Ruthenium aldehyde hydrogenation

Palladium is usually the prefeired metal of choice for aromatic aldehyde hydrogenation in neutral non-polar solvents such as hexane, DMF, or ethyl acetate (5-100 °C and 1-10 bar) although ruthenium, which is less active, can be considered and run in aqueous alcohol at similar temperatures and pressures. If higher pressures are accessible ruthenium may be preferable because of its lower (historical) cost. Its use has recently been reviewed [4]. Although platinum and rhodium could... 

Three pathways for the ruthenium-catalyzed hydrogenations of ketones, aldehydes, and a,(3-unsaturated aldehydes with several catalyst systems were studied the direct insertion, the migratory insertion, and the concerted hydrogen transfer highlighting the catalyst/substrate configurations that lead to stereoselective differentiation. The contribution of computational chemistry were essential to differentiate these mechanisms of ruthenium-catalyzed reactions and to rationalize the experimentally observed stereochemical outcome. 

Chemoselective Ruthenium-Catalyzed Hydrogenation of Ketones and Aldehydes. 53... 

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 most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Hydrogenation of substrates having a polar multiple C-heteroatom bond such as ketones or aldehydes has attracted significant attention because the alcohols obtained by this hydrogenation are important building blocks. Usually ruthenium, rhodium, and iridium catalysts are used in these reactions [32-36]. Nowadays, it is expected that an iron catalyst is becoming an alternative material to these precious-metal catalysts. 

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

Gordon used a household microwave oven for the transfer hydrogenation of benz-aldehyde with (carbonyl)-chlorohydridotris-(triphenylphosphine)ruthenium(II) as catalyst and formic acid as hydrogen donor (Eq. 11.43) [61]. An improvement in the average catalytic activity from 280 to 6700 turnovers h-1 was achieved when the traditional reflux conditions were replaced by microwave heating. 

The formation of metal-oxygen bonds has previously been found to occur for the stoichiometric hydrogenation of CO to methanol with metal hydrides of the early transition metals (20). Moreover, in ruthenium-phosphine catalyzed hydrogenation (with H2) of aldehydes and ketones, metal-oxygen bonded catalytic intermediates have been proposed for the catalytic cycle and in one case isolated (21,22). 

This finding is the consequence of the distribution of various ruthenium(II) hydrides in aqueous solutions as a function of pH [RuHCl(mtppms)3] is stable in acidic solutions, while under basic conditions the dominant species is [RuH2(mtppms)4] [10, 11]. A similar distribution of the Ru(II) hydrido-species as a function of the pH was observed with complexes of the related p-monosulfo-nated triphenylphosphine, ptpprns, too [116]. Nevertheless, the picture is even more complicated, since the unsaturated alcohol saturated aldehyde ratio depends also on the hydrogen pressure, and selective formation of the allylic alcohol product can be observed in acidic solutions (e.g., at pH 3) at elevated pressures of H2 (10-40 bar [117, 120]). (The effects of pH on the reaction rate of C = 0 hydrogenation were also studied in detail with the [IrCp (H20)3]2+ and [RuCpH(pta)2] catalyst precursors [118, 128].)... 

CO Subsequently a migratory insertion will take place. Oxidative addition of H2 will be faster at the electron rich metal centre and thus the aldehyde will form. Hydrogenation takes place at ruthenium (added as Ru3(CO)i2) as indeed catalyst systems containing cobalt only are known to give 3-hydroxypropanal as the product. 

Kinetic experiments on the hydrogenation of prenal and citral were first carried out in the batch reactor with variation of aldehyde concentration, hydrogen partial pressure, ruthenium concentration and reaction temperature (Table 7). Stirring was provided at 2000 rpm in order to be sure that the overall reaction rate was determined by kinetics. 

More recently, using the cyclometallated iridium C,(7-benzoate derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP, catalytic carbonyl crotylation employing 1,3-butadiene from the aldehyde, or alcohol oxidation was achieved under transfer hydrogenation conditions [274]. Carbonyl addition occurs with roughly equal facility from the alcohol or aldehyde oxidation level. However, products are obtained as diastereomeric mixtures. Stereoselective variants of these processes are under development. It should be noted that under the conditions of ruthenium-catalyzed transfer hydrogenation, conjugated dienes, including butadiene, couple to alcohols or aldehydes to provide either products of carbonyl crotylation or p,y-enones (Scheme 16) [275, 276]. 

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