oxonium ion

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

Alkyloxonium

Hydronium is one of a series of oxonium ions with the formula R_''n''H_3−''n''O⁺.
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−−R₁R₂).

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

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

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=−R′ which forms a resonance structure with the fully-fledged carbocation R−−O−R′ and is therefore especially stable:

Gold-stabilized species

An unusually stable oxonium species is the gold complex tris riphenylphosphinegold(I)xonium tetrafluoroborate, Ph₃PAu)₃OBF₄], where the intramolecular aurophilicity, aurophilic interactions between the gold atoms are believed responsible for the stabilisation of the cation. This complex is prepared by treatment of Ph₃PAuCl with Ag₂O in the presence of NaBF₄:

:3 Ph₃PAuCl + Ag₂O + NaBF₄ → Ph₃PAu)₃Osup>+ F₄sup>− + 2 AgCl + NaCl

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

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''.

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, l(pftb)₄sup>−, pftb = perfluoro-''tert''-butoxy) as the counteranion. 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 l(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.

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

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Oxycations