Oxonium ion

In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge.[1] The simplest oxonium ion is the hydronium ion (H3O+).[2]

Alkyloxonium

Hydronium is one of a series of oxonium ions with the formula RnH3−nO+. Oxygen is usually pyramidal with an sp3 hybridization. Those with n = 1 are called primary oxonium ions, an example being protonated alcohol (e.g. methanol). In acidic media, the oxonium functional group produced by protonating an alcohol can be a leaving group in the E2 elimination reaction. The product is an alkene. Extreme acidity, heat, and dehydrating conditions are usually required. Other hydrocarbon oxonium ions are formed by protonation or alkylation of alcohols or ethers (R−C−+O−R1R2).

Secondary oxonium ions have the formula R2OH+, an example being protonated ethers.

Tertiary oxonium ions have the formula R3O+, an example being trimethyloxonium.[3] Tertiary alkyloxonium salts are useful alkylating agents. For example, triethyloxonium tetrafluoroborate (Et
3O+
)(BF−
4), a white crystalline solid, can be used, for example, to produce ethyl esters when the conditions of traditional Fischer esterification are unsuitable.[4] It is also used for preparation of enol ethers and related functional groups.[5][6]

general pyramidal
oxonium ion		skeletal formula of the
trimethyloxonium cation		ball-and-stick model
of trimethyloxonium		space-filling model
of trimethyloxonium

Oxatriquinane and oxatriquinacene are unusually stable oxonium ions, first described in 2008. Oxatriquinane does not react with boiling water or with alcohols, thiols, halide ions, or amines, although it does react with stronger nucleophiles such as hydroxide, cyanide, and azide.

Oxocarbenium ions

Another class of oxonium ions encountered in organic chemistry is the oxocarbenium ions, obtained by protonation or alkylation of a carbonyl group e.g. R−C=+O−R′ which forms a resonance structure with the fully-fledged carbocation R−+C−O−R′ and is therefore especially stable:

Gold-stabilized species

An unusually stable oxonium species is the gold complex tris[triphenylphosphinegold(I)]oxonium tetrafluoroborate, [(Ph3PAu)3O][BF4], where the intramolecular aurophilic interactions between the gold atoms are believed responsible for the stabilisation of the cation.[7][8] This complex is prepared by treatment of Ph3PAuCl with Ag2O in the presence of NaBF4:[9]

3 Ph3PAuCl + Ag2O + NaBF4 → [(Ph3PAu)3O]+[BF4]− + 2 AgCl + NaCl

It has been used as a catalyst for the propargyl Claisen rearrangement.[10]

Relevance to natural product chemistry

Complex bicyclic and tricyclic oxonium ions have been proposed as key intermediates in the biosynthesis of a series of natural products by the red algae of the genus Laurencia.[11]

Several members of these elusive species have been prepared explicitly by total synthesis, demonstrating the possibility of their existence.[11] The key to their successful generation was the use of a weakly coordinating anion (Krossing's anion, [Al(pftb)4]−, pftb = perfluoro-tert-butoxy) as the counteranion.[12] As shown in the example below, this was executed by a transannular halide abstraction strategy through the reaction of the oxonium ion precursor (an organic halide) with the silver salt of the Krossing's anion Ag[Al(pftb)4]•CH2Cl2, generating the desired oxonium ion with simultaneous precipitation of inorganic silver halides. The resulting oxonium ions were characterized comprehensively by nuclear magnetic resonance spectroscopy at low temperature (−78 °C) with support from density functional theory computation.

These oxonium ions were also demonstrated to directly give rise to multiple related natural products by reacting with various nucleophiles, such as water, bromide, chloride, and acetate.[13][14][15]

See also
  • Acylium ion, a type of oxonium ion with the structure R–C≡O+
  • Onium ion, a +1 cation derived by protonation of a hydride (includes oxonium ions)
  • Pyrylium, a subtype of oxonium ion
  • Sulfonium, a sulfur analog that can be chiral

References
  1. March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (4th ed.), New York: Wiley, p. 497
  2. Olah, George A. (1998). Onium Ions. John Wiley & Sons. p. 509. ISBN 9780471148777.
  3. Olah, George A. (1993). "Superelectrophiles". Angew. Chem. Int. Ed. Engl. 32 (6): 767–788. doi:10.1002/anie.199307673.
  4. Raber, Douglas J.; Gariano Jr, Patrick; Brod, Albert O.; Gariano, Anne L.; Guida, Wayne C. (1977). "Esterification Of Carboxylic Acids With Trialkyloxonium Salts: Ethyl And Methyl 4-acetoxybenzoates". Org. Synth. 56: 59. doi:10.15227/orgsyn.056.0059.
  5. Struble, Justin R.; Bode, Jeffrey W. (2010). "Synthesis Of A N-mesityl Substituted Aminoindanol-derived Triazolium Salt". Org. Synth. 87: 362. doi:10.15227/orgsyn.087.0362.
  6. Hegedus, Lous S.; Mcguire, Michael A.; Schultze, Lisa M. (1987). "1,3-Dimethyl-3-methoxy-4-phenylazetidinone". Org. Synth. 65: 140. doi:10.15227/orgsyn.065.0140.
  7. Schmidbaur, Hubert (2000). "The Aurophilicity Phenomenon: A Decade of Experimental Findings, Theoretical Concepts and Emerging Application". Gold Bulletin. 33 (1): 3–10. doi:10.1007/BF03215477.
  8. Schmidbaur, Hubert (1995). "Ludwig Mond Lecture: High-Carat Gold Compounds". Chem. Soc. Rev. 24 (6): 391–400. doi:10.1039/CS9952400391.
  9. Bruce, M. I.; Nicholson, B. K.; Bin Shawkataly, O.; Shapley, J. R.; Henly, T. (1989). "Synthesis of Gold-Containing Mixed-Metal Cluster Complexes". In Kaesz, Herbert D. (ed.). Inorganic Syntheses. Vol. 26. John Wiley & Sons, Inc. pp. 324–328. doi:10.1002/9780470132579.ch59. ISBN 9780470132579.
  10. Sherry, Benjamin D.; Toste, F. Dean (2004). "Gold(I)-Catalyzed Propargyl Claisen Rearrangement" (PDF). Journal of the American Chemical Society. 126 (49): 15978–15979. CiteSeerX 10.1.1.604.7272. doi:10.1021/ja044602k. ISSN 0002-7863. PMID 15584728.
  11. Sam Chan, Hau Sun; Nguyen, Q. Nhu N.; Paton, Robert S.; Burton, Jonathan W. (2019-10-09). "Synthesis, Characterization, and Reactivity of Complex Tricyclic Oxonium Ions, Proposed Intermediates in Natural Product Biosynthesis". Journal of the American Chemical Society. 141 (40). A full list of references encompassing the contributions from Braddock, Snyder, Murai, Suzuki, Fukuzawa, Burton, Kim, and Fox are available inside.: 15951–15962. doi:10.1021/jacs.9b07438. ISSN 0002-7863. PMID 31560524. S2CID 203580092.
  12. Krossing, Ingo (2001). "The Facile Preparation of Weakly Coordinating Anions: Structure and Characterisation of Silverpolyfluoroalkoxyaluminates AgAl(ORF)4, Calculation of the Alkoxide Ion Affinity". Chemistry – A European Journal. 7 (2): 490–502. doi:10.1002/1521-3765(20010119)7:2<490::aid-chem490>3.0.co;2-i. ISSN 1521-3765. PMID 11271536.
  13. Wang, Bin-Gui; Gloer, James B.; Ji, Nai-Yun; Zhao, Jian-Chun (March 2013). "Halogenated Organic Molecules of Rhodomelaceae Origin: Chemistry and Biology". Chemical Reviews. 113 (5): 3632–3685. doi:10.1021/cr9002215. ISSN 0009-2665. PMID 23448097.
  14. Zhou, Zhen-Fang; Menna, Marialuisa; Cai, You-Sheng; Guo, Yue-Wei (2015-02-11). "Polyacetylenes of Marine Origin: Chemistry and Bioactivity". Chemical Reviews. 115 (3): 1543–1596. doi:10.1021/cr4006507. ISSN 0009-2665. PMID 25525670.
  15. Wanke, Tauana; Philippus, Ana Cláudia; Zatelli, Gabriele Andressa; Vieira, Lucas Felipe Oliveira; Lhullier, Cintia; Falkenberg, Miriam (2015-11-01). "C15 acetogenins from the Laurencia complex: 50 years of research – an overview". Revista Brasileira de Farmacognosia. 25 (6): 569–587. doi:10.1016/j.bjp.2015.07.027. ISSN 0102-695X.
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