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.

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&minus;). TPAP is synthesized by oxidizing RuCl3 to RuO4&minus; 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.