Alcohols 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). 

S. Venkatesan, A. S. Kumar, J.-M. Zen, A Rugged Lead-ruthenate pyrochlore Membrane Catalyst for Highly Selective Oxidation of Alcohols, J. Mol. Catal. A Chem. 250 (2006) 87-93. 

H. B. Friedrich, N. Singh, A Study of Poly (4-vinylpyridine)-Supported Ruthenate in the Oxidation of Alcohols, Catal. Lett. 110 (2006) 61—70. 

As mentioned above for RuO (1.2.7.10) and [RuO ]" (1.3.4.6) there are reports of Ru-catalysed oxidations for which the nature of the active catalyst or catalyst precursor is unclear but is probably predominantly [RuO ]. Electronic and Raman spectroscopy have been used to establish the nature of the catalytic species, but incorrectly fran.y-[Ru(0H)2(0)3] " rather than [RuO ] " was the formula ascribed to the ruthenate solute [212, 222]. Examples in which [RuO ] is the catalytic species include oxidations of nucleosides by RuCl3/K3(S20g)/aq. M KOH (Fig. 2.11) [547], and of primary alcohols oxidised to aldehydes RuClj or Ru03/Na(C10)/aq. base [551]. 

Potassium dioxalato-nitroso-pyridino-ruthenate yields with quinine hydrochloride twro isomeric quinine salts, a dextro- and a lsevo-modi-fication, which differ in solubility. The isomers are separated by fractional crystallisation. The dextro-salt is the less soluble, and crystallises in long needles with rotation of [a]n+252 in a solution of equal volumes of alcohol and chloroform. The ltevo-salt is more soluble, 1 Charoimat, Oonipt. rend., 1924, 178, 1423. 

Lead tetraacetate-Manganese(II) acetate, 157 Osmium tetroxide, 222 Potassium ruthenate, 259 Samarium(II) iodide, 270 reagents specific for primary alcohols Osmium tetroxide, 222 reagents specific for benzylic alcohols Cetyltrimethylammonium permanganate, 69... 

Polymer supported sodium ruthenate is able to catalyze the oxidation of alcohols with iodosobenzene or tetrabutylammonium periodate in CH2CI2.8 It is not clear whether the primary oxidant is ruthenate or perruthenate. 

In fact, equilibria between ruthenium ions in different oxidation states in aqueous solution add complexity to the mechanistic analysis of these oxidations. Thus, Burke and Healy presented mechanistic evidences9 suggesting that putative oxidations of alcohols with ruthenate ion are in fact produced by perruthenate originated by dismutation of ruthenate. 

Griffith, Ley et al.n discovered that, in variance with the instability and complex behaviour of perruthenate and ruthenate ions in aqueous solution, TPAP in organic media is quite stable and behaves as a very good oxidant for alcohols. Normally, it is employed in catalytic quantities in dry CH2CI2 with addition of TV-methylmorpholine /V-oxide (NMO) as the secondary oxidant. Catalytic TPAP in the presence of NMO is able to oxidize alcohols to adehydes and ketones under very mild conditions in substrates adorned by complex functionalities, and it has become one of the routine oxidants for alcohols in most Synthetic Organic Chemistry laboratories. 

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. 

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 Aquo-chlor-ruthenate, K2Ru(OH2)CI5, results1 on boiling a slightly acidified solution of potassium chlor-ruthenite, K2RuC15, with alcohol as also by heating the hydrated sesquioxide with hydrochloric acid and alcohol. 

Caesium Aquo-chlor-ruthenate, Cs2Ru(OH2)Cl3, is an interesting salt inasmuch as it is isomeric with the first chlor-ruthenite to be discovered, namely, Cs2RuCl5. HaO vide supra). It is obtained 3 as a buff precipitate on adding alcohol to the blue filtrate from electrically reduced solutions of ruthenium trichloride and caesium chloride. On crystallisation, rose-coloured prisms are obtained. These are soluble in water, and may be recrystallised unchanged from aqueous hydrochloric acid. 

Oxidation. This reagent effects the oxidation of primary alcohols to carboxylic acids under alkaline conditions. Exposure of the hydroxy carboxylate salt derived from 1 to sodium ruthenate in 1 (V sodium hydroxide solution afforded the diacid 2, in which the substituent at Ce had epimerized to the thermodynamically favored 8-configuration. ... 

Sodium ruthenate, Na2Ru04, is prepared in situ from ruthenium tetroxide (in solution in carbon tetrachloride) and 1 M sodium hydroxide by shaking for 2 h at room temperature. The reagent remains in the aqueous layer, which acquires bright-orange color [937]. It oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones and is comparable with but stronger than potassium ferrate [937]. 

Potassium ruthenate, K2RUO4, is prepared in situ from ruthenium trichloride and aqueous persulfate. The reagent catalyzes persulfate oxidations of primary alcohols to acids, secondary to ketones, and primary amines to nitriles or acids at room temperature in high yields [196],... 

Sodium ruthenate [957] and potassium ruthenate [196 oxidize allylic and benzylic alcohols to carboxylic acids at room temperature. Cinnamyl alcohol is transformed into cinnamic acid with sodium ruthenate in 1 M sodium hydroxide at 10 °C after 1 h in 70% yield [957]. In oxidations with potassium ruthenate, only catalytic amounts can be used in the presence of a persulfate, which reoxidizes the reduced ruthenium salt [196. ... 

High yields of ketones result from the gentle oxidation of alcohols with compounds of ruthenium. Ruthenium tetroxide oxidizes cyclohexanol to cyclohexanone in carbon tetrachloride at room temperature in 93% yield [940], Instead of the rather expensive ruthenium tetroxide, which is required in stoichiometric amounts, catalytic amounts of ruthenium trichloride may be used in the presence of sodium hypochlorite as a reoxidant with the same results [701]. Sodium ruthenate [937] and potassium ruth-enate [196], which are prepared from ruthenium dioxide and sodium periodate in sodium hydroxide and from ruthenium trichloride and potassium persulfate, respectively, also effect oxidations to ketones at room temperature. 

The mechanism of the Ley oxidation is complex and the exact nature of the species involved in the catalytic cycle is unknown. The difficulty in establishing an exact mechanism arises from the fact that the complexes of Ru ", Ru ", Ru , Ru and Ru are all capable of stoichiometrically oxidizing alcohols to carbonyl compounds. The TRAP reagent can oxidize alcohols stoichiometrically as a three-electron oxidant and can also be used as a catalyst when a co-oxidant is present (e.g., NMO, TMAO, or hydroperoxides). Data suggests that the oxidation proceeds via the formation of a complex between the alcohol and TRAP (ruthenate ester). It was also found that the stoichiometric oxidation of isopropyl alcohol with TRAP is autocatalytic and the catalyst is suspected to be colloidal RUO2. Small amounts of water decrease the degree of autocatalysis. This observation is supported by the finding that the addition of molecular sieves improves the efficiency of the reaction. 

Electro-Organic Oxidation Properties. Table I lists some results for the electro-oxidation of primary alcohols and propylene on leadsubstituted lead ruthenate. Propylene was cleaved with nearly 100% selectivity to acetic acid and CO2. In borate buffer at pH 9 the oxidation of propylene also occurred, and the selectivity to acetate and CO2, based on the amount of carbonate isolated, was also close to 100%. Dissolved ethanol and propanol were both converted with high selectivity to the corresponding carboxylic acid salts in alkaline electrolyte. In contrast, Pt black (also shown in Table I) oxidized ethanol to CO2 and then rapidly deactivated. 

LEAD RUTHENATES OXIDIZE ALCOHOLS AND CLEAVE PROPYLENE ... 

Table II shows results for the electro-oxidation of secondary alcohols and ketones. In alkaline electrolyte, secondary butanol was not oxidized to methyl ethyl ketone but was cleaved to acetate. Similarly methyl ethyl ketone was cleaved to acetate, although some CO2 and propionate formed, indicative of cleavage on the other side of the carbonyl group. Butanediol (2 ) went to acetate yielding less CO2. At pH 9 in borax buffer 2 Trtanol went exclusively to methyl ethyl ketone at 89% conversion, suggesting that enolization in alkali is a necessary part of the cleavage process. Cyclohexanol and cyclohexanone were both converted to adipic acid. Figure 12 summarizes the various types of electro-organic oxidations, thus far discussed, which are observed to occur on lead ruthenate in alkaline electrolyte.

A particularly satisfactory ruthenium catalyst was prepared as follows. Commercial ruthenium powder was fused with a mixture of potassium hydroxide and potassium nitrate (1 part ruthenium, 10 parts potassium hydroxide, 1 part potassium nitrate) preferably in a silver crucible and stirred with a silver spatula. Pusion was complete after 1 to 2 hours. After cooling, the fused mass was dissolved in water a deep red solution of potassium ruthenate resulted, which was heated to boiling. Methyl alcohol was added dropwise to the boiling solution. The reduction of potassium ruthenate to ruthenium dioxide began with the addition of the first drops and went rapidly to completion. The precipitate settled after a few hours. It was washed on a fritted glass plate, first with water acidified with nitric acid and then with distilled water. Finally the catalyst was dried at 110°C. The reduction to metal proceeds just as smoothly under synthesis conditions as by a hydrogen treatment, which latter is usually required with catalysts of the iron group. 

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