Methanol homologation, ruthenium

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. 

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. 

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). 

Table II. Methanol homologation with ruthenium catalysts...

Besides cobalt, other catalyst metals haw also been examined for the methanol homologation reaction [26], More detailed studies have been reported for iron [85]. ruthenium [86], and rhodium [87]. whereas little is known about nickel [88]. osmium, iridium, and palladium [5]. 

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... 

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

The synergistic effect of ruthenium and rhodium in methanol homologation was observed at 100 atm synthesis gas pressure, whereas ruthenium or rhodium chloride alone is inactive for ethanol synthesis.No enhancement of ethanol production was observed with the mixed-metal compounds [HRuRh3(CO)i2], [HRuRh3(CO)io(PPh3)2], [H2Ru2Rh2(CO),2], and [PPN][RuRh5(CO),6] as catalyst precursors. This is consistent with the cluster decomposition found to occur in all the experiments. ... 

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. 

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. 

Alcohol Homologation Solvent and promoter effects on the cobalt carbonyl catalysed methanol homologation have been studied under synthesis gas pressure.The main product in a methanol/hydrocarbon two-phase system is 1,1-dimethoxyethane (ca. 70 selectivity).Using similar iodide promoted cobalt catalysts, R2C 0Me)2 and dimethylcarbonate are converted to acetaldehyde with up to 87 selectivity.Ruthenium in the presence of Co, 12 and dppe improves the ethanol selectivity in the homologation of dimethylether. Best results are achieved in inert solvents with high dielectric constants, e.g. sulfolane (e = 44), and with BF3 as activator. 

G Braca, C. Sbrana, G. Valentmi, C- Andrich and G. Gregorio Carbonytation and Homologation of Methanol, Methyl Ethers and Esters in the presence of Ruthenium Catalysis (Fundamental Research in Homogeneous Catalysis v. 3. ed.. M. Tsutsui), pp. 221 238. Plenum Prevs (1979). 

When ethanol is produced, methanol is formed in the first step, and is then homologated. Dombek reported that ruthenium complexes are effective for the production of ethylene glycol at 340 atm and below, especially in the presence of iodide (Eq. 11.4) [11]. 

Homologation is the one-carbon extension reaction of organic compounds such as alcohols and carboxylic esters, and is very important. Cobalt, rhodium, and ruthenium complexes are known to be efficient catalysts. Methanol and methyl ester can be converted to ethanol and ethyl ester, respectively, using Ru/F [28] and Ru/Co [29] catalysts (Eq. 11.9). 

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. 

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). 

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