Methanols alkyl halide carbonylation

Rhodium catalyzed carbonylations of olefins and methanol can be operated in the absence of an alkyl iodide or hydrogen iodide if the carbonylation is operated in the presence of iodide-based ionic liquids. In this chapter, we will describe the historical development of these non-alkyl halide containing processes beginning with the carbonylation of ethylene to propionic acid in which the omission of alkyl hahde led to an improvement in the selectivity. We will further describe extension of the nonalkyl halide based carbonylation to the carbonylation of MeOH (producing acetic acid) in both a batch and continuous mode of operation. In the continuous mode, the best ionic liquids for carbonylation of MeOH were based on pyridinium and polyalkylated pyridinium iodide derivatives. Removing the highly toxic alkyl halide represents safer, potentially lower cost, process with less complex product purification. 

Historically, the rhodium catalyzed carbonylation of methanol to acetic acid required large quantities of methyl iodide co-catalyst (1) and the related hydrocarboxylation of olefins required the presence of an alkyl iodide or hydrogen iodide (2). Unfortunately, the alkyl halides pose several significant difficulties since they are highly toxic, lead to iodine contamination of the final product, are highly corrosive, and are expensive to purchase and handle. Attempts to eliminate alkyl halides or their precursors have proven futile to date (1). 

In this manuscript, we will chronicle the discoveiy and development of these non-alkyl halide containing processes for the rhodium catalyzed carbonylation of ethylene to propionic acid and methanol to acetic acid when using ionic liquids as solvent. 

The rhodium catalyzed carbonylation of ethylene and methanol can be conducted in the absence of added alkyl halide if the reactions are conducted in iodide based ionic liquids or molten salts. In the case of ethylene carbonylation, the imidazolium iodides appeared to perform best and operating in the absence of ethyl iodide gave improved selectivities relative to processes using ethyl iodide and ionic hquids. In the case of... 

A recent patent has described the carbonylation of methanol to give acetic acid using a palladium-based system (119). The system requires alkyl halide promoters and electron-rich nitrogen ligands (e.g., 2,2 -bipyridine) and operates in the ranges 125-250°C and 20-210 atm. There is insufficient information available to allow discussion of pathways involved. 

Heck and Breslow found that alkyl halides, sulfates, and sulfonates undergo carboalkoxylation in the presence of carbon monoxide, an alcohol, a base, and a catalytic amount of sodium cobalt carbonylate, as illustrated for the reaction of 1-iodooctane in methanol with the strongly basic hindered amine dicyclohexylethyl-amine as base. Use of an unhindered amine leads to formation of the amide. 

Quaternary ammonium salts derived from ephedrine have been used as catalysts for the addition of dialkylzinc to carbonyl compounds (Section D.1.3.1.4.) and are useful as phase-transfer catalysts for alkylation of carbonyl compounds17 and reductions18. N-Benzyl-A -methylephedri-nium salts 10 have found varied application they are easily prepared from A -methylephedrine 913 by reaction with benzyl halide in toluene1 or chloroform/methanol (1 1)18 in high yield. Ref 18 also gives the preparation of other ephedrinium and pseudoephedrinium salts. 

Halide-promoted heterogeneous catalysts for carbonylation are analogues to homogeneous carbonylation catalyzed by metal carbonyls, that is, the Reppe reaction (11). The first step of the Reppe reaction involves the oxidative addition of alkyl halide promoter to carbonyl metal, for example, Rh(I) complex (Fig. 1). This step is followed by methyl migration, bonding of carbon monoxide to give a coor-dinatively saturated Rh(III) complex, and subsequent decomposition of this complex in the presence of methanol to yield a carbonylated product and regenerate the promoter and the catalyst. 

Vinyl halides (example 17, Table VII) were first observed by Kroper to form acrylic esters by reaction with carbon monoxide under pressure and tetracarbonylnickel in methanol at 100°C. These reactions were later shown to occur under much milder conditions. Highly stereospecific reactions were observed c/s-vinyl halides gave cis-carbonylation products and trans-vinyl halides trans-carbonylation products (example 18, Table VII). Retention of configuration of alkyl substrates in carbonylation seems to be a general feature in carbon monoxide chemistry (193a). 

Reaction rates have first-order dependence on both metal and iodide concentrations. The rates increase linearly with increased iodide concentrations up to approximately an I/Pd ratio of 6 where they slope off. The reaction rate is also fractionally dependent on CO and hydrogen partial pressures. The oxidative addition of the alkyl iodide to the reduced metal complex is still likely to be the rate determining step (equation 8). Oxidative addition was also indicated as rate determining by studies of the similar reactions, methyl acetate carbonylation (13) and methanol carbonylation (14). The greater ease of oxidative addition for iodides contributes to the preference of their use rather than other halides. Also, a ratio of phosphorous promoter to palladium of 10 1 was found to provide maximal rates. No doubt, a complex equilibrium occurs with formation of the appropriate catalytic complex with possible coordination of phosphine, CO, iodide, and hydrogen. Such a pre-equilibrium would explain fractional rate dependencies. 

Homogeneous hydrogenation of carbon dioxide to methanol is catalyzed by ruthenium cluster anions in the presence of halide anions. The catalyst system was Ru3(CO)i2 and alkyl iodides in A -methylpyrrolidone (NMP) solution at 513 K. Some methane was also formed. FT-IR spectra of the reactions allowed identification of several ruthenium carbonyl anions. 

Carbonylation and decarbonylation reactions of alkyl complexes in catalytic cycles have been reviewed . A full account of the carbonylation and homologation of formic and other carboxylic acid esters catalysed by Ru/CO/I systems at 200 C and 150-200 atm CO/H2 has appeared. In a novel reaction, cyclobutanones are converted to disiloxycyclopentenes with hydrosilane and CO in the presence of cobalt carbonyl (reaction 4) . The oxidative addition of Mel to [Rh(CO)2l2] in aprotic solvents (MeOH, CHCI3, THF, MeOAc), the rate determining step in carbonylation of methyl acetate and methyl halides, is promoted by iodides, such as Bu jN+I", and bases (eg 1-methylimidazole) . A further kinetic study of rhodium catalysed methanol carbonylation has appeared . The carbonylation of methanol by catalysts prepared by deposition of Rh complexes on silica alumina or zeolites is comparable with the homogeneous analogue . 

The use of halomethyl vinylsilanes would appear to offer significant promise. As allylic halides, they should be sufficiently reactive to permit regiospecific alkylation as vinylsilanes, they can be converted, via a,/3-epoxysilanes, into carbonyl compounds. The conditions for this last transformation, with simple a, 8-epoxysilanes, are rather vigorous, requiring hot methanol/ sulfuric acid. 

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