Hydroxy-, Alkoxy- and Aminocarbonylations of C-X Bonds

We will begin with the carbonylation of Mel which in situ is generated from MeOH for acetic acid production because of its industrial importance. Acetic acid is an important chemical commodity with a wide range of appUcations in organic chemistry. In organic synthesis, acetic acid is mainly used as a raw material for vinyl acetate monomers and acetic anhydride synthesis, as well as a solvent for producing terephthalic acid from xylene via the oxidation process. In 1998 the world s capacity of acetic acid production was approximately 7.8 milUon tons, of which more than 50 % were produced by BP-Amoco and Celanese. 

The first commercialized homogeneous methanol carbonylation route to acetic acid was established at BASF in 1955, using a homogeneous Ni catalyst. In 1960 BASF developed an improved process it used an iodide-promoted CO catalyst and operated at an elevated temperature (230 °C) and pressure (600 bar) [2]. In 1970, Monsanto commercialized an improved homogeneous methanol carbonylation process using a methyl-iodide-promoted Rh catalyst [3-5]. This process operated at much milder conditions (180-220 °C, 30-40 bar) than the BASF process and performed much better [6]. Celanese and Daicel further improved the Monsanto 

Belter and X.-F. Wu, Transition Metal Catalyzed Carbonylation Reactions, DOI 10.1007/978-3-642-39016-6 2, Springer-Verlag Berlin Heidelberg 2013 

Inherent in the homogeneous system, however, are drawbacks relating to catalyst solubility hmitations and the loss of expensive Rh metal due to precipitation in the separation sections. Therefore, inmiobilization of the Rh complex on a support has been the topic of significant research as its heterogeneous catalyst properties. Moreover, Chiyoda and UOP have jointly developed a heterogeneous Rh catalyst system for the methanol carbonylation process to produce acetic acid [14-16]. 

Rhodium-catalyzed carbonylation of methanol is known as the Monsanto process, which has been studied extensively. From the reaction mechanism aspect, the study of kinetics has proved that the oxidative addition of methyl iodide to the [Rh(CO)2l2] is the rate-determining step of the catalytic cycle. It was also observed that acetyl iodide readily adds to [Rh(CO)2l2], indicating that the acetyl iodide must be scavenged by hydrolysis in order to drive the overall catalytic reaction forward. An alternative to sequential reductive elimination and the hydrolysis of acetyl iodide is the nucleophilic attack of water on the Rh acetyl complex and the production of acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via the reductive elimination of acetyl iodide from the rhodium center. The addition of iodide salts, especially lithium iodide, can realize the reaction run at lower water concentrations thus, byproduct formation via the water gas shift reaction is reduced, subsequently improving raw materials consumption and reducing downstream separation. In addition to the experimental studies of the catalytic mechanism, theoretical studies have also been carried out to understand the reaction mechanism [17-20]. 

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