Rhodium-catalyzed carbonylations

In the late 1960s, workers at Monsanto began studies into the carbonyla-tion of methanol to acetic acid. The process they developed (9-11), now known worldwide as the Monsanto acetic acid process, is based on an iodide-promoted rhodium catalyst system. Because of the high efficiency and selectivity of the reaction (typical commercial operating conditions are 150-200°C and 30-100 atm, giving selectivities 99% based on CH3OH), 

Relative Rate and Product Distribution Data for the Iodide-Promoted Rhodium-Catalyzed Carbonylations of Higher Alcohols at 175°C and 48 atm CO Pressure  

Alcohol Major products (selectivity) Relative rates 

1-Propanol Butanoic acid (80%) 2-Methyl propanoic acid (20 %) 0.2 

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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. 

Ethylene Carbonylation. The classical rhodium catalyzed carbonylation of ethylene to propionic acid (Eqn. 1) used ethyl iodide or HI as a co-catalyst (6). In the presence of excess ethylene and CO the process could proceed further to propionic anhydride (Eqn. 2). While additional products, such as ethyl propionate (EtC02Et), diethyl ketone (DEK), and ethanol were possible (See Eqns. 3-5), the only byproduct obtained when using a rhodium-alkyl iodide catalyst was small amounts (ca. 1-1.5%) of ethyl propionate. (See Eqns. 3-5.)... 

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... 

This chapter will address the development of selected stereoselective rhodium-catalyzed carbonylation reactions and their application to problems in organic synthesis. It is in no way intended to serve as a comprehensive review of rhodium-catalyzed carbonylation chemistry. The focus, rather, is on the development of stereoselective rhodium-catalyzed carbonylation reactions for use in the synthesis of stereochemically complex natural products, particularly polyketides. 

In the first rhodium-catalyzed carbonylative silylcarbocyclization (CO-SiCaC), which was reported in 1992 [12, 13], silylcyclopentenone 9 was isolated as a minor product in the silylformylation of 1-hexyne 8 (Scheme 7.4). Under optimized conditions using Et3SiH and ( BuNC)4RhCo(CO)4 as the catalyst at 60°C, 9 is formed in 54% yield [13]. A possible mechanism proposed for this intermolecular CO-SiCaC is shown in Scheme 7.4 [13]. In this mechanism, the formation of 9 is proposed to proceed via in-... 

In anhydrous mixtures, the rhodium catalyzed carbonylation is enhanced by the presence of hydrogen. Introduction of hydrogen to a rhodium catalyzed carbonylation of methyl acetate increases the reaction rate and maintains catalyst stability (26) when the hydrogen partial pressure is rather low. It leads to reduced products formation, e.g. acetaldehyde and ethylidene diacetat with higher hydrogen partial pressure, in excess of 50 psi (27, 28). This is a clear indication that hydrogen is added to the coordination sphere of the rhodium catalyst. However, in the case of methanol carbonylation, the presence of hydrogen does not enhance the reaction rate or lead... 

The intermolecular version of the above reaction has also been reported (391). In the first example, a rhodium-catalyzed carbonyl yhde cycloaddition with maleimide was smdied. However, only enantioselectivities of up to 20% ee were obtained... 

The most common oxidation states and corresponding electronic configurations of rhodium are +1 (tf8), which is usually square planar although some five coordinate complexes are known, and +3 (T) which is usually octahedral. Dimeric rhodium carboxylates are +2 (oxidation states —1 (industrial applications include rhodium-catalyzed carbonylation of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to tf-butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Rather than converting methanol direcdy to ethanol, two processes have been announced that go through the intermediate step of converting the methanol to acetic acid by rhodium-catalyzed carbonylation. 

Alkyl 2-phenylethynylbenzoates 779 can undergo rhodium-catalyzed carbonylation and cyclizations under water-gas shift reaction conditions to form indeno[l,2-r] isocoumarins (Scheme 207) <1997TL4989, 1999JM0211>. 

A tetrahydrofuran fused with a seven-membered ring was obtained from an enyne through a [5+2] cycloaddition reaction catalyzed by [(C10H8)Rh(COD)]+ SbF6 complex <02AG(E)4550>. Rhodium-catalyzed carbonylative alkene-alkyne coupling reactions... 

WATER-GAS SHIFT REACTION AND RHODIUM-CATALYZED CARBONYLATION 63... 

Rhodium-catalyzed carbonylation of alcohols such as ethanol, n-propanol, and f-propanol have been studied. Unlike methanol carbonylation, central to all... 

How would the byproduct formation in the rhodium-catalyzed carbonylation reaction be affected by the absence of CH3I in an acidic pH ... 

Scheme 2. Mechanism for the iodide-promoted rhodium-catalyzed carbonylation of 1-propanol to account for the formation of isomeric butyric acids. From Refs. 24 and 31.

Figure 22-9 Simplified cycle for the rhodium catalyzed carbonylation of methanol. The oxidative addition of iodomethane to A is the rate-limiting step.

Another approach, developed by Chiyoda/UOP, uses a rhodium catalyst heterogenized on a polymeric cation exchange resin. This takes advantage of the fact that the rhodium catalyzed carbonylation involves anionic complexes (see Section 4.2.5 below). The Chiyoda/UOP Acetica process employs a cross-linked polyvinylpyridine which is quaternized by methyl iodide to generate cationic pyridinium sites and which hold the anionic rhodium complexes by electrostatic interactions. The polymer support is tolerant of elevated temperatures and the ionic attachment of the catalyst is quite robust, resulting in only... 

Rhodium-Catalyzed Carbonylation of Methyl Acetate to Acetic Anhydride... 

Table 6 Comparison of cobalt- and rhodium-catalyzed carbonylation of methanol to acetic acid...

Several further experimental and theoretical studies of the oxidative addition of methyl iodide to Rh(I) complexes have been reported, in part because of its importance in the rhodium-catalyzed carbonylation of methanol (see Carbonylation Processes by Homogeneous Catalysis) to... 

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