Per-ruthenates

The oxidation of alcohols to carbonyl compounds has been studied by several authors and a variety of methods have been used. Papers concerned vith such oxidations are illustrated (Scheme 3.26). Good results have been obtained using pyridinium chlor-ochromate (PCC) adsorbed onto silica gel for the selective oxidation of unsaturated substrates e.g. terpene [135] and furanyl derivatives [136]. Steroidal homoallylic alcohols can be converted to the corresponding 4-ene-3,6-diones using tetrapropylammo-nium per-ruthenate (TPAP) in catalytic amounts [137]. In this case, the oxidising agent is N-methyl morpholine N-oxide (NMO). 

Griffith, W., S. Ley, G. Whitcombe, and A. White (1987) Preparation and use of tetra-n-butylammonium per-ruthenate (TBAP reagent) and tetra-n-propylammonium per-ruthenate (TpAP reagent) as new catalytic oxidants for alcohols. Chemical Communications, 1625-1627. 

Fig. 17.15. Mechanism of the TPAP oxidation of an atcohot to an aldehyde (TPAP stands for tetrapropylammonium per-ruthenate). The effective oxidant is a Ru(VII) oxide, other than in Figure 17.12 where a Ru(VIII) oxide is employed. Here, the stoichiometrically used oxidizing agent is N-methylmorpholin-/V-oxide ("NMO"), whereas in Figure 17.12 NaI04 is used.

If no iV-methylmorpholine-iV-oxidc were added the ruthenium(V) acid would be converted into RuOz. In that case, Ru(VII) would be a three-electron oxidizing agent just like Cr(VI) (Figure 14.10). Such a conversion of Ru(V) into Ru(IV) could in principle occur, since Ru(V) also oxidizes alcohols. This oxidation presumably would proceed via an a-hydroxylated radical as discussed for the Cr(IV) oxidation of alcohols (Fig 14.10, center). Yet, there is no indication for such a radical pathway to occur when the reaction is carried out in the presence of A-methylmorpholine-A-oxide. Hence, it appears that A-methylmorpholine-A-oxide reoxidizes the ruthenium(V) acid to per-ruthenate faster than the ruthenium(V) acid could attack an alcohol molecule. 

Potassium per-ruthenate, KRu04, is analogous to, but not isomor-phous with, potassium permanganate. 

Ruthenium tetroxide is permanent when kept in sealed tubes in the dry state and protected from light. Exposed to light it assumes a brown colour, but the brown product is soluble in alkali, yielding a ruthenate. Presumably the coloration is due to partial reduction. Owing to its ready reduction by organic substances whereby a black precipitate of finely divided ruthenium is obtained, potassium per-ruthenate has been found useful for histological microscopy. ... 

In solution potassium ruthenate is of a deep orange colour, but in dilute solution assumes a green colour, due to formation of per-ruthenate. Hydrochloric acid effects the precipitation of an oxide, chlorine being simultaneously evolved. The solution stains the skin black owing to separation of oxide. 

Sodium Per-ruthenate, NaRu04.H20, is prepared in an analogous manner to the potassium salt as small, black, crystalline lamellae. It is considerably more soluble in water than the potassium analogue, and yields a dark green solution. Heated to 440° C. water and oxygen are evolved.4... 

But +b occurs in ruthenium and osmium compounds, such asCsRuFgand NaOsFg, and +7 in the per-ruthenates, KRUO4. Rh is found in the ion [Rh(PhNC)4]+. Ir and Ir occur in Na IrFg and IrF respectively, and Ir and Pd in Ir (NH3)5, (Watt, 1953) and Pd(NH3)4. Complexes in which the metal has charge +4 and a half-filled d shell increase in stability on passing downwards from Co through Rh to Ir 6 co-ordination is usual. 

Polymer-supported per-ruthenate reagents have some advantages over the solution-phase chemistry, not least in the ease of purification by simple filtra-... 

TPAP can be used as an effective catalyst for the aerobic oxidation of alcohols to give the corresponding carbonyl compounds [Eq. (34)] [71]. A polymer supported per-ruthenate (PSP) and a perruthenate immobihzed within MCM-41 can be used for heterogeneous oxidation of alcohols [72]. 

The orange-colored solution, containing, among other things, potassium ruthenate, was treated with nitric acid, whereupon a black precipitate of osmium dioxide containing from fifteen to twenty per cent of ruthenium oxide was thrown down as a velvety deposit. Klaus distilled this with aqua regia, taking care to condense the osmium tetroxide. The residue... 

Luminescence titrations further demonstrate that the ruthenated duplex behaves as a 15 mer bearing one intercalator (76). As free [Ru(phen)2(dppz)]2+ is added to a solution of unmetallated 15 mer duplex, the luminescence increases linearly until the emission reaches saturation at about three equivalents of ruthenium(II) per duplex, consistent with competitive binding of [Ru(phen)2(dppz)]2+ to the 15-mer duplex and an average binding site size of a little more than four base pairs. When the analogous experiment is conducted with the ruthenated duplex, saturation of luminescence occurs after almost two equivalents of [Ru(phen)2(dppz)]2+ are added. This comparison indicates that the covalently bound ruthenium(II) complex is not displaced by additional intercalators. 

The noise index NI in dB is measured by a band-pass filter (bandwidth of 1 kHz between 618 Hz and 1618 kHz) [51]. A noise index NI = 0 dB corresponds to 1 pV noise voltage per 1 V DC voltage. For ruthenate-based TFRs, the noise index exhibits an increase proportional to the sheet resistivity and a decrease proportional to the resistor area [47,52]. 

---