Carbonyl-based systems

Analogous carbonylation reactions using nickel and iron carbonyl based systems also produce alkanecarboxylic acids [11, 13, 14]. The mechanism of the conversion of benzyl halides into arylacetic acids using iron pentacarbonyl is not as well defined as it is for reactions promoted by nickel or molybdenum carbonyl complexes. Iron... 

Addition-elimination reactions are not exclusive to esters. In fact, these reactions can occur with any carbonyl-based system where the leaving group is a weaker nucleophile... 

Amidocarbonylation converts aldehydes into amido-substituted amino acids, which have many important industrial applications ranging from pharmaceuticals to detergents and metal-chelating agents.588 Two catalyst systems have been developed, a cobalt-based system and, more recently a palladium-based system. In the cobalt system, alkenes can be used as the starting material, thus conducting alkene-hydroformylation, formation of hemi-amidal and carbonylation in one pot as... 

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

Abstract The basic principles of the oxidative carbonylation reaction together with its synthetic applications are reviewed. In the first section, an overview of oxidative carbonylation is presented, and the general mechanisms followed by different substrates (alkenes, dienes, allenes, alkynes, ketones, ketenes, aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, phenols, amines) leading to a variety of carbonyl compounds are discussed. The second section is focused on processes catalyzed by Pdl2-based systems, and on their ability to promote different kind of oxidative carbonylations under mild conditions to afford important carbonyl derivatives with high selectivity and efficiency. In particular, the recent developments towards the one-step synthesis of new heterocyclic derivatives are described. 

REACTIONS OF THE CHLOROFORM/BASE SYSTEM WITH CARBONYL COMPOUNDS... 

In many respects the apparently analogous reduction of nitroarenes with triruthenium dodecacarbonyl under basic phase-transfer conditions is superior to that of the iron carbonyl-mediated reductions. However, the difference in the dependence of the two processes on the concentration of the aqueous sodium hydroxide and the pressure of the carbon monoxide suggests that they may proceed by different mechanisms. Although the iron-based system is most effective under dilute alkaline conditions in the absence of carbon monoxide, the use of 5M sodium hydroxide is critical for the ruthenium-based system, which also requires an atmosphere of carbon monoxide [11]. The ruthenium-based reduction has been extended to the... 

The different ligand properties compared to phosphine ligands can be seen when comparing the NHC complexes with known analogous phosphine chelators. The differences become obvious from the coordination polyhe-dra and the electronic properties. The TIMEN hgands lead to high spin carbonyl complexes while the phosphine-based systems form diamagnetic complexes. 

The second metal, for example, the promoter, may also be added by subsequent impregnation of binary sulfide. When a nonreactive promoter precursor, for example, metal nitrate, is used it is necessary to resulfide the impregnated sulfide in order to decompose the precursor. Another variation of this method consists in using reactive promoter precursors that will react with the surface of the binary sulfide. In this case, further treatment of the catalyst may not be required. Good precursors include metal carbonyls and metal alkyls (32, 33). The precursor decomposition approach been most widely applied to the MoS2-based systems. However, it has also been extended to the mixed noble-metal sulfides by Breysse and co-workers (34) at Lyon following the work of Passaretti et al (35). 

This review summarizes some earlier qualitative work as well as recent quantitative studies of redistribution equilibria and describes the principles underlying the mathematical treatment of such equilibria as well as the general implications of these equilibria with respect to general chemistry. In line with the general objective of the Advances in Organometallic Chemistry series, this article is limited to carbon-metal-bonded systems, metal hydrides, metal carbonyl compounds, metallocenes, and similar complexes. Excluded therefore are halogen-, sulfur-, nitrogen, and phosphorus-based systems.Various aspects of redistribution reactions were reviewed previously (42, 74, 87, 88,150,186, 285, 286, 288). 

The cross-coupling of aryl halides and enolates is a powerful method to prepare a-arylated carbonyl compounds that are difficult to access through classic organic chemistry [29]. (NHC)Pd(allyl)Cl species [30] were the first NHC-bearing complexes used as pre-catalysts for the a-arylation of ketones [31,32]. More recently, novel (IPr)Pd(acac)Cl complexes have shown remarkable catalytic activity in this transformation [33]. From aryl chlorides, excellent yields were obtained after short reaction times at 60 °C, the lowest temperature reported to date with a carbene-based system (Table 1). 

In our present discussions, 1,2- and 1,4-additions to carbonyl systems were introduced. However, these reactions were not presented in the context of specific carbonyl-based functional groups. Expanding upon this concept, the three types of functional groups generally used in addition reactions to carbonyls are aldehydes, ketones, and esters. 

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. 

Efforts to optimize rhodium-based systems for methanol carbonylation led to the development of new supporting ligands containing phosphorus and sulfur donor atoms, both thiolates and thioethers, such as those used in the preparation of complexes (20) and (21). Ligands such as 2-diphenylphosphinothiolate have been shown to give rise to complexes that exhibit higher activities, up to four times faster, for the carbonylation of methanol compared to [Rh(CO)2l2] . ... 

The Cativa process is based on a promoted iridium catalyst, and offers a considerable improvement over the rhodium-based system as a result of increased catalyst stability at lower water concentrations, decreased by-product formation, higher rates of carbonylation, high selectivity (>99% based upon methanol), and improved yields on carbon monoxide. This is a more cost-effective process for methanol carbonylation owing to lower energy consumption and fewer purification requirements. Implementation of this new process has now been achieved in four plants worldwide. 

Although the carbonylation of methanol using an iodide-promoted iridium complex was first reported by Monsanto researchers Roth and Pauhk in 1968, and its mechanism studied by Forster and others, it was the rhodium system that was initially developed for commercialization. A more complex mechanism for iridium, involving both anionic and neutral intermediates was discovered, but it would take over twenty years to coimnercialize an iridium-based system for methanol carbonylation (Scheme 21). In the Cativa process, the iridium complex is promoted by two distinct... 

SF4 forms adducts with main group inorganic fluorides, and these have been variously described as simple Lewis acid-base systems (with SF4 behaving as the base) or as ionic systems such as [Sp3]+ [BF4] . The reactions of SF4 with organic molecules have been widely studied. The most important reaction is the conversion of a carbonyl gronp to a diflnoride (eqnation 51). 

A Ni°-based system related to those already described in connection with syntheses of both cyclobut-enediones and several five-membered heterocycles (Sections 9.4.2.1, 9.4.3.3 and 9.4.3.5) is also capable of catalyzing pyridone synthesis from isocyanates and alkynes. However, the structures found for the isolable metallacyclic intennediates in this system imply a completely different insertion sequence isocyanate first, followed by the two alkynes (Scheme 35). As a consequence, the regiochemistry found in the products of reaction of unsymmetrical alkynes is the reverse of that typical of Co larger substituents end up at positions 4 and 6. Thus, starting wiA the carbonyl carbon of the isocyanate, each carbon-caib-on bond forming event is strongly regioselective for the less-hindered alkyne caibon (equation 46). The... 

One of the early efforts at a systematic study of carbonyl systems in acid solvents has historical importance. Murty and Seshadri published a series of papers describing their Raman investigations of solutions of carbonyl compounds in various solvents (1471-1476). Among the carbonyl bases were esters, aldehydes, and carboxylic acids. The solvents included phenol, various alcohols, water, chloroform, ethers, and carbon tetrachloride. 

A, A -Dialkylhydrazones are often converted into the carbonyl form by a variety of oxidative methods. The respective compounds (like 100) shown in Scheme 93, derived from proline-based systems (RAMP and SAMP), are widely used as potent chiral auxiliaries and have provided a very versatile method for the diastereoselective a-alkylation of ketones. SAMP and RAMP hydrazones can be cleaved with O3, by reductive techniques and by hydrolysis with strong acids. 

Recently, a more stable Rh catalyst for methanol carbonylation based on the crosslinked polyvinylpyridine system has been disclosed in which the degree of crosslinking of the resin support is as high as 60 % [115c-e]. This catalyst improvement is the basis for the potential development of a commercial methanol carbonylation acetic acid process named Acetica . This process is being offered for license by Chiyoda and UOR Even with this announcement, there are still considerable doubts whether heterogenized carbonylation catalyst systems can compete with the low-water homogeneous Rh- and Ir-catalyzed processes (cf. Sections 2.1.1 and 3.1.1.3). 

Amidocarbonylation is a recently developed, organometallic-catalyzed route to amino acid generation - particularly A(-acyl a-amino acids - using either aldehydes or alkenes as starting materials and synthesis gas as an integral building block. The two principal classes of reaction are illustrated in eqs. (1) and (2). Both syntheses offer the opportunity to introduce two functionalities, amido and carboxylate, simultaneously where an amide is the co-reactant. Homogeneous amidocarbonylation catalysts are typically cobalt carbonyl-based, or utilize transition-metal binary systems, e. g. cobalt-rhodium, cobalt-palladium, and cobalt-iron. 

In the first reported direct A -carbonylation of nitroaromatics to isocyanates, simple Pd- or Rh-based systems were used to catalyze the reaction of aromatic mononitro compounds with carbon monoxide [11, 12]. Later, it became possible to work without the drastic reaction conditions that had been required initially, by using Lewis acid co-catalysts [13], Various catalysts and catalyst mixtures, normally based on Ru, Rh, or Pd complexes with co-catalysts, were described in numerous patents and publications [1, 3, 14—16], The careful choice of the composition of the triad consisting of metal salt, co-catalysts and ligand (preferably aromatic amines) led to efficient catalyst systems [14a-e] for the direct reductive carbonylation process. A quite active Pd-phenanthroline-H system with noncoordinating carboxylic acids such as 2,4,6-trimethlybenzoic acid as proton source is worth mentioning [14 d]. 

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