Carbonylic acids, formation

The advent of a large international trade in methanol as a chemical feedstock has prompted additional purchase specifications, depending on the end user. Chlorides, which would be potential contaminants from seawater during ocean transport, are common downstream catalyst poisons likely to be excluded. Limitations on iron and sulfur can similarly be expected. Some users are sensitive to specific by-products for a variety of reasons. Eor example, alkaline compounds neutralize MTBE catalysts, and ethanol causes objectionable propionic acid formation in the carbonylation of methanol to acetic acid. Very high purity methanol is available from reagent vendors for small-scale electronic and pharmaceutical appHcations. 

Nitrosobenzenes react with the carbonyl group of aldehydes to yield hydroxamic acids 73, according to reaction 20. Recently, the reactions between some X-substituted nitrosobenzenes (X = H, p-Me, p-C 1, m-Cl, p-Br) and formaldehyde were reported194 in order to investigate the mechanism of the hydroxamic acid formation. The mechanism reported in Scheme 9 involves a first equilibrium yielding the zwitterionic intermediate 74 which rearranges (by acid catalysis) into hydroxamic acid 75. The presence of a general acid catalysis, the substituent effect (p values of the Hammett equation equal —1.74),... 

Under acidic conditions, electrophilic addition occurs first, namely protonation of the carbonyl and formation of the conjugate acid. The conjugate acid, with a full positive charge, is now a more reactive electrophile than the original uncharged carbonyl... 

It was found that a nickel-activated carbon catalyst was effective for vapor phase carbonylation of dimethyl ether and methyl acetate under pressurized conditions in the presence of an iodide promoter. Methyl acetate was formed from dimethyl ether with a yield of 34% and a selectivity of 80% at 250 C and 40 atm, while acetic anhydride was synthesized from methyl acetate with a yield of 12% and a selectivity of 64% at 250 C and 51 atm. In both reactions, high pressure and high CO partial pressure favored the formation of the desired product. In spite of the reaction occurring under water-free conditions, a fairly large amount of acetic acid was formed in the carbonylation of methyl acetate. The route of acetic acid formation is discussed. A molybdenum-activated carbon catalyst was found to catalyze the carbonylation of dimethyl ether and methyl acetate. 

Figure 7 shows the results of methyl acetate carbonylation in the presence of water. Methanol and dimethyl ether were formed up to 250 C suggesting that hydrolysis of methyl acetate proceeded. With increasing reaction temperature, the yield of acetic acid increased remarkably, while those of methanol and dimethyl ether decreased gradually. Figure 8 shows the effects of partial pressures of methyl iodide, CO, and methyl acetate in the presence of water. The rate of acetic acid formation was 1.0 and 2.7 order with respect to methyl iodide and CO, respectively. Thus, the formation of acetic acid from methyl acetate is highly dependent on the partial pressure of CO. This suggests that acetic acid is formed by hydrolysis of acetic anhydride (Equation 6) which is formed from methyl acetate and CO rather than by direct hydrolysis of methyl acetate. 

Toward the synthesis of zaragozic acids, a novel family of fungal metabolites that has been shown to be picomolar competitive inhibitors of squalene synthease, Hodgson s group and Hashimoto s group have used cyclic carbonyl ylide formation/[3 + 2]-cycloaddition approach. " In Hashimoto s synthesis, the 2,8-dioxabicyclo[3,2,l]octane core... 

Steam may have a positive effect on the activity according to Komaro-vskii et al. [179], A doubling of the reaction rate was observed by adding up to 20% steam to the oxidation of a mixture of n-butenes in a flow reactor over a Bi/Mo catalyst of unknown composition at 420—480°C. The same authors [179] also studied the influence of the oxygen concentration, which was found to have no effect on the kinetics at 02/butene > 0.4. Furthermore, a rather complex set of kinetic equations was derived to describe side reactions (isomerization, and formation of carbonyls, acids and furan). 

While most of the above carbonylations are carried out at pressures greater than 40 atm (isocyanate and acetic acid formations are exceptions), decarbonylations are low pressure reactions. Decarbonylation of acyl halides catalyzed by (3P)2RhCOCl leads either to halides (65) (Reaction 19)... 

The conversion of methanol to ethanol with carbon monoxide and hydrogen has attracted considerable attention. Further carbonylation to higher alcohols occurs much more slowly, but acetic acid formation is a competing reaction and this leads to ester formation. Using CoI2 in presence of PBu 3 as catalyst, the selectivity to ethanol was improved by addition of the borate ion B4072. 399 This was attributed to an enhanced carbene-like nature of an intermediate cobalt-acyl complex by formation of a borate ester (equation 76). This would favour hydrogenolysis to... 

In a brief survey of other simple binary carbonyls we find that the compounds M(CO)6(M = Cr, Mo, W) and Ru3(CO)12 have only minimal catalytic activity for autooxidizing alcohols or ketones. The compounds Fe(CO)5 and Fe3(CO)12 are decomposed completely when we try to use them as catalysts. When the compound Mn2(CO)i0 is used, there is a considerable enhancement in acid formation. During this reaction there is extensive decomposition to manganese dioxide, and we believe that this compound is the one primarily involved in the catalytic oxidation. 

Intermolecular cycloaddition also proceeds smoothly. The 2,8-dioxabicyclo-[3,2.1]octane core system 379 of zaragozic acid 380 was constructed by the intramolecular carbonyl ylide formation from 376 catalysed by Rh2(OAc)4, followed by intermolecular 1,3-dipolar cycloaddition of the electron-deficient dipolarophile 377 as shown by 378 as a single diastereomer out of four possible diastereomers [124],... 

In the presence of alkyl halides and base, alkyltetracarbonylcobalt complexes are formed with Co2(CO)8 these species [RCo(CO)4] carbonylate a wide range of aryl halides or heterocyclic halides to various products, which depend upon the specific conditions. In the presence of alcohols, carboxylic esters are formed. Under phase transfer conditions and with iodomethane, mixtures of methyl ketone and carboxylic acid formation are realized (equation 207). In the presence of sodium sulfide or NaBH4 in water-Ca(OH)2 (equation 208) good amounts of double carbonylation are realized under very mild conditions412-414. 

The rearrangement of acetals of 2-haloalkyl aryl ketones is a well-documented process yielding esters of 2-arylalkanoic acids by 1,2-aryl shift (equation 7). The mechanism of this rearrangement is reminiscent of other semipinacol rearrangements. Loss of the halogen (usually assisted by Lewis acid), yields a carbocation (4), which then undergoes a 1,2-aryl shift with carbonyl group formation. 

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