Denis Forster

Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes

Publisher Summary The chapter focuses on the mechanistic pathways in the catalytic carbonylation of methanol by rodium and iridium complexes. Kinetic studies of the rhodium-catalyzed methanol carbonylation reaction show a remarkably simple behavior. The iodide promoter can be charged to the reaction in several different forms without marked differences in the reaction rate being noted. Many different types of rhodium compounds can be charged to the reaction, and at typical reaction temperatures of 1500–2000C, they function as effective catalysts. The generation of the initial metal–carbon bond in the catalytic cycle by reaction of methyl iodide with a metal carbonyl, containing species has been proposed as a key step in both the cobalt and rhodium catalyzed systems. Iridium is also an excellent homogeneous catalyst for the carbonylation of methanol under relatively mild reaction conditions. There are apparently complex interactions among solvent, water, iodide form, and carbon monoxide pressure, and this complicates interpretation. Some general kinetic observation can be studied as (1) the reaction rate is strongly dependent on water concentration, with a decrease in rate being observed at higher water levels; (2) in reaction media containing appreciable concentrations of iodide ion, the reaction rate increases with increasing carbon monoxide pressure; (3) the reaction rate is not first order with respect to methyl iodide concentration as in the rhodium system, but shows an optimum level; and (4) at low iodide levels using methyl acetate as the substrate with low levels of water present, the reaction rate is inversely dependent on carbon monoxide pressure.

Homogeneous catalytic reactions of methanol with carbon monoxide

Abstract The reactions between methanol and carbon monoxide or synthesis gas in the presence of cobalt, rhodium and iridium catalysts are reviewed. The differences between the systems are discussed with reference to the pathways at the metal center. A new interpretation of the mechanism of the cobalt-catalyzed methanol carbonylation reaction is proposed. These systems also exhibit varying amounts of competitive side-reactions, in particular the water-gas shift reaction and alcohol hydrogenolysis. The mechanisms of these side-reactions are discussed.

Mechanistic Aspects of Transition-Metal-Catalyzed Alcohol Carbonylations

Publisher Summary This chapter focuses on the nature of the transformations at the metal center, especially with regard to oxidation state and formation of the initial alkyl-, alkoxy-, or carboalkoxy-metal bond from saturated precursors, and discusses the mechanistic aspects of transition-metal-catalyzed alcohol carbonylations. Prerequisite to any catalytic activity is the ability of the metal center to interact effectively with alcohols or alcohol-derived precursors. There are several ways in which this can occur, and most of these are observed or postulated in at least one catalytic scheme. In order to understand the specific reactivities, though, the reader should be familiar with some fundamental aspects of organo-transition-metal chemistry. Homologation reactions, which are believed to usually proceed by way of aldehyde intermediates, are also discussed in the chapter but only as they pertain to the incorporation of the CO into the metal-carbon bonds. A frequent theme in alcohol carbonylations by transition metals is the use of a halide or pseudo-halide promoters or cocatalysts. Despite major problems of corrosion associated with its use, iodide is almost always found to be most effective in this capacity. This is because the halide serves several purposes, for each of which iodide is ideally suited.

Reaction of iodocarbonylrhodium ions with methyl iodide. Structure of the rhodium acetyl complex: [Me3PhN+]2[Rh2I6(MeCO)2(CO)2]2−

A rhodium acetyl complex [Me 3 PhN + ] 2 [Rh 2 I 6 (MeCO) 2 (CO) 2 ] 2− has been prepared and structure determined by X-ray diffraction: the RhC (acetyl) bond length is 2.062(23) A and the dimeric anion is held together by RhIRh bridges with unequal RhI bond lengths.

Mechanistic Pathways in the Catalysis of Olefin Hydrocarboxylation by Rhodium, Iridium, and Cobalt Complexes

Abstract Carboxylic acids can be synthesized by reacting olefins with carbon monoxide and water in the presence of a variety of transition transition metal catalysts: (1) Metals which have been employed as catalysts for this reaction include nickel, as first reported [1] by Reppe for the synthesis of acrylic and propionic acids from acetylene and ethylene, cobalt, iron, rhodium, ruthenium, palladium, and platinum [2]. The earlier studies of this reaction employed nickel, cobalt, and iron catalysts and required rather severe operating conditions, viz., 200-300 atm and 200-300°C. More recently the use of rhodium [3], iridium [4], platinum [5], palladium [6], and pyridine-promoted cobalt [7] catalysts has been reported. These latter systems all function at relatively mild reaction conditions (see Table 1).

Studies into the mechanism of the rhodium-catalyzed carbonylation of isopropyl alcohol

On etudie la carbonylation du propanol-2 catalyse par rhodium/HI dans un systeme solvant acide acetique-eau a 170°C

HYDROGENOLYSIS OF AROMATIC GLYCOLS : AN EXAMPLE OF MULTIFUNCTIONAL CATALYSIS BY HCO(CO)4

Abstract HCo(CO)4 catalyzes the hydrogenolysis of diaryl glycols to diarylethanes with very high selectivity at temperatures of 180–220 °C. The reaction pathway was determined by mass spectral identification of intermediate and byproduct molecules and can be rationalized by a multiple-step reaction involving preliminary carbonium ion rearrangement followed by various hydrogenation and homologation reactions. The cobalt catalyzes at least five different types of transformation in order to generate the products observed.

Interconversion Reactions of the Halocarbonyls of Iridium

Abstract Carbonylation routes for the preparation of iridium halocarbonyl anions are described. A series of interconversion reactions among the various complexes is presented