Editing Oxonium ion

{{Short description|Cation containing an oxygen atom with 3 bonds and 1+ formal charge}}

In [[chemistry]], an '''oxonium ion''' is any [[cation]] containing an [[oxygen]] atom that has three [[chemical bond|bonds]] and 1+ [[formal charge]].<ref>{{citation |author=March, Jerry |author-link=Jerry March |year=2007 |title=Advanced Organic Chemistry: Reactions, Mechanisms, and Structure |edition=4th |location=New York |publisher=Wiley |page=497}}</ref> The simplest oxonium ion is the [[hydronium]] ion ({{chem2|H3O+}}).<ref name=OlahBk>{{cite book |first=George A. |last=Olah |year=1998 |title=Onium Ions |publisher=John Wiley & Sons |pages=509 |isbn=9780471148777}}</ref>

==Alkyloxonium==
Hydronium is one of a series of oxonium ions with the formula R<sub>''n''</sub>H<sub>3−''n''</sub>O<sup>+</sup>.
Oxygen is usually pyramidal with an sp<sup>3</sup> [[orbital hybridisation|hybridization]]. Those with ''n''&nbsp;=&nbsp;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 [[elimination reaction|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 [[alcohol (chemistry)|alcohol]]s or [[ether]]s (R−C−{{overset|+|O}}−R<sub>1</sub>R<sub>2</sub>).

Secondary oxonium ions have the formula R<sub>2</sub>OH<sup>+</sup>, an example being protonated ethers.

Tertiary oxonium ions have the formula R<sub>3</sub>O<sup>+</sup>, an example being [[trimethyloxonium tetrafluoroborate|trimethyloxonium]].<ref>{{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}}</ref> Tertiary [[alkyl]]oxonium [[salt (chemistry)|salts]] are useful [[alkylating agent]]s. 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.<ref>{{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}}</ref> It is also used for preparation of enol ethers and related functional groups.<ref>{{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}}</ref><ref>{{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}}</ref>
{| class="wikitable" style="margin:1em auto; text-align:center;"
|[[File:Oxonium-ion-2D.png|100px]]||[[File:Trimethyloxonium-2D-skeletal.png|100px]]||[[File:Trimethyloxonium-3D-balls.png|100px]]||[[File:Trimethyloxonium-3D-vdW.png|100px]]
|-
|general [[trigonal pyramidal molecular geometry|pyramidal]]<br>oxonium ion||[[skeletal formula]] of the<br>[[trimethyloxonium]] cation||[[ball-and-stick model]]<br>of trimethyloxonium||[[space-filling model]]<br>of trimethyloxonium
|}

[[Oxatriquinane]] and [[oxatriquinacene]] are unusually stable oxonium ions, first described in 2008. Oxatriquinane does not react with boiling water or with [[Alcohol (chemistry)|alcohol]]s, [[thiol]]s, [[halide ion]]s, or [[amine]]s, although it does react with stronger [[nucleophile]]s 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={{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|center|250px]]

==Gold-stabilized species==
[[File:Triphenylphosphinegoldoxonium.png|right|frameless]]
An unusually stable oxonium species is the gold complex tris[triphenylphosphinegold(I)]oxonium tetrafluoroborate, [(Ph<sub>3</sub>PAu)<sub>3</sub>O][BF<sub>4</sub>], where the intramolecular [[aurophilicity|aurophilic interactions]] between the gold atoms are believed responsible for the stabilisation of the cation.<ref>{{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}}</ref><ref>{{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}}</ref> This complex is prepared by treatment of Ph<sub>3</sub>PAuCl with [[silver oxide|Ag<sub>2</sub>O]] in the presence of [[sodium tetrafluoroborate|NaBF<sub>4</sub>]]:<ref>{{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}}</ref>

:3&nbsp;Ph<sub>3</sub>PAuCl + Ag<sub>2</sub>O + NaBF<sub>4</sub> → [(Ph<sub>3</sub>PAu)<sub>3</sub>O]<sup>+</sup>[BF<sub>4</sub>]<sup>−</sup> + 2&nbsp;AgCl + NaCl

It has been used as a catalyst for the propargyl [[Claisen rearrangement]].<ref>{{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}}</ref>

==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 product]]s by the red algae of the genus ''[[Laurencia]]''.<ref name=":0">{{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}}</ref>
[[File:Oxoniumionlaurencia2.png|center|900x900px]]

Several members of these elusive species have been prepared explicitly by total synthesis, demonstrating the possibility of their existence.<ref name=":0" /> The key to their successful generation was the use of a [[Non-coordinating anion|weakly coordinating anion]] (Krossing's anion, [Al(pftb)<sub>4</sub>]<sup>−</sup>, pftb = perfluoro-''tert''-butoxy) as the counteranion.<ref>{{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}}</ref> As shown in the example below, this was executed by a transannular halide abstraction strategy through the reaction of the oxonium ion precursor (an [[Halocarbon|organic halide]]) with the silver salt of the Krossing's anion Ag[Al(pftb)<sub>4</sub>]•CH<sub>2</sub>Cl<sub>2</sub>, generating the desired oxonium ion with simultaneous precipitation of inorganic [[silver halide]]s. The resulting oxonium ions were characterized comprehensively by [[nuclear magnetic resonance spectroscopy]] at low temperature (−78&nbsp;°C) with support from [[density functional theory]] computation.
[[File:Wiki fig 2.png|center|600x600px]]

These oxonium ions were also demonstrated to directly give rise to multiple related [[natural product]]s by reacting with various [[nucleophile]]s, such as water, bromide, chloride, and acetate.<ref>{{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 }}</ref><ref>{{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}}</ref><ref>{{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=C<sub>15</sub> 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}}</ref>
[[File:Wiki fig 3.png|center|800x800px]]

==See also==
* [[Acyl group#Acylium_cations,_radicals,_and_anions|Acylium ion]], a type of oxonium ion with the structure R–C≡O<sup>+</sup>
*[[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==
{{Reflist}}

[[Category:Oxonium compounds| ]]
[[Category:Oxycations]]