Rhodium introduction

Reaction pathways There is general agreement as to the nature of the catalyst precursor (26. 30). It is well admitted that even under CO low pressure Rh(III)-Y was reduced to the monovalent rhodium dicarbonyl attached to the zeolite framework via one or more oxide ions irrespective of the rhodium introduction procedure onto the zeolite. 

A related but distinct rhodium-catalyzed methyl acetate carbonylation to acetic anhydride (134) was commercialized by Eastman in 1983. Anhydrous conditions necessary to the Eastman acetic anhydride process require important modifications (24) to the process, including introduction of hydrogen to maintain the active [Rhl2(CO)2] catalyst and addition of lithium cation to activate the alkyl methyl group of methyl acetate toward nucleophilic attack by iodide. 

The pronounced acidity of the bridgehead hydrogen atoms in 4 (R = H) facilitates the regio-selective introduction of electrophiles. Rearrangements of 4 (R = H, Me, CHO, C02Me) catalyzed by dicarbonyldichlororhodium(I) lead to 4-substituted 1-benzothiepins 5,10 except in the case of R = Me where a mixture (1 1.3) of 3- and 4-methyl-l-benzothiepin is obtained (total yield 98 %). In the case of the dimethyl-substituted derivative 8 (R1 = R2 = Me), however, the rhodium(I)-catalyzed isomerization reaction leads to the thiophene isomer. 

As with rhodium (and cobalt), introduction of five ammonia molecules is relatively straightforward, but the sixth substitution is difficult, requiring more forcing conditions. One versatile route involves the formation of the pentammine triflate complex ion [Ir(NH3)5(03SCF3)]2+, where the labile triflate group is readily replaced by water, then by a range of anionic ligands [148]. 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

Despite the very attractive properties of the rhodium-based system, no commercial plants used it because the low stability of the catalyst meant that the catalyst separation problem prevented commercialisation. Very recently, this situation has changed with the introduction of rhodium-based plant by Sasol in South Africa which uses technology developed by Kvaemer Process Technology (now Davy Process Technology). This batch continuous plant produces medium-long chain aldehydes and the separation is carriedoutbylow pressure distillation [16-18]... 

A breakthrough in hydro formylation was achieved with the introduction of a tri-arylphosphine-modified, in particular triphenylphosphine-modified, rhodium catalyst. [5] This innovation provided simultaneous improvements in catalyst stability, reaction rate and process selectivity. Additionally, products could be separated from catalyst under hydro formylation conditions. One variant is described as Gas Recycle (Figure 2.1) since the products are isolated from the catalyst by vaporization with a large recycle of the reactant gases. [6] The recycle gas is chilled to condense butanals. 

The reaction of aryldiazoacetates with cyclohexene is a good example of the influence of steric effects on the chemistry of the donor/acceptor-substituted rhodium carbenoids. The Rh2(reaction with cyclohexene resulted in the formation of a mixture of the cyclopropane and the G-H insertion products. The enantios-electivity of the C-H insertion was high but the diastereoselectivity was very low (Equation (31)). 0 In contrast, the introduction of a silyl group on the cyclohexene, as in 15, totally blocked the cyclopropanation, and, furthermore, added sufficient size differentiation between the two substituents at the methylene site to make the reaction to form 16 proceed with high diastereoselectivity (Equation (32)).90 The allylic C-H insertion is applicable to a wide array of cyclic and acyclic substrates, and even systems capable of achieving high levels of kinetic resolution are known.90... 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

The sulfamate ester variant of this chemistry has already been shown to be a very powerful protocol for the syntheses of 1,3-amino alcohols and related /3-amino acids (Equation (90)), as well as iminium ion equivalents (Equation (91)). The further showcases of this chemistry are the total syntheses of the bromopyrrole alkaloids, manzacidins A and C (Scheme 13).234 The cyclic sulfamidate 129 was obtained diastereospecifically from sulfamate 128 using intramolecular rhodium-catalyzed G-H insertion. It was then found to react with sodium azide in NfN-dimethylformamide at room temperature after introduction of the Boc-activating group to afford the 1,3-diamino precursor 130 in 78% yield over 3 steps. Four subsequent manipulations afford the target structure 131. 

Some transition metal catalysts induce the living polymerization of various acetylenic compounds.68,69 Such polymerizations of phenylacetylene catalyzed by rhodium complexes are used in conjunction with a quantitative initiation and introduction of functional groups at the initiating chain end (Scheme 16).70 The catalyst is prepared from an [RhCl(nbd)]2/Ph2C=C(Ph)Li/PPh3 mixture and proceeds smoothly to give quantitatively the polymer 54 with a low polydispersity ratio. 

The first example of asymmetric rhodium-catalyzed 1,4-addition of organoboron reagents to enones was described in 1998 by Hayashi and Miyaura. Significant progress has been made in the past few years. This asymmetric addition reaction can be carried out in aqueous solvent for a broad range of substrates, such as a,/ -unsaturated ketones, esters, amides, phosphonates, nitroalkenes. The enantioselectivity is always very high (in most cases over 90% ee). This asymmetric transformation provides the best method for the enantioselective introduction of aryl and alkenyl groups to the / -position of these electron-deficient olefins. 

Introduction of two different chalcogen elements to the C-C unsaturated bond is of particular interest from both synthetic and mechanistic viewpoints. Therefore, extensive studies have been carried out on the development of the (RE)2/(R E )2 binary system without using RE-E R compounds, which are difficult to prepare. (Z)-l-Seleno-2-thio-1-alkenes are produced regio- and stereoselectively when a mixture of diaryl disulfides and diaryl diselenides is subjected to a rhodium-catalyzed reaction with alkynes (Equation (68)).193... 

Transition-metal catalysts are, in general, more active than the MPVO catalysts in the reduction of ketones via hydrogen transfer. Especially, upon the introduction of a small amount of base into the reaction mixture, TOFs of transition-metal catalysts are typically five- to 10-fold higher than those of MPVO catalysts (see Table 20.7, MPVO catalysts entries 1-20, transition-metal catalysts entries 21-53). The transition-metal catalysts are less sensitive to moisture than MPVO catalysts. Transition metal-catalyzed reactions are frequently carried out in 2-propanol/water mixtures. Successful transition-metal catalysts for transfer hydrogenations are based not only on iridium, rhodium or ruthenium ions but also on nickel [93], rhenium [94] and osmium [95]. It has been reported that... 

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. 

Extension of the rhodium-catalyzed allylic alkylation to a-subshtuted enolates was found to facilitate the introduction of an additional stereogenic center (Eq. 5). 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

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