Carbonylation reaction, organic cycles

Spectroscopic Studies of the Organic Cycles of Carbonylation Reactions... 

Spectroscopic Studies of the Organic Cycles of Carbonylation Reactions 219 MeOAc + H2O AcOH + MeOH (3)... 

The carbonylation reaction in the Hoechst process involves the use of PdCl2(PPh3)2 as the precatalyst, a CO pressure of about 50 bar, and a temperature of about 130°C. It is performed in a mixture of an organic solvent and hydrochloric acid. The mechanism at a molecular level is not known with certainty. On the basis of the known chemistry of palladium, a speculative catalytic cycle is shown in Fig. 4.12. 

Sulfur cycling is affected in a variety of ways, including UV photoinhibition of organisms such as bacterioplankton and zooplankton that affect sources and sinks of DMS and UV-initiated CDOM-sensitized photoreactions that oxidize DMS and produce carbonyl sulfide. Metal cycling also interacts in many ways with UVR via direct photoreactions of dissolved complexes and of metal oxides and indirect reactions that are mediated by photochemically-produced ROS. Photoreactions can affect the biological availability of essential trace nutrients such as iron and manganese, transforming the metals from complexes that are not readily assimilated into free metal ions or metal hydroxides that are available. Such photoreactions can enhance the toxicity of metals such as copper and can initiate metal redox reactions that transform non-reactive ROS such as superoxide into potent oxidants such as hydroxyl radicals. 

In the last six chapters we discussed the transition metal catalyzed carbonylative activation of organohalogen (C-X, X = I, Br, Cl, OTf, etc.) compounds. They all have one common point in their reaction mechanism taking a palladium catalyst, for example, the reactions start with Pd(0) and then go to Pd(II) after an oxidative addition. To summarize, the reactions all go through Pd(0) to Pd(II) and a Pd(0) cycle. But for oxidative carbonylation reactions, the reactions go through Pd(ll) to Pd(0) and a Pd(II) cycle. Clearly, oxidative carbonylations need additional oxidants to reoxidize the Pd(0) to Pd(II), and various organic nucleophiles were applied as substrates in the presence of CO. One of the most obvious advantages for oxidative carbonylation reactions is the oxidative addition step can be avoid which is more reluctant under CO atmosphere. 

The competition between the catalytic carbonylation of organic nitro compounds and other chemical routes for the synthesis of a variety of organic compounds has not yet come to an end, but many progresses have been done in the former field. We also like to emphasize that this type of research does not only involve relevant industrial problems to be solved, but it opens a research field where the academic interests (mechanism of the reactions, isolation of the intermediates in the catalytic cycles, synthesis of model compounds and so on) can find a lot of opportunities. 

The formation of isomeric aldehydes is caused by cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These cobalt organic compounds isomerize rapidly into a mixture of isomer position cobalt organic compounds. The primary cobalt organic compound, carrying a terminal fixed metal atom, is thermodynamically more stable than the isomeric internal secondary cobalt organic compounds. Due to the less steric hindrance of the terminal isomers their further reaction in the catalytic cycle is favored. Therefore in the hydroformylation of an olefin the unbranched aldehyde is the main reaction product, independent of the position of the double bond in the olefinic educt ( contrathermodynamic olefin isomerization) [49]. 

The direct reductive amination (DRA) is a useful method for the synthesis of amino derivatives from carbonyl compounds, amines, and H2. Precious-metal (Ru [130-132], Rh [133-137], Ir [138-142], Pd [143]) catalyzed reactions are well known to date. The first Fe-catalyzed DRA reaction was reported by Bhanage and coworkers in 2008 (Scheme 42) [144]. Although the reaction conditions are not mild (high temperature, moderate H2 pressure), the hydrogenation of imines and/or enam-ines, which are generated by reaction of organic carbonyl compounds with amines, produces various substituted aryl and/or alkyl amines. A dihydrogen or dihydride iron complex was proposed as a reactive intermediate within the catalytic cycle. 

Coupling of organostannanes with halides in a carbon monoxide atmosphere leads to ketones by incorporation of a carbonylation step.249 The catalytic cycle is similar to that involved in the coupling of alkyl or aryl halides. These reactions involve Reactions involving a migration of one of the organic substituents to the carbonyl carbon, followed by... 

In a slightly less convenient procedure, but one which has general versatility, carbonylation of aryl (or vinyl) palladium compounds produces aryl, heteroaryl, and vinyl carboxylic acids. As with the other procedures, immediate upon its formation, the carboxylate anion migrates to the aqueous phase. Consequently, haloaromatic acids can be obtained from dihaloarenes, without further reaction of the second halogen atom, e.g. 1,4-dibromobenzene has been carbonylated (90% conversion) to yield 4-bromobenzoic acid with a selectivity for the monocarbonylation product of 95%. Additionally, the process is economically attractive, as the organic phase containing the catalyst can be cycled with virtually no loss of activity and ca. 4000 moles of acid can be produced for each mole of the palladium complex used [4],... 

In common with many catalysed reactions, the important features of carbonylation process chemistry may be associated with different aspects of the catalytic cyde. Broadly, process activity may vary either because (i) more of the catalyst is present in the active form, (ii) the activity of the catalyst in the active form is enhanced or inhibited or, less commonly, (iii) the rate controlling step does not involve the catalyst. The process selectivity may vary because of side reactions (i) occurring through the active catalyst cycle, (ii) involving inactive catalyst, or (iii) taking place because of the organic chemistry of the systems. Examples of all these contributions to overall process effidency are found in the various commerdal carbonylation processes. 

In this reaction, the initially introduced Se02, insoluble in organic solvent, is converted to soluble SeCO by CO (30 atm) under reflux in toluene, which is the active catalyst for the reductive carbonylation of nitrobenzene or nitropyridine. The metallic selenium (insoluble) is also converted to SeO under CO pressure. After completion of the reaction, soluble selenium catalyst solution was readily recovered by simple filtration and reused. The recovered catalyst solution was used for five cycles without loss of activity. 

Substituted benzyl chlorides were carbonylated using a Pd/tppts catalyst in aqueous/organic two phase systems under basic reaction conditions to afford the sodium salts of the corresponding phenylacetic acids. After acidification the phenylacetic acid dissolved in the organic phase and could be readily separated from the Pd/tppts catalyst contained in the aqueous phase (Figure 12) 466-468 TOFs up to 21 h 1 (turnover number, TON=165) and phenylacetic acid yields up to 94% were obtained at 70°C, 1 bar CO, tppts/Pd=10, NaOH/substrate=3/2 in an aqueous/toluene (1/1) two phase system in a batchwise procedure.466 The TOFs were improved to a maximum of 135 h 1 (TON=1560) in a continuous operation mode. Palladium catalysts modified with binas (Table 2 25) exhibited low catalytic activity (TONs up to 140) in the carbonylation of benzyl chloride 466 In strongly acidic media (pH=l) the Pd/25 catalyst was active and remained stable during the reaction which contrasts with Pd/tppts where palladium black was observed. However, the catalyst was completely deactivated after three cycles.466... 

The mechanism of the Montedison reaction has been studied in some detail, and tentative mechanisms have been offered. The proposed catalytic cycle is shown in Fig. 4.11. The biphasic reaction medium consists of a layer of diphenyl ether and that of aqueous alkali. In the presence of alkali, the precatalyst Co2(CO)8 is converted into 4.7. The sodium salt of 4.7 is soluble in water but can be transported to the organic phase, that is, a diphenyl ether layer by a phase-transfer catalyst. The phase-transfer catalyst is a quaternary ammonium salt (R4N+X ). The quaternary ammonium cation forms an ion pair with [Co(CO)4]. Because of the presence of the R groups, this ion pair, [R4N]+[Co(CO)4], is soluble in the organic medium. In the nonaqueous phase benzyl chloride undergoes nucleophilic attack by 4.7 to give 4.40, which on carbonylation produces 4.41. The latter in turn is attacked by hydroxide ion transported from the aqueous phase, to the organic phase again by the phase-transfer catalyst. The product phenyl acetate and 4.7 are released in the aqueous phase as the sodium or quaternary ammonium salts. 

Low-water operation can be accomplished with modifications to the process which include significant changes in the catalyst system [23]. The main catalytic cycle for high-water methanol carbonylation is still operative in the low-water process (see Section 2.1.2.1.1), but at low water concentration two other catalytic cycles influence the carbonylation rate. The incorporation of an inorganic or organic iodide as a catalyst co-promoter and stabilizer allows operation at optimum methyl acetate and water concentrations in the reactor. Carbonylation rates comparable with those realized previously at high water concentration (ca. 10 molar) are demonstrated at low reaction water concentrations (less than ca. 4 molar) in laboratory, pilot plant, and commercial units, with beneficial catalyst stability and product selectivity [23]. With this proprietary AO technology, the methanol carbonylation unit capacity at the Celanese Clear Lake (TX) facility has increased from 270 X 10 metric tons per year since start-up in 1978 to 1200 X 10 metric tons acetic acid per year in 2001 with very low capital investment [33]. This unit capacity includes a methanol-carbonylation acetic acid expansion of 200 X 10 metric tons per year in 2000 [33]. 

The mechanism of the Li/Rh-catalyzed carbonylation of methyl acetate was formally described by a catalyst cycle by Zoeller et al. based on kinetic measurements under high pressure [41b]. The organic reaction cycle is combined with the rhodium catalyst cycle in the Li/Rh catalyst system (Figure 3). 

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