Perruthenate ion

Just as Ru(VIII) chemistry is dominated by RuO, Ru(VII) chemistry is dominated by salts of the perruthenate ion, [RuO ] , particularly TPAP, ( Pr N)[RuO ]. Another similarity between RuO and [RuO ]" in solution is their reluctance to expand their coordination spheres beyond tetrahedral. The principal salts are listed first. There are few Ru(VII) complexes apart from [RuO ] . 

As expected, inorganic perruthenates, like sodium perruthenate (NaRu04) or potassium perruthenate (KRu04), are soluble in water and insoluble in apolar organic solvents. On the other hand, the perruthenate ion (Ru04) is unstable in aqueous solution because it produces the oxidation of water according to the following Equation.55... 

A milestone in the routine employment of perruthenate in the oxidation of alcohols was established with the publication by Griffith, Ley et al. in 1987 on the catalytic use of tetra- -propylammonium perruthenate (TPAP).11 The presence of the tetra- -propylammonium cation renders this compound soluble in apolar media and allows the existence of a high concentration of perruthenate ion in organic solvents. The tetra- -propylammonium perruthenate is easily prepared and can be employed catalytically in CH2CI2 solution in the oxidation of alcohols to ketones and aldehydes, using /V-methyl morpholine A-oxide (NMO) as the secondary oxidant. 

Sodium hypochlorite can be used as secondary oxidant in the presence of TPAP but in this case the primary oxidant is reported to be Ru04, instead of the perruthenate ion.70 This oxidizing system is much more energetic than the standard TPAP/NMO system and is able to transform ethers into esters.71... 

It is open to speculation whether the same mechanism would apply to the more common oxidations with TPAP, in which the perruthenate ion operates in an apolar environment. 

Functional groups able to withstand TPAP oxidations include esters, ethers, amides, epoxides, alkynes, urethanes and even alkenes.61b It is quite remarkable that alkenes are resistant to TPAP because they are known to react with aqueous perruthenate ions.79... 

The effective oxidant in the TPAP oxidation of alcohols is the perruthenate ion, a Ru(VII) compound. This compound is employed only in catalytic amounts hut is continuously replenished (see below). The mechanism of the alcohol — aldehyde oxidation with TPAP presum-... 

Mechanism The mechanism of the oxidation of alcohols with TPAP corresponds to the non-radical Cr(VI) oxidation. The perruthenate ion reacts with alcohols to form the ester of ruthenium(VII) acid A, which on elimination gives an aldehyde and reduced ruthe-nium(V) acid B (Scheme 7.10). It is believed that NMO reoxidizes the ruthenium(V) acid to perruthenate faster than the ruthenium(V) acid could attack on alcohol molecule. 

The difficulties associated with the chromium reagents (stoichiometric amounts required harsh conditions low selectivity) [95-97] led to the development of polymer-supported perruthenate 124 by Ley et al. [98]. These efforts were built upon a large body of experience with soluble salts of the perruthenate ion, e.g., tetra-propylammonium permthenate (TRAP) [99, 100]. The reagent 124 was prepared by prolonged exposure of anion-exchange resin (125) to an aqueous solution of potassium perruthenate 126. 

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