroscopy. The role of DBU is manifold: it deaggregates LiCl or (2A), it deprotonates (1b) within (I), and it facilitates proton exchange between (2) and (1b), the relative importance of such effects depending upon the DBU concentration.