oxonium ion

Hydronium is one of a series of oxonium ions with the formula R_nH_3−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−{{overset|+|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.{{cite journal |title=Superelectrophiles |author=Olah, George A. |year=1993 |journal=Angew. Chem. Int. Ed. Engl. |volume=32 |issue=6 |pages=767–788 |doi=10.1002/anie.199307673}} Tertiary alkyloxonium salts are useful alkylating agents. For example, triethyloxonium tetrafluoroborate ({{Chem|Et|3|O|+}})({{Chem|BF|4|-}}), a white crystalline solid, can be used, for example, to produce ethyl esters when the conditions of traditional Fischer esterification are unsuitable.{{cite journal |last1=Raber |first1=Douglas J. |last2=Gariano Jr |first2=Patrick |last3=Brod |first3=Albert O. |last4=Gariano |first4=Anne L. |last5=Guida |first5=Wayne C. |year=1977 |title=Esterification Of Carboxylic Acids With Trialkyloxonium Salts: Ethyl And Methyl 4-acetoxybenzoates |journal=Org. Synth. |volume=56 |page=59 |doi=10.15227/orgsyn.056.0059}} It is also used for preparation of enol ethers and related functional groups.{{cite journal |last1=Struble |first1=Justin R. |last2=Bode |first2=Jeffrey W. |year=2010 |title=Synthesis Of A N-mesityl Substituted Aminoindanol-derived Triazolium Salt |journal=Org. Synth. |volume=87 |page=362 |doi=10.15227/orgsyn.087.0362 |doi-access=free}}{{cite journal |last1=Hegedus |first1=Lous S. |last2=Mcguire |first2=Michael A. |last3=Schultze |first3=Lisa M. |year=1987 |title=1,3-Dimethyl-3-methoxy-4-phenylazetidinone |journal=Org. Synth. |volume=65 |page=140 |doi=10.15227/orgsyn.065.0140}}

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.

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={{overset|+|O}}−R′ which forms a resonance structure with the fully-fledged carbocation R−{{overset|+|C}}−O−R′ and is therefore especially stable:

File:Carbonyl-oxonium-resonance-2D-skeletal.png

File:Triphenylphosphinegoldoxonium.png

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.{{cite journal |first=Hubert |last=Schmidbaur |title=The Aurophilicity Phenomenon: A Decade of Experimental Findings, Theoretical Concepts and Emerging Application |journal=Gold Bulletin |year=2000 |volume=33 |issue=1 |pages=3–10 |doi=10.1007/BF03215477 |doi-access=free}}{{cite journal |first=Hubert |last=Schmidbaur |title=Ludwig Mond Lecture: High-Carat Gold Compounds |journal=Chem. Soc. Rev. |year=1995 |volume=24 |issue=6 |pages=391–400 |doi=10.1039/CS9952400391}} This complex is prepared by treatment of Ph₃PAuCl with Ag₂O in the presence of NaBF₄:{{cite book |last1=Bruce |first1=M. I. |author-link1=Michael Ian Bruce |last2=Nicholson |first2=B. K. |last3=Bin Shawkataly |first3=O. |last4=Shapley |first4=J. R. |last5=Henly |first5=T. |chapter=Synthesis of Gold-Containing Mixed-Metal Cluster Complexes |year=1989 |title=Inorganic Syntheses |title-link=Inorganic Syntheses |volume=26 |pages=324–328 |editor-last=Kaesz |editor-first=Herbert D. |publisher=John Wiley & Sons, Inc. |doi=10.1002/9780470132579.ch59 |isbn=9780470132579}}

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

It has been used as a catalyst for the propargyl Claisen rearrangement.{{cite journal |last1=Sherry |first1=Benjamin D. |last2=Toste |first2=F. Dean |author-link2=F. Dean Toste |year=2004 |title=Gold(I)-Catalyzed Propargyl Claisen Rearrangement |journal=Journal of the American Chemical Society |volume=126 |issue=49 |pages=15978–15979 |doi=10.1021/ja044602k |pmid=15584728 |citeseerx=10.1.1.604.7272 |issn=0002-7863 |url=http://www.cchem.berkeley.edu/toste/publications/jacs12615978.pdf}}

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.{{cite journal |last1=Sam Chan |first1=Hau Sun |last2=Nguyen |first2=Q. Nhu N. |last3=Paton |first3=Robert S. |last4=Burton |first4=Jonathan W. |date=2019-10-09 |others=A full list of references encompassing the contributions from Braddock, Snyder, Murai, Suzuki, Fukuzawa, Burton, Kim, and Fox are available inside. |title=Synthesis, Characterization, and Reactivity of Complex Tricyclic Oxonium Ions, Proposed Intermediates in Natural Product Biosynthesis |journal=Journal of the American Chemical Society |volume=141 |issue=40 |pages=15951–15962 |doi=10.1021/jacs.9b07438 |pmid=31560524 |s2cid=203580092 |issn=0002-7863 |url=https://ora.ox.ac.uk/objects/uuid:0188aa06-1672-434d-a33b-9c6862ff937a}}

File:Oxoniumionlaurencia2.png

Several members of these elusive species have been prepared explicitly by total synthesis, demonstrating the possibility of their existence. 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.{{cite journal |last=Krossing |first=Ingo |date=2001 |title=The Facile Preparation of Weakly Coordinating Anions: Structure and Characterisation of Silverpolyfluoroalkoxyaluminates AgAl(ORF)4, Calculation of the Alkoxide Ion Affinity |journal=Chemistry – A European Journal |volume=7 |issue=2 |pages=490–502 |doi=10.1002/1521-3765(20010119)7:2<490::aid-chem490>3.0.co;2-i |pmid=11271536 |issn=1521-3765}} 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.

File:Wiki fig 2.png

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.{{cite journal |last1=Wang |first1=Bin-Gui |last2=Gloer |first2=James B. |last3=Ji |first3=Nai-Yun |last4=Zhao |first4=Jian-Chun |date=March 2013 |title=Halogenated Organic Molecules of Rhodomelaceae Origin: Chemistry and Biology |journal=Chemical Reviews |volume=113 |issue=5 |pages=3632–3685 |doi=10.1021/cr9002215 |pmid=23448097 |issn=0009-2665 |url=http://ir.yic.ac.cn/handle/133337/9046|url-access=subscription }}{{cite journal |last1=Zhou |first1=Zhen-Fang |last2=Menna |first2=Marialuisa |last3=Cai |first3=You-Sheng |last4=Guo |first4=Yue-Wei |date=2015-02-11 |title=Polyacetylenes of Marine Origin: Chemistry and Bioactivity |journal=Chemical Reviews |volume=115 |issue=3 |pages=1543–1596 |doi=10.1021/cr4006507 |pmid=25525670 |issn=0009-2665}}{{cite journal |last1=Wanke |first1=Tauana |last2=Philippus |first2=Ana Cláudia |last3=Zatelli |first3=Gabriele Andressa |last4=Vieira |first4=Lucas Felipe Oliveira |last5=Lhullier |first5=Cintia |last6=Falkenberg |first6=Miriam |date=2015-11-01 |title=C₁₅ acetogenins from the Laurencia complex: 50 years of research – an overview |journal=Revista Brasileira de Farmacognosia |volume=25 |issue=6 |pages=569–587 |doi=10.1016/j.bjp.2015.07.027 |issn=0102-695X |doi-access=free}}

File:Wiki fig 3.png

• 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

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Category:Oxycations