Ruthenium-catalyzed homologation

Wender and coworkers conclude that cobalt-catalyzed benzyl alcohol homologation involves the intermediate formation of car-bonium ions (8). However, since the methyl cation (CH3+) is unstable and difficult to form (9), it is more likely that methanol homologation to ethanol proceeds via nucleophilic attack on a protonated methyl alcohol molecule. Protonated dimethyl ether and methyl acetate forms have been invoked also by Braca (10), along with the subsequent formation of methyl-ruthenium moieties, to describe ruthenium catalyzed homologation to ethyl acetate. 

The principal competing reactions to ruthenium-catalyzed acetic acid homologation appear to be water-gas shift to C02, hydrocarbon formation (primarily ethane and propane in this case) plus smaller amounts of esterification and the formation of ethyl acetate (see Experimental Section). Unreacted methyl iodide is rarely detected in these crude liquid products. The propionic acid plus higher acid product fractions may be isolated from the used ruthenium catalyst and unreacted acetic acid by distillation in vacuo. 

I/Ru ratio critical, 34 112 proposed mechanism, 34 112 ruthenium-carbonyl complexes, 34 113 species involved, 34 110-113 -catalyzed homologation, 34 115 proposed mechanism, 34 115 compensation behavior of, 26 285, 286 complex catalyst... 

In 1981, Texaco announced the ruthenium/H2/CO-catalyzed homologation of carboxylic acids. Homologation refers to a chain-extension reaction that increases the carbon number of the carboxylic acid see Homologation Reaction. The particular reaction that was initially studied was the conversion of acetic acid to propionic acid. The proposed mechanism shown in Scheme 22 is based on a Ru VRtf cycle, similar to that suggested by Knifton and coworkers at Texaco. 

Nevertheless there remains several possible routes to the formation of ethyl esters from CO/H2 (see Scheme A). The direct production of ethanol (path c) can be discounted in our systems since both methanol and ethanol are generated in significant concentrations at high propionic acid conversions (see Table VI, expt. 2 and 8). Path (d) appears less likely in view of the relatively slow rates of I-free, ruthenium-catalyzed, methanol homologation (55), relative to esterification. Path (a), also eq. 24, and (b), however, could represent parallel reaction paths where at high acid levels (and therefore low acid conversions) the ester route (a) might be expected to predominate (we see little or no evidence for methanol under those conditions). Preliminary results with stronger aliphatic acid coreactants, such as trifluoroacetic acid, are also in accord with these conclusions. 

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

To achieve, then, high acetic acid selectivity directly from synthesis gas (eq. 1) it is necessary to balance the rates of the two consecutive steps of this preparation - ruthenium-carbonyl catalyzed methanol formation (10) (Figures 2 and 5) and cobalt-carbonyl catalyzed carbonylation to acetic acid (Figure 6) - such that the instantaneous concentration of methanol does not build to the level where competing secondary reactions, particularly methanol homologation (7, H), ester homologation (12, 13), and acid esterification (1 ), become important. 

It has been reported ( ) that homogeneous ruthenium- or cobalt-iodide-based complexes catalyze the homologation of esters of carboxylic acid to their next higher homologues, for instance ... 

Methanol homologation - The strong acid hydride HRu(00)3X3, present in the catalytic ruthenium iodide solutions for the methanol homologation, is able to directly protonate the substrate and produce the methyl and successively the acetyl intermediates for the homologation to ethanol (eq. 2). It also catalyzes etherification to dimethyl ether (eq. 3). 

The ruthenium-iodide-catalyzed carbonylation of alcohols is greatly complicated by the facility with which the same system catalyzes the competitive water-gas shift and homologation reactions. The resulting inability to totally isolate the reaction(s) of interest necessitates that conclusions are based on observations which are less direct than in other systems discussed in this article. Further work, aimed at determining the nature of the proposed transformations, perhaps through the use of model compounds, would appear to be required to unravel the finer mechanistic details of the system. 

Methanol homologation catalyzed by ruthenium has been studied by Braca etal. [86, 89, 90]. Catalyst systems such as Ru(acac)3/Nal and Ru(C0)4lj/NaI have been shown to be active. In contrast to cobalt catalysts, no reaction occurs in the absence of 1" and a proton supplier is needed. As can be taken from Table XI, the reaction is higidy selective to C -products and no higlter products are formed. Due to the high hydrogenation activity of ruthenium, however, methane and ethane arc formed as side products in considerable amounts as well as dimethyl ether. Thus, the overall yield of ethanol is limited. The same catalyst systems have also been shown to be active in the homologation/carbonylation of ethers and esters. 

Methanol homologation using carbon dioxide catalyzed by ruthenium-cobalt bimetallic complex system... 

The ruthenium-cobalt bimetallic complex system catalyzes the homologation of methanol with carbon dioxide and hydrogen in the presence of iodide salts. A synergistic effect is found between these two metals. The yield of ethanol is also affected by the Lewis acidity of the iodide salt, lithium iodide being most effective. The reaction profile shows that methanol is homologated with CO formed by the hydrogenation of CO2. 

Ruthenium-cobalt bimetallic complex catalyzed methanol homologation with CO2 ... 

There are two possible pathways to homologate methanol with carbon dioxide the CO2 insertion path and CO insertion path (Scheme 2). As for the former, Fukuoka et al. reported that the cobalt-ruthenium or nickel bimetallic complex catalyzed acetic acid formation from methyl iodide, carbon dioxide and hydrogen, in which carbon dioxide inserted into the carbon-metal bond to form acetate complex [7]. However, the contribution of this path is rather small because no acetic acid or its derivatives are detected in this reaction. Besides, the time course... 

Diphosphine ligands of the type Ar2P(CH2)nPAr2 (n = 2 4) were originally found by Moloy and Wegman [85,86] to be effective in the rhodium-complex-catalyzed reductive carbonylation of methanol to acetaldehyde (Equation (10)) when synthesis gas (CO + H2) was used instead of pure CO as the feed gas. With a ruthenium trichloride cocatalyst, in situ hydrogenation of the aldehyde to ethanol resulted in the overall homologation reaction shown in Equation (11). 

A number of researchers have now examined ruthenium-cobalt catalyzed methanol homologation to ethanol (60-62). Doyle concludes (56) that the ruthenium and cobalt moieties act independently, with the cobalt species responsible for the formation of 2-oxygenates like acetaldehyde and ruthenium reducing the aldehyde intermediate to ethanol. However, in our work - even with CO-rich syngas (8) -acetaldehyde is never more than a trace product. Mixed ruthenium-cobalt carbonyls are now well documented (56,62), but in these melt studies there is no direct spectroscopic evidence for their formation. 

Ruthenium catalysts also have been reported to catalyzed the homologation of methylacetate to ethyl acetate [60]. 

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