Ruthenium tetroxide

Ruthenium tetroxide (RuO₄) is a yellow, diamagnetic tetrahedral ruthenium compound. As expected for a charge-neutral symmetrical oxide, it is quite volatile. The analogous OsO₄ is more widely used and better known. One of the few solvents in which it forms stable solutions is CCl₄.

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Preparation

RuO₄ is prepared by oxidation of ruthenium(III) chloride with NaIO₄. In a typical reactions featuring RuO₄ as the oxidant, many forms of ruthenium usefully serve as precursors to RuO₄, e.g. oxides hydrates or hydrated chloride.
Since RuO₄ will readily decompose explosively at slightly elevated temperatures, most laboratory do not synthesise it directly, nor is it commercially available through major chemical vendors such as Sigma-Aldrich. Most laboratories instead use an anionic derivative from a salt of "TPAP", or tetrapropylammonium perruthenate (Pr₄N⁺ RuO₄^-). TPAP is synthesized by oxidizing RuCl₃ to RuO₄^- by NaBrO₃ and countering it with the tetrapropylamine cation.

Properties and Uses

RuO₄ oxidizes virtually any hydrocarbon. For example, it will oxidize adamantane to 1-adamantanol. It is used in organic synthesis to oxidize alkynes to 1,2-diketones and primary alcohols to carboxylic acids. When used in this fashion, the ruthenium tetroxide is used in catalytic amounts and regenerated by the addition of sodium periodate to ruthenium(III) chloride and a solvent mixture of acetonitrile, water and carbon tetrachloride.

Because it is such an aggressive oxidant, reaction conditions are mild, generally room temperature. Although a strong oxidant, RuO₄ oxidations do not perturb stereocenters that are not oxidized. Illustrative is the oxidation of the following diol to a carboxylic acid:

Oxidation of epoxy alcohols also occurs without degradation of the epoxide ring:



Under milder condition, oxidative reaction yields aldehydes instead.

RuO₄ readily converts secondary alcohols into ketones. Although similar results can be achieved with other cheaper oxidants such as chromium(VI) based compounds or DMSO-based oxidants, RuO₄ is ideal when a very vigorous oxidant is needed but a mild conditions must be maintained.

RuO₄ readily cleaves double bonds to yield carbonyl products. Osmium tetroxide, a more familiar oxidant that is structurally similar to RuO₄, does not cleave double bonds.

In terms of practical details, the substrate to be oxidized is typically dissolved in solvent such as CCl₄, and acetonitrile is added as an aiding ligand to the catalytic cycle. Ether can then be added to precipitate and recover the ruthenium pre-catalyst.

Oxidative Catalyst and Mechanism

Although used as an oxidant, RuO₄ is occasionally used as a catalytic for other oxidant. For an oxidation of cyclic alcohols with RuO₄ as a catalyst and bromate as a base, RuO₄ first takes up an extra O and H:

RuO₄ + OH^- → HRuO₅^-

Then HRuO₅^- complexes with the cyclic alcohol and form a metal coordination complex (denoted C₁ here):

HRuO₅^- + (CH₂CH₂)_nCHOH → C₁ + OH^-

The Ru complex is then attracted by a bromate in which the oxidation of the coordinated alcohol take place:

C₁ + BrO₃^- → (CH₂CH₂)_nC=O + HRuO₅^-

In which HRuO₅^- is reformed as the catalyst, and the cyclic alcohol is oxidized into a cyclic ketone.

References

• Cotton, S. A. "Chemistry of Precious Metals," Chapman and Hall (London): 1997. ISBN 0-7514-0413-6
• Martin, V. S., Palazón, J. M., 'Ruthenium(VIII) Oxide', Encyclopedia of Reagents for Organic Synthesis 2001. (Article)
• Farmer, V., Welton, T., The oxidation of alcohols in substituted imidazolium ionic liquids using ruthenium catalysts Royal Society of Chemistry, 2002.
• Singh, B., Srivastava, S., Kinetics and Mechanism of Ruthenium tetroxide Catalysed Oxidation of Cyclic Alcohols by Bromate in a Base Transition Met. Chem., 1991, 16, 466-468.
• Courtney, JL., Swansbor, KF. Ruthenium tetroxide Oxidation Reviews of Pure and Applied Chemistry 1972, 22, 47-