Solution Structures of Lithium Enolates, Phenolates, Carboxylates, and Alkoxides in the Presence of N,N,N′,N′-Tetramethylethylenediamine: A Prevalence of Cyclic Dimers

The method of continuous variation was used to characterize lithium enolates, phenolates, carboxylates, and alkoxides solvated by N,N,N′,N′-tetramethylethylenediamine (TMEDA). The method relies on characterizing an ensemble of homo- and heteroaggregates using 6Li NMR spectroscopy. A combination of aggregate counts and symmetries, nearly statistical distributions, and quantitative parametric fits revealed that cyclic dimers are the dominant forms. Nonstatistical distributions favoring heteroaggregated dimers were observed when hindered enolates and carboxylates were mixed with unhindered enolates. Hindered (tertiary) alkoxides form higher aggregates (possibly hexamers), whereas hindered lithium phenolates appear to form TMEDA-solvated monomers.