Synthesis, physical properties, and practical applications of perfluorinated alkylmorpholines and alkylpiperidines obtained by electrochemical fluorination

Objectives. The work set out to describe the main physicochemical properties of new perfluorinated compounds (perfluoro-N-butylmorpholine, perfluoro-N-butylpiperidine, and perfluoro-N-ethylpiperidine) developed at the Russian Research Center “Applied Chemistry” (GIPH), and to evaluate the potential of their use as perfluorinated technical liquids by studying the temperature dependencies of their density, viscosity, and heat capacity in low temperature regions, as well as their applicability as media for syntheses involving strong Lewis acids using tris(pentafluoroethyl)difluorophosphorane as an example.

Methods. Perfluorinated tertiary amines (perfluorotriethylamine, perfluorotributylamine, perfluoro-N-butylmorpholine, perfluoro-N-butylpiperidine, perfluoro-N-ethylpiperidine) were synthesized by electrochemical fluorination in anhydrous hydrogen fluoride. The composition, purity, and structure of the perfluoroamines were determined by gas–liquid chromatography (Kristall 2000M) and confirmed by nuclear magnetic resonance spectroscopy (Bruker AVANCE III HD 400 MHz) and chromatography–mass spectrometry (Agilent Technologies 7890B/5977A). The thermal characteristics of the samples were determined by differential scanning calorimetry (NETZSCH DSC 214 Polyma). The density and kinematic viscosity of the samples were studied using a Stabinger viscometer (Anton Paar Stabinger Viscometer SVM 3000). Metal tris(pentafluoroethyl)trifluorophosphates were obtained by the fluorination of tris(pentafluoroethyl)difluorophosphorane by fluorides of the alkali metals—lithium, sodium, and potassium.

Results. Electrochemical fluorination produced tert-amines of various structures: perfluorotriethylamine, perfluorotributylamine, perfluoro-N-butylmorpholine, perfluoro-N-butylpiperidine, and perfluoro-N-ethylpiperidine with current yields from 29 to 61%. The dependencies of the heat capacity, density, and kinematic viscosity of new perfluorinated alkylmorpholines and alkylpiperidines in comparison with noncyclic perfluoroamines were studied over a wide temperature range. In the media of the obtained perfluoroamines, tris(pentafluoroethyl)difluorophosphorane salts of lithium, sodium, and potassium were synthesized at a yield of 38 to 95%.

Conclusions. The analysis of the physicochemical properties of heterocyclic perfluorinated tert-amines obtained in this work (perfluoro-N-butylmorpholine, perfluoro-N-butylpiperidine, and perfluoro-N-ethylpiperidine) indicates that these compounds are not inferior to noncyclic perfluorinated tert-amines in a number of characteristics, thus indicating their potential use as perfluorinated technical fluids thanks to their high heat capacity and potentially low conductivity. The key parameters determining their applicability as media for syntheses involving perfluorinated reagents are identified as physicochemical similarity, high density, and low viscosity at subzero temperatures.

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