Rhodium catalysts review

Reduction of a carbon-carbon double bond will produce a chiral product if the olefin is (unsymmetrically) geminally disubstituted. Although hundreds of catalysts having chiral ligands have been synthesized and screened with a number of alkene structural types (reviews ref. [65,97-107]), the present discussion will focus on only one the reduction of acetamido cinnamates using soluble rhodium catalysts (reviews ref. [97,100,108-110]). The development of chiral bisphosphine ligands and the herculean effort that led to the elucidation of the mechanism of this reaction make it an important example for study, since we now know that the major enantiomer of the product arises from a minor (often invisible) component of a pre-equilibrium [109,111]. This aspect of chemical reactivity is an important lesson whose importance cannot be overemphasized when we strive to understand the... 

Platinum complexes with chiral phosphorus ligands have been extensively used in asymmetric hydroformylation. In most cases, styrene has been used as the substrate to evaluate the efficiency of the catalyst systems. In addition, styrere was of interest as a model intermediate in the synthesis of arylpropionic acids, a family of anti-inflammatory drugs.308,309 Until 1993 the best enantio-selectivities in asymmetric hydroformylation were provided by platinum complexes, although the activities and regioselectivities were, in many cases, far from the obtained for rhodium catalysts. A report on asymmetric carbonylation was published in 1993.310 Two reviews dedicated to asymmetric hydroformylation, which appeared in 1995, include the most important studies and results on platinum-catalogued asymmetric hydroformylation.80,81 A report appeared in 1999 about hydrocarbonylation of carbon-carbon double bonds catalyzed by Ptn complexes, including a proposal for a mechanism for this process.311... 

The ruthenium-, rhodium-, and palladium-catalyzed C-C bond formations involving C-H activation have been reviewed from the reaction types and mechanistic point of view.135-138 The activation of aromatic carbonyl compounds by transition metal catalyst undergoes ortho-alkylation through the carbometallation of unsaturated partner. This method offers an elegant way to activate C-H bond as a nucleophilic partner. The rhodium catalyst 112 has been used for the alkylation of benzophenone by vinyltrimethylsilane, affording the monoalkylated product 110 in 88% yield (Scheme 34). The formation of the dialkylated product is also observed in some cases. The ruthenium catalyst 113 has shown efficiency for such alkylation reactions, and n-methylacetophenone is transformed to the ortho-disubstituted acetophenone 111 in 97% yield without over-alkylation at the methyl substituent. 

This brief review can provide only a snapshot of the state of art. The older literature up to 1980 is covered by the still important review by Falbe [3]. A concise overview up to 2002 can be found in Cornils and Flerrmann [4]. Hy-droformylation with rhodium catalysts is covered by the book of van Leeuwen and Claver [5]. Ungvary reviews new developments in hydroformylation, mainly in scientific papers, every year [6-9]. 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

The above results were reviewed in 1974 (5). Since then the main advances in the field have been the achievement of asymmetric hydro-carbalkoxylation (see Scheme I, X = -OR) using palladium catalysts in the presence of (-)DIOP (6), the use of other diphosphines as asymmetric ligands in hydroformylation by rhodium (7), and the achievement of the platinum-catalyzed asymmetric hydroformylation (8, 9). Further work in the field of asymmetric hydroformylation with rhodium catalysts has been directed mainly towards improving optical yields using different asymmetric ligands (10), while only very few efforts were devoted to asymmetric hydroformylation catalyzed by cobalt or other metals (11, 12) and it will be discussed in a modified form in this chapter. 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

Wilkinson catalyst, tris (triphenylphosphine) chlororhodium (I). With this in mind, it will be helpful to review briefly a few facts about [(C6H5)3P] 3RhCl before discussing the asymmetric rhodium catalysts in greater detail. 

Scurrell (201) recently briefly reviewed the literature on heterogenized homogeneous rhodium catalysts for methanol carbonylation up to 1976. 

A review about the rearrangement and cycloaddition of carbonyl ylides generated from a-diazo compounds is available <2001CSR50>. Enantioselective intramolecular cyclopropanations of allyl 2-diazo-3-silanyloxybut-3-enoates to yield cyclopropyl 7-butyrolactones have been investigated with a variety of chiral rhodium catalysts. The best results were obtained with Rh2(PTTL)4, where enantioselectivity culminated at 89% ee (Equation 99) <2005TA2007>. 

Introduction. Homogeneous catalytic hydrogenation with cationic rhodium catalysts has been extensively explored by Schrock and Osborn. Use of these complexes in stereoselective organic synthesis has been a topic of more recent interest, and has been recently reviewed. The reagent of choice for many of these directed hydrogenations has continued to be [Rh(nbd)(dppb)]BF4 (1). 

In another approach for the activation of hemiacetals, a phosphine/Cu(II) complex [433] and a rhodium catalyst have been proposed recently [434]. A detailed discussion on the use of hemiacetals in glycosylation reactions can be found in a recent review [435]. 

Soon after the initial discovery of the hydroformylation activity of RhH(CO)(PPh3)3 it was found that ligands have a profound influence on the activity and selectivity of the rhodium catalysts. The majority of the numerous published patents and publications are concerned with the effect of ligands and it is therefore impossible to review this matter here. Instead we have made a choice that includes a study of the electronic and steric effects of phosphines and phosphites and a few examples of bidentate ligands. A few trends will be briefly summarized. 

An important requirement for all homogeneous catalytic processes is that the dissolved catalyst must be separated from the liquid product and recycled to the reactor without significant catalyst loss the need is acute when the metal is as expensive as rhodium. One approach to aid this separation process is to immobilize (anchor) the soluble catalyst on a solid support in order to confine the catalyst to the reactor and overcome the need for a catalyst recycle step. A number of types of solid support have been employed to anchor rhodium catalysts for use in methanol carbon-ylation with liquid- or gas-phase reactants. These were reviewed by Howard et al. in 1993 [8] and include activated carbon, inorganic oxides, zeolites, and a range of polymeric materials. 

Several reviews discussing general aspects of this topic are available154- l 55,159. In the first applications the rhodium catalyst was attached to non-cross-linked polystyrene via chiral phosphane ligands such as Diop9. More efficiency is achieved with platinum catalysts attached to cross-linked polystyrene/ Diop systems138. 

These unexpected results were rationalized and extended to related systems by Doyle (see his review, 1992), but we do not discuss his explanations here because this would require too much space, as other experimental data must be included. More recently, Doyle et al. (1993 a) also investigated the effectiveness of various rhodium catalysts with chiral ligands for enantiocontrol in CH insertion reactions (see Sect. 8.8). 

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