Rhodium complexes overview

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Jones reported an overview of the use of tris(pyrazolyl)borate rhodium complexes for the activation of arene and alkane C-H bonds, in particular detailing the fundamental studies with [Rh(CNR)(R)H(Tp )j. Selected example is reported in Fig. 8.12.154 Jones and coworkers155 also carried out some experiments in order to verify the involvement of... 

From this overview it also appears that in most cases, catalytic tests have been performed with catalysts formed in situ from a metal precursor and the desired chiral phosphine, according to usual procedures, while specific catalyst design has been done only sporadically. Relevant examples are the DuPHOS/diene rhodium complexes mentioned in Fig. 10.31 (Sect. 10.3.2) and the platinum/NHC/phos-phine derivatives C4 which allowed highly enantioselective platinum promoted cycloisomerizations to be carried out (Fig. 10.44). 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

A recurrent theme in the chemistry of rhodium and iridium poly(pyrazolyl)borate complexes is their capacity to activate hydrocarbon C-H bonds, both intra and intermolecularly, under photochemical and/or thermal conditions. This has increasingly become the subject of detailed investigations aimed at elucidating the salient mechanistic features, not least the extent to which the Tp ligand participates, through its capacity for variable denticity. This body of work (to 2001) was recently reviewed by Slugovc and Carmona,and many details are described elsewhere herein. This section thus seeks to provide a concise overview and... 

Because of the immense importance of phosphites as ligands, not only in rhodium-catalyzed hydroformylation, several recent reviews have dealt with these compounds and provided quite complete collections of individuals [2, 15], In contrast to these overviews, in this chapter we will focus on some issues that are seldom in the focus. This concerns the synthesis of alcohols that are required as the alcohol component for the synthesis of phosphorous acid triesters. This has never been considered in detail. However, their availability is an important criterion for chemists dealing with large-scale applications and therefore has economic consequences. Some general synthesis protocols of phosphites together with some typical examples will also be considered. The complexation behavior of phosphites with rhodium will also be discussed briefly. Some remarks about the stability of ligands and Rh catalysts will close this chapter. 

Myrcene is a very abundant acyclic monoterpene available from the essential oils of various plants including wild thyme and hops. Recently, an excellent overview on the manufacture and transformation of this natural product was given by Behr and Johnen [125]. Commercially, myrcene is produced by the pyrolysis of pinenes [126]. The rhodium-catalyzed hydroformylation of myrcene gives usually a mixture of fragrance aldehydes in more than 90% combined yields (Scheme 6.37) [127, 128]. The main aldehyde, which accounted for 70 - 80% of the mass balance, results from the reaction with the less substituted C=C bond through the formation of a T) -allyl rhodium intermediate complex [127]. The reaction was also performed in a toluene/water biphasic system using the water-soluble TPPTS ligand and a cationic surfactant [84]. 

The mechanism of the cobalt- (BASF), rhodium- (Monsanto), and iridium- (Cativa) catalyzed reaction is similar but the rate-determining steps differ and different intermediate catalyst complexes are involved. In all three processes two catalytic cycles occur. One cycle involves the metal carbonyl catalyst (II) and the other the iodide promoter (i). For a better overview only the catalytic cycle of the rhodium-catalyzed Monsanto process is presented in detail (Figure 6.15.4). Initially the rhodium iodide complex is activated with carbon monoxide by forming the catalytic active [Rhi2(CO)2] complex 4. Further the four-coordinated 16-electron complex 4 reacts in the rate-determining step with methyl iodide by oxidative addition to form the six-coordinated 18-electron transition methyl rhodium (I II)... 

This overview will concentrate exclusively on the use of dirhodium(II) complexes as these are the most active catalysts for this transformation. The preparation of rhodium(II) carboxylate complexes was first reported in 1960 by the group of Chernyaev following the reaction of rhodium(III) chloride in refluxing formic acid. Their ability to decompose diazo compounds for the formation of a metaUocarbene was then discovered by Teyssie and coworkers a decade later. This seminal study has opened the vast domain of dirhodium(II)-catalyzed carbene additions that has proved highly successful. ° Their use in catalytic nitrene addition, though less extensively investigated, has also led to significant achievements that are summarized below with an emphasis on the latest developments made in the last 5 years. 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

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