Hydrogenolysis of ethyl acetate to ethanol

Bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on siHca and prepared by reaction of M with tetra(butyl)tin display catalytic properties quite different from those of the monometallic catalysts. Indeed, they are very selective for the hydrogenolysis of ethyl acetate to ethanol [110, 112-115]. For example, while the... 

Hydrogenolysis of esters to aldehydes or alcohols needs high temperatures and high pressures. Moreover, it leads to the formation of acids, alcohols, and hydrocarbons. In contrast, bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on sihca are very selective for the hydrogenolysis of ethyl acetate into ethanol [181]. For example while the selectivity to ethanol is 12% with Ru/Si02, it increases up to 90% for a Ru-Sn/Si02 catalyst with a Sn/Ru ratio of 2.5 [182]. In addition, the reaction proceeds at lower temperatures than with the classical catalysts (550 K instead of temperatures higher than 700 K). The first step is the coordination of the ester to the alloy (Scheme 46), and most probably onto the tin atoms. After insertion into the M - H bond, the acetal intermediate decomposes into acetaldehyde and an ethoxide intermediate, which are both transformed into ethanol under H2. 

V-(9-Fluorenylmethoxycarbonyl)-0-(2,3,4-tri-0-benzoyl- 3-D-xylopyranosyl)-L-serine 15. The Fmoc O-xylosyl serine benzyl ester 13 (1.0 g, 1.2 mmol) is stirred in methanol (40 mL) at room temperature and subjected to hydrogenolysis for 18 h under atmospheric pressure using palladium-charcoal (0.2 g, 5%) as the catalyst. The educt 13 dissolves slowly. The catalyst is filtered off, and the solvent evaporated in vacuo. If the residue is not pure according to thin-layer chromatography (TLC), it is dissolved in 2 mL of ethyl acetate and purified by chromatography on a short column of silia gel 60. The byproducts are eluted with petroleum ether-ethyl acetate the product 15 with methanol yield 0.85 (92%) mp 109°C, [cr]D -12.6° (c 0.3, CH3OH) Rf 0.64 (toluene-ethanol, 1 2). 

There are different methods to cleave benzyl ether bonds. The most common one is hydrogenolysis with palladium on carbon or platinum as catalysts under H2 atmosphere. The standard solvents are ethanol or ethyl acetate. Pd is the preferred and milder one, because the use of Pt at any rate results in aromatic ring hydrogenation. Also a number of methods have been developed in which hydrogen is generated in situ, e. g. from cyclo-hexene, -hexadiene or formic acid (see Chapter 7). 

IsolaHon of anUnes. Amino sugars have been isolated and purified as the N-carbobenzoxy derivatives. The group is removed by hydrogenolysis. Thus Winter-steiner and co-workers isolated crude mycosamine hydrochloride from hydrolysis of the antibiotic nystatin, but were unable to induce the material to crystallize. Reaction with carbobenzoxy chloride in aqueous sodium carbonate and extraction with ethyl acetate gave an oil and, when a solution of this in warm ethanol was chilled and scratched, colorless needles in clusters soon deposited. 

Palladium catalysts, usually Pearlman s catalyst [42] and Pd/C are used for the hydrogenolysis of benzyl-nitrogen bonds. Acetic acid, ethyl acetate, ethanol, or methanol are frequently used as solvents. Sometimes a small amount of a strong acid is added to the reaction mixture [54-56]. The bond rupture needs the adsorption of carbon [57]. The hydrogenolysis of C-N bonds occurs with inversion on both Pd and Ni [2,58]. 

The conversions of acetic anhydride to acetaldehyde and of ethylidene diacetate to ethyl acetate both involve hydrogenolysis of C—O bonds, whereas the hydrogenation of acetaldehyde to ethanol involves 0=0 reduction. An appropriate choice of hydrogenolysis versus hydrogenation catalyst functions should enable discrimination between the reaction pathways and the development of highly selective processes to both ethyl acetate and propionic acid, respectively. However, a clear disadvantage common to both is the requirement for recycling of stoichiometric quantities of acetic acid (see the next section). 

This reaction can be quite complex and can result in the formation of numerous products including acetaldehyde, ethanol, ethyl acetate, ethane, CH4 and CO. As a result, this network of reactions provided an excellent opportunity to examine the role a support can play, because with Pt/Si02 catalysts at low conversions only hydrogenolysis occurred to form CH4 and CO, whereas with Pt/Ti02 catalysts ethanol (50%), ethyl acetate (30%) and ethane (20%) were produced [33]. In situ characterization using IR spectroscopy (DRIFTS) under reaction conditions combined with TPD and TPR (Temperature Programmed Desorption and Reduction, respectively) led to the identification of acyl and acetate species on the catalyst [34] however, only the acyl species was a reactive intermediate at lower temperatures because the acetate species was too stable. This valuable information was incorporated into the reaction model. 

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