Structure Determination Using the Method of Continuous Variation: Lithium Phenolates Solvated by Protic and Dipolar Aprotic Ligands

Abstract

The method of continuous variation (MCV) in conjunction with 6Li NMR spectroscopy was used to characterize four lithium phenolates solvated by a range of solvents including N,N,N′,N′-tetramethylethylenediamine, Et2O, pyridine, protic amines, alcohols, and highly dipolar aprotic solvents. Dimers, trimers, and tetramers were observed, depending on the precise lithium phenolate-solvent combinations. Competition experiments (solvent swaps) provide insights into relative solvation energies propensities toward mixed solvation.

Method of Continuous Variation: Characterization of Alkali Metal Enolates Using 1H and 19F NMR Spectroscopies

“…The parametric fits shown have been described previously. 24 The mole fraction, X A , is what we call the measured mole fraction—the mole fraction derived from the relative integrations rather than the intended mole fractions. Ascertaining the mole fraction from the integrations renders the method robust by providing more accurate values for X A as well as eliminating problems arising from unwanted impurities, standard experimental error, and multiple aggregation states.…”

Solution Structures of Lithium Amino Alkoxides Used in Highly Enantioselective 1,2-Additions

“…Warming of the samples to 60–70 °C, however, elicited the hoped-for rapid intraaggregate Li–Li site exchanges. We have examined structures in the limit of rapid intraaggregate exchange before, 12,20a but the temperatures required for vicinal amino alkoxides are remarkably high.…”

“…12 The most compelling assignments stem from structurally related ROLi/R′OLi pairs. At the outset, however, we thought that pairing structurally very different alkoxides would be required to obtain sufficient resolution in the 6 Li NMR spectra.…”

“…In conjunction with 6 Li– 15 N double-labeling studies, the application of MCV is extended to distinguish lithium hexamethyldisilazide-lithium amino alkoxide dimers 11 and the corresponding ladders 12 16 —a structural ambiguity arising from opaque O–Li linkages that has dogged us for many years. 12b,17 We also provide a more nuanced view of the benefits of catalytic lithium salts on the enantioselectivities observed by DuPont investigators.…”

“…NMR samples were prepared using protocols described previously. 12c 6 Li NMR spectra were typically recorded on a 500 or 600 MHz spectrometer with the delay between scans set to >5 x T1 to ensure accurate integrations. Chemical shifts are reported relative to a 0.30 M 6 LiCl/MeOH standard at −80 °C.…”