Ruthenium carboxylic acids

Internal alkynes are oxidized to acytoins by thalliuin(III) in acidic solution (A. McKil-lop, 1973 G.W. Rotermund, 1975) or to 1,2-diketones by permanganate or by in situ generated ruthenium tetroxide (D.G. Lee, 1969, 1973 H. Gopal, 1971). Terminal alkynes undergo oxidative degradation to carboxylic acids with loss of the terminal carbon atom with these oxidants. 

The conversion of primary alcohols and aldehydes into carboxylic acids is generally possible with all strong oxidants. Silver(II) oxide in THF/water is particularly useful as a neutral oxidant (E.J. Corey, 1968 A). The direct conversion of primary alcohols into carboxylic esters is achieved with MnOj in the presence of hydrogen cyanide and alcohols (E.J. Corey, 1968 A,D). The remarkably smooth oxidation of ethers to esters by ruthenium tetroxide has been employed quite often (D.G. Lee, 1973). Dibutyl ether affords butyl butanoate, and tetra-hydrofuran yields butyrolactone almost quantitatively. More complex educts also give acceptable yields (M.E. Wolff, 1963). 

The extent of coupling is also influenced by the solvent. In the hydrogenation of aniline over ruthenium oxide, coupling decreased with solvent in the order methanol > ethanol > isopropanol > t-butanol. The rate was also lower in the lower alcohols, probably owing to the inhibiting effect of greater concentrations of ammonia (44). Carboxylic acid solvents increase the amount of coupling (42). 

Hydroxy-5-oxo-3,5-seco-4-norandrostane-3-carboxylic acid has been prepared by ozonolysis of testosterone2-4 or of testosterone acetate, followed by alkaline hydrolysis,5 and by the oxidation of testosterone acetate with ruthenium tetroxide.9... 

Ruthenium complexes containing this ligand are able to reduce a variety of double bonds with e.e. above 95%. In order to achieve high enantioselectivity, the reactant must show a strong preference for a specific orientation when complexed with the catalyst. This ordinarily requires the presence of a functional group that can coordinate with the metal. The ruthenium-BINAP catalyst has been used successfully with unsaturated amides,23 allylic and homoallylic alcohols,24 and unsaturated carboxylic acids.25... 

Ruthenium tetroxide is a potent oxidant, however, and it readily attacks carbon-carbon double bonds.19 Primary alcohols are oxidized to carboxylic acids, methyl ethers give methyl esters, and benzyl ethers are oxidized to benzoate esters. 

Most ruthenium-initiated ROMP studies have been performed using (233) and strained cyclo-olefinic monomers such as norbornene688 and cyclobutenes,689 although several reports on the polymerization of 8-membered rings have also appeared.690-692 A wide range of functionalities are tolerated, including ethers, esters, amines, amides, alcohols, carboxylic acids, and ketones. 

Mild allylic oxidation of the A-2-crotyl-substituted thiadiazolidinone 1,1-dioxide 140 by sodium metaperiodate/ ruthenium trichloride hydrate (RuC13) gave the aldehyde 141. Excess oxidizing agent afforded the carboxylic acid 142 (Equation 26) <1999EJ02275>. 

Although related reactions have also been done under low pressures/ very low rates of product formation are observed (8/10/11). We have found/ however, that a ruthenium carbonyl catalyst is quite active for converting H2/CO to methanol under moderate pressures (below 340 atm). More significantly, we also discovered that an ethylene glycol product could be obtained from this catalyst by use of carboxylic acid promoters or solvents (12) This remarkable and intriguing promoter effect deserved, we felt, further mechanistic investigation... 

In this paper we disclose the syngas homologation of carboxylic acids via ruthenium homogeneous catalysis. This novel homologation reaction involves treatment of lower MW carboxylic acids with synthesis gas (C0/H2) in the presence of soluble ruthenium species, e.g., Ru02, Ru3(C0)12, H4Ru4(C0)12, coupled with iodide-containing promoters such as HI or an alkyl iodide (1). 

The scope and mechanism of carboxylic acid homologation is examined here in relation to the structure of the carboxylic acid substrate, the concentrations and composition of the ruthenium catalyst precursor and iodide promoter, synthesis gas ratios, as well as 13C labelling studies and the spectral identification of ruthenium iodocarbonyl intermediates. 

Deuteration studies with acetic acid-d4 (99.5% atom D) as the carboxylic acid building block, ruthenium(IV) oxide plus methyl iodide-d3 as catalyst couple and 1/1 (C0/H2) syngas, were less definitive (see Table III). Typical samples of propionic and butyric acid products, isolated by distillation in vacuo and glc trapping, and analyzed by NMR, indicated considerable scrambling had occurred within the time frame of the acid homologation reaction. 

Some of the more important features of our novel syngas homologation of aliphatic carboxylic acids, catalyzed by ruthenium, include ... 

Metatheses of 1,7-octadienes containing various functional groups are catalysed by ruthenium carbene complexes of the type 248. For instance, the alcohol 249 (R = CH2OH), the aldehyde 249 (R = CHO) and the carboxylic acid 249 (R = CO2H) are all converted into the corresponding cyclohexenes 250 in 82-88% yields (equation 127) and the heterocycles 252 (n = 0, 1 or 2) are efficiently produced from the amides 251 (equation 128)123. 

Ruthenium catalysts have also been used in this context.200,201 In particular, the cationic ruthenium complex, CpRu(CH3CN)3PF6, in conjunction with carboxylic acid ligand 3, has been used to achieve the remarkably chemoselective allylation of a variety of alcohols via dehydrative condensation with allyl alcohol (Equation (50)).202 It is worth noting that this transformation proceeds with 0.05 mol% catalyst loading and does not require the use of excess allyl alcohol. 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

Undheim [24] described the stereoselective synthesis of cyclic 1-amino-l-carboxylic acid using ruthenium-catalyzed enyne metathesis. His plan is shown... 

BINAP itself has been shown to be effective for the reduction of a,/ -unsatu-rated carboxylic acids [8, 36, 177, 215-220], but Hg-BINAP often provides higher ee-values [193, 194]. The ruthenium complex with P-Phos provides high selectiv-... 

Aldehyde 244 reacts with manganese dioxide and sodium cyanide in ethanol to give ethyl ester 245 (Scheme 19), while oxidation of alcohol 12 with sodium peroxodisulfate in the presence of a catalytic amount of ruthenium chloride furnishes the carboxylic acid 246 (Scheme 19) <1998CPB287>. 

---