Reductive carbonylation methyl acetate

The Pd-catalyzed reductive carbonylation of methyl acetate with CO and H2 affords acetaldehyde. The net reaction is the formation of acetaldehyde from MeOH, CO, and H2P4]. Methyl formate (109) is converted into AcOH under CO pressure in the presence of Lil and Pd(OAc)2[95],... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

X-ray structure analyses of Rh(COCH3)(I)2(dppp) (14) and [Rh(I I)(I)(//-I)(dppp)]2 (15), where dppp l,3-bis(diphenylphosphino) propane, were reported. Unsaturated complex (14) possesses a distorted five-coordinate geometry that is intermediate between sbp and tbp structures.69 Under CO pressure, the rhodium/ionic-iodide system catalyzes either the reductive carbonylation of methyl formate into acetaldehyde or its homologation into methyl acetate. By using labeled methyl formate (H13C02CH3) it was shown that the carbonyl group of acetaldehyde or methyl acetate does not result from that of methyl formate.70... 

By adjusting the C0 H2 ratio, catalytic systems for the reductive carbonylation of methyl acetate can be tuned to the production of acetic anhydride, ethylidene diacetate or acetaldehyde. 

While the direct carbonylation is well accepted by industry, the reductive and oxidative carbonylations are still in the research and development stage. Using Texaco technology (j, 7/ ) the combined synthesis of ethene and ethanol is feasible via homologation of acids according to Figure 3. Ethene can also be obtained from the reductive carbonylation of methyl acetate to ethyl acetate followed by pyrolysis (2 ). Both routes, so far, lack selectivity. 

Methyl acetate is the principal by-product in the reductive carbonylation of methanol. As indicated in Table I, decreasing the Hp/CO increases the methyl acetate selectivity. In the limit of pure CO, methyl acetate is obtained in 90-95% selectivity. 

Methyl acetate probably originates from the reaction of methanol with the intermediate cobalt-acyl complex. The reaction leading to the formation of acetaldehyde is not well understood. In Equation 8, is shown as the reducing agent however, metal carbonyl hydrides are known to react with metal acyl complexes (20-22). For example, Marko et al. has recently reported on the reaction of ri-butyryl- and isobutyrylcobalt tetracarbonyl complexes with HCo(CO) and ( ). They found that at 25 °C rate constants for the reactions with HCo(CO) are about 30 times larger than those with however, they observed that under hydroformylation conditions, reaction with H is the predominant pathway because of the greater concentration of H and the stronger temperature dependence of its rate constant. The same considerations apply in the case of reductive carbonylation. Additionally, we have found that CH C(0)Co(C0) L (L r PBu, ... 

The single step conversion of methyl acetate to ethylidene diacetate is catalyzed by either a palladium or rhodium compound, a source of iodide, and a promoter. The mechanism is described as involving the concurrent generation of acetaldehyde and acetic anhydride which subsequently react to form ethylidene diacetate. An alternative to this scheme involves independent generation of acetaldehyde by reductive carbonylation of methanol or methyl acetate, or by acetic anhydride reduction. The acetaldehyde is then reacted with anhydride in a separate step. 

Reductive Carbonylation of Methyl Acetate One Step Synthesis ... 

The formation of EDA in a one step reaction that involves the reductive carbonylation of methyl acetate has been achieved (equation 2) ( 1). Alternatively, the substrate of reductive carbonylation can be dimethyl ether instead of methyl acetate which simply integrates into the reaction an initial carbonylation step leading to EDA. 

Concurrent with acetic anhydride formation is the reduction of the metal-acyl species selectively to acetaldehyde. Unlike many other soluble metal catalysts (e.g. Co, Ru), no further reduction of the aldehyde to ethanol occurs. The mechanism of acetaldehyde formation in this process is likely identical to the conversion of alkyl halides to aldehydes with one additional carbon catalyzed by palladium (equation 14) (18). This reaction occurs with CO/H2 utilizing Pd(PPh )2Cl2 as a catalyst precursor. The suggested catalytic species is (PPh3)2 Pd(CO) (18). This reaction is likely occurring in the reductive carbonylation of methyl acetate, with methyl iodide (i.e. RX) being continuously generated. 

An alternative scheme to simultaneous formation of acetaldehyde and acetic anhydride could entail the carbonylation of methyl acetate to acetic anhydride which is subsequently reduced to acetaldehyde and acetic acid. The reaction of acetaldehyde with excess anhydride would form EDA. In fact, Fenton has described production of EDA by the reduction of acetic anhydride using both rhodium and palladium salts as catalysts when modified with triphenylphosphine (26). Two possible mechanisms for the reduction are postulated in equation 16. 

When the anhydride was placed under the reductive carbonylation conditions, EDA was produced along with methyl acetate and acetic acid. However, the rate of EDA formation was substantially lower than usual methyl acetate conversions. Also, a mechanism incorporating Fenton s reduction cannot account for excess acetaldehyde along with EDA formation. This cannot be the major path to EDA. 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Reductive Carbonylation of Methanol. As discussed earlier, rhodium based catalysts are capable of catalyzing the reductive carbonylation of methyl acetate to ethylidene diacetate ( 1), as well as the carbonylation of methyl acetate to acetic anhydride (16). These reaction proceed only, wjjen, tjie reaction environment... 

B. Acetaldehyde From Methyl Acetate Reductive Carbonylation. A... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The carbonylation of methyl acetate to acetic anhydride is likely to become an industrial process in the near future 424,427 RhCl3-3H20 is typically used as catalyst precursor and an iodide promoter is used. A mechanistic study indicated that methyl iodide formed from the ester and HI is carbonylated as in acetic acid synthesis (Scheme 26). The resulting acyl, perhaps via reductive elimination of acetyl iodide, converts the acetic acid formed in the ester cleavage to acetic anhydride.428 430 [RhI(CO)(PPh3)2] also catalyzes the reaction though apparently more slowly than complex (95).430,431 The mechanism of this reaction is given in Scheme 27. 

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