Preparation and applications of perfluoroalkoxides bearing α-fluorines

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Hexafluoropropylene oxide

Partially fluorinated ethers, known as hydrofluoroethers, have been introduced as inert replacements of chlorofluorocarbons and predominately find use as refrigerants and solvents. However, these materials are not inert as purported but readily methylate Group 15 nucleophiles. Not only are hydrofluoroethers reactive, when the reaction is performed under a moisture-free environment, the sole reaction products from the interaction of tertiary amines are quaternary ammonium perfluoroalkoxides bearing α-fluorines. This general route allows for the single-step synthesis of a highly under-represented organofluorine functional group from commercially available reagents negating the use of air-sensitive reagents, anhydrous fluorides, and often highly toxic, expensive precursors. Hence, 30 new perfluoroalkoxides have been prepared from a series of methoxy hydrofluoroethers from CH3OCF3 through to CH3OC4F9 including clinical inhalation anaesthetic methoxyflurane. The products were isolated in 9-99% yield with product conversions commensurate with increasing length of the fluorinated segment of the hydrofluoroether in the presence of sterically unhindered, nucleophilic tertiary amines. Thermal analysis of isolated tetramethylammonium perfluoropropoxides and butoxides indicated stability to 150 degree Celsius before partially decomposing under vacuum at 180 degree Celsius to NMe3 and CH3F as evidenced by thermogravimetric analysis. In two divergent studies, a series of experiments were devised to develop a functional group tolerant protocol for the trifluoromethoxylation of arenes, a current and largely unsolved synthetic problem. A wide range of strategies towards C(aryl)–OCF3 bond formation were attempted using the prepared tetraalkylammonium trifluoromethoxides including both metal and non-metal-mediated protocols. By considering traditional cross-coupling methods with state-of-the-art Ni0-NiII, Pd0-PdII, CuI-CuIII, AgI-AgII, and AuI-AuIII manifolds, it was determined that β-fluorination of aryl precursors from the coordinated OCF3 ligand outcompetes C-O bond formation at temperatures necessary for reductive elimination. The poor nucleophilicity, high moisture sensitivity, and propensity for β-fluoride elimination from -OCF3 precluded its use as a general synthetic building block for many metal-mediated cross-couplings. In a second study, the efficacy of the ostensibly fluoride-free tetraalkylammonium perfluoroalkoxides were tested as initiators for the anionic ring-opening polymerization of the perfluorooxirane monomer, hexafluoropropylene oxide (HFPO). While these perfluoroalkoxides did not outperform the industry optimized CsF/tetraglyme conditions, it was demonstrated that tetraalkylammonium perfluoroalkoxides can successfully ring-open HFPO, provide a direct study of the cation, and a means to measure chain transfer while providing oligomers with DPn = 1-3. The inoperative chain transfer process is likely a multi-faceted problem involving physical properties of the polymerization process. For instance, the phase-transfer mechanism and solvent choice play a critical role in propagation of polymeric based perfluoroalkoxides. Initial synthetic efforts were undertaken to design oligio(HFPO) terminated ethylene glycol surfactants or soluble oligio(HPFO) hydrofluoroethers to overcome the need for polar organic solvents required to dissolve the ionic initiators.

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This thesis may be printed or downloaded for non-commercial research and scholarly purposes. Copyright remains with the author.
Daniel Leznoff
Science: Department of Chemistry
Thesis type: 
(Dissertation) Ph.D.