Improved Synthesis of Pinacol Dimethyl Ether – A Superior Solvent for Lithium–Oxygen Batteries

Abstract

Pinacol dimethyl ether (2,3-dimethoxy-2,3-dimethylbutane) was reported to be a superior solvent for lithium-oxygen batteries because of lacking α-H-atoms, thus, shutting down the major decomposition pathway in such devices. An improved Williamson ether synthesis of this solvent is reported starting from pinacol and methyl iodide. Crucial for the success of this double alkylation reaction was the choice of methyl lithium as the base.

Keywords

electrolyte - ether - alkylation - lithium-oxygen battery - solvent

• References and Notes

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• 11 2,3-Dimethoxy-2,3-dimethylbutane (PDE) Under exclusion of air and moisture (N₂ atmosphere) and cooling (ice-water bath), a solution of MeLi (55 mL of a 3 mol/L solution in diethoxymethane, 165 mmol, 2 equiv) was added over a period of 10 min to a solution of pinacol (10.0 g, 84.6 mmol) in abs. THF (50 mL). Towards the end of this period, gas evolution is observed. The cooling bath was removed, and the mixture was stirred for further 1 h at ambient temperature. After addition of MeI (26.3 mL, 60.0 g, 422 mmol, 5 equiv) the mixture was heated to reflux (oil-bath temperature 70 °C) for 20 h. After cooling to ambient temperature, a sat. aq Na₂S₂O₃ solution (90 mL) was added, and the mixture extracted with Et₂O (3 × 90 mL). The combined organic layers were dried ­(Na₂SO₄) and evaporated to yield a crude material, which consists to 25% of the monoether together with 75% of the title compound (by GLC analysis). The mixture was transferred to a small distillation flask and treated with NaH (2.0 g of a 60% dispersion in mineral oil, 50 mmol). After stirring for 2 h at ambient temperature, the mixture was distilled at 50 mbar through a 10 cm Vigreux column to yield 5.80 g (39.7 mmol, 47%) of the title compound in the main fraction (bp 60 °C at 50 mbar). All spectroscopic data are in accordance with the literature.⁷