Front Cover: α‐Fluoroalcohols: Synthesis and Characterization of Perfluorinated Methanol, Ethanol and n‐Propanol, and their Oxonium Salts (Chem. Eur. J. 63/2018)
Amanda F. BaxterJonas SchaabJoachim HeggeThomas SaalMonica VasiliuDavid A. DixonRalf HaigesKarl O. Christe
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Perfluoroalcohols: Although thermally unstable, these alcohols can be readily accessed through equilibria of the corresponding acyl fluorides with HF in anhydrous HF solution. These equilibria can be shifted to the side of the alcohols by trapping them as oxonium salts, ethers or alkoxides, thus making these reactions synthetically useful. More information can be found in the Communication by K. O. Christe et al. on page 16742. Perfluoroalcohols: Although thermally unstable, these alcohols can be readily accessed through equilibria of the corresponding acyl fluorides with HF in anhydrous HF solution. These equilibria can be shifted to the side of the alcohols by trapping them as oxonium salts, ethers or alkoxides, thus making these reactions synthetically useful. More information can be found in the Communication by K. O. Christe et al. on page 16742.Keywords:
Oxonium ion
Anhydrous
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Tetramethylammonium
An oxonium ion with hydroxymethyl and methylidene substituents is found in the salts 1-MF6 (M=As, Sb), which can be isolated from solutions of formaldehyde in superacids HF/MF5. Their formation was at first surprising, since NMR spectroscopy indicates that protonated monofluoromethanol is present in solution.
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Oxonium ions (H+·(H2O)n) can be stabilised and isolated in the solid state by using crown ethers of differing sizes. Crown ethers can act as very good hydrogen bond acceptors for the binding of oxonium ions. It has been reasonably well established that 18C6 binds the H3O+ oxonium ion almost selectively in the cavity of the crown, while for the smaller crown ethers 12C4 and 15C5 the macrocycles generally span successive oxonium ions of varying nuclearity (H3O+, H5O2+, H7O3+ and H9O4+) to form hydrogen bonded polymers. Crown ethers larger than 18C6, e.g. 21C7 and 24C8 can bind H5O2+ ions on the face of the crowns, while 30C10 appears to prefer two molecules of H3O+ in the cavity. It is the intention of this perspective to review the literature involving the solid state interactions of crown ether complexes involving oxonium ions.
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Despite numerous studies of trialkyloxonium ions in the literature, investigations into the chemistry of allylic, benzylic, propargylic and allenylic oxonium ions are rare. Existing reports on well characterized allylic and benzylic oxonium ions invariably construct these species based on constrained tricyclic oxatriquinane/oxatriquinacene scaffolds, and only one example of an unconstrained but thermally highly unstable benzylic oxonium ion has been reported in the literature without detailed characterization. Herein we report an investigation on a collection of allylic, benzylic and hitherto unknown propargylic and allenylic oxonium ions prepared on unconstrained scaffolds by a general, modular and unified strategy. Permutation of the substitution pattern of these oxonium ions allowed the extension of the strategy for the syntheses of various doubly-substituted oxonium ions. Most of these oxonium ions could be characterized at room temperature by NMR, and a series of unexpected reactions and chemical behavior pertinent to these species are briefly described.
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An oxonium cation complexed by a noncyclic polyether has been synthesized and structurally characterized; the oxonium cation is complexed assymetrically with short–ether bonds and the oxonium cation is found in the thermodynamically preferred pyramidal configuration.
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An efficient method for the synthesis of fluoroaromatics with tetramethylammonium fluoride(TMAFanh.) as fluorination agent via the fluorodenitration reaction was reported.Fluoroaromatics were obtained from TMAFanh.and nitroaromatics by simply stirring them together at 80~100 ℃ for 1~5 h in an anhydrous solution of DMF in 19.4%~99.0% yields.The nucleophilic fluorinating method has the advantages of mild reaction conditions,high yields,easy work-up and no phenol or ether side-products were detected in the reaction.
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A new approach to synthesis of functional derivatives of 7,8-dicarba- nido -undecaborate anion based on ring-opening of its cyclic oxonium derivatives [10-(CH 2 ) 4 O-7,8-C 2 B 9 H 11 ] and [10-O(CH 2 CH 2 ) 2 O-7,8-C 2 B 9 H 11 ] with various nucleophiles was developed. Both cyclic oxonium derivatives can be obtained as single isomers by reaction of the parent anion [7,8-C 2 B 9 H 12 ] - with mercury(II) chloride in the corresponding solvents. Mechanism of formation of the cyclic oxonium derivatives of 7,8-dicarba- nido -undecaborate anion was proposed. A series of 7,8-dicarba- nido -undecaborate derivatives with terminal carboxylic and azide functions were prepared by the ring-opening reactions of the cyclic oxonium derivatives with substituted phenolate and azide ions, respectively.
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