[14931-42-1] · K2O4Ru · Potassium Ruthenate · (MW 243.27)
(powerful oxidant capable of oxidizing primary alcohols to carboxylic acids and secondary alcohols to ketones;1 in combination with persulfate can oxidize nitro compounds2 and activated halides3 and will selectively oxidize allylic alcohols under phase-transfer conditions4)
Alternate Name: potassium tetraoxoruthenate(VI).
Physical Data: d 3.07 g cm-3.
Solubility: sol H2O; insol organic solvents.
Form Supplied in: green-black crystals (anhyd), deep red crystals (hyd); not commercially available.
Preparative Method: K2RuO4 has been prepared according to the method of Krauss.5 To molten KOH in a nickel crucible is added equimolar amounts of Ru and KNO3. The mixture is cooled, dissolved in H2O, and K2RuO4 crystallized by slow evaporation over P2O5.
Handling, Storage, and Precautions: unstable in all but basic media and should be prepared and used as such. The toxicological properties of Ru have not been thoroughly investigated.
K2RuO4 functions as a two-electron oxidant in the oxidation of aliphatic alcohols, allylic alcohols, diols, aldehydes, halides, and nitro compounds. The reactions are generally performed in aqueous base using catalytic [RuO4]2- with persulfate (S2O82-) as the secondary oxidant. A typical workup procedure involves either extraction of the solution with ether if the product is a ketone, or acidification of the solution to pH 2 with H2SO4, followed by ether extraction if carboxylic acids are produced. The organic extracts are combined and concentrated before passage through a Celite pad to remove any residual Ru. The solvent is then removed under reduced pressure to give the pure ketone or carboxylic acid. Alternatively, phase-transfer conditions1,4 may be used to effect the oxidation of alkyl halides and alcohols.
Primary alcohols are oxidized to carboxylic acids by the [RuO4]2-/S2O82- system (eq 1).1,3 Under phase-transfer conditions3 this reagent gives aldehydes in good yields (eq 2).
The rate of oxidation of a series of substituted benzyl alcohols varies according to the electron-withdrawing or -donating capacity of the substituent; the oxidation rate decreases in the order 4-NO2 > 4-H > 4-OMe.1 Similar rate effects have been noted for oxidations with CrO36 and [FeO4]2-.7
Secondary alcohols are oxidized to ketones by catalytic ruthenate in a similar manner (eq 3).1,3 However, oxidation of secondary alcohols under phase-transfer conditions results in poor yields of the desired ketone.3
Oxidation of mandelic acid and hydrobenzoin gives benzoic acid arising from additional carbon-carbon bond cleavage.1 However, this only occurs with oxidatively sensitive substrates, as oxidation of methyl mandelate proceeds to give the corresponding glyoxylate with no such cleavage.1
Unlike other two-electron oxidants such as Chromium(VI) Oxide, Ruthenium(VIII) Oxide and Osmium Tetroxide,8 catalytic ruthenate exhibits low reactivity towards double bonds and as such is particularly useful for the oxidation of allylic and homoallylic alcohols.1,9 Thus allyl alcohol1 and trans-cinnamyl alcohol (eq 4)1,9 are cleanly oxidized to the corresponding unsaturated acids.
These are oxidized to their corresponding dicarboxylic acids under similar conditions (eq 5).1,10
As expected, aldehydes are oxidized to carboxylic acids under both aqueous1,3,9 and phase-transfer3 conditions.
The best known methods for the oxidation of alkyl halides are the noncatalytic Sommelet or Kornblum reactions.11 Ruthenate may be used catalytically to effect the (large scale) oxidation of allylic and benzylic primary alkyl halides to acids and secondary halides to ketones.3
Catalytic ruthenate oxidizes primary nitroalkanes to carboxylic acids3 and on one occasion has been used for the production of a ketone from a secondary nitro compound (eq 6).2
Although several reagents for the selective oxidation of allylic and benzylic alcohols have been reported,12 these oxidants suffer from variable selectivity and lack of general applicability. For example, the widely used reagent Manganese Dioxide oxidizes aliphatic alcohols in certain cases13 and even CrVI reagents such as Bis(tetrabutylammonium) Dichromate and 4-(Dimethylamino)pyridinium Chlorochromate oxidize primary and secondary alcohols to a significant extent.12b,c However, catalytic ruthenate under phase-transfer conditions is a selective oxidant for allylic and benzylic alcohols in the presence of aliphatic alcohols (eq 7).4
Steven V. Ley & Ray Leslie
University of Cambridge, UK