Rhodium carboxylates activity

For example, cycloaddition of carbethoxycarbene (from ethyl diazoacetate as precursor) on XXXIV (R=methyl, Cl, Br) in the presence of various rhodium carboxylates yields cyclopropanecarboxylic acid esters (pyrethroids) that are useful for their insecticidal activity [56] (slightly volatile, biodegradable, non toxic for mammalians). 

Iwasawa et al. [21] also reported chelation-assisted reactions in an article entitled Rhodium(I)-Catalyzed Direct Carboxylation of Arenes with CO2 via Chelation-Assisted C-H Bond Activation, in which the cyclometalation reactions proceed easily and form cyclometalation intermediates. The metal atoms are active centers in their intermediates. Hence, the active metal atom reacts easily with inert carbon dioxide to give carboxylic acid derivatives. Examples include the cyclometalation of 2-phenylpyridine as a substrate in the presence of a rhodium intermediate. Carbon dioxide can be inserted into the rhodium-phenyl carbon bond, and a methyl ester is formed with TMSCH2N2 from a rhodium carboxylate, as shown in Eq. (6.5). The reaction mechanism is proposed as shown in Scheme 6.2 [21]. 

When a large excess of carboxylic acid is present, rhodium carboxylate is not converted into hydride [1]. A preferred dionate is 2,2,6,6-tetramethyl-3,5-heptanedionate (dipivaloylmethane) especially because its solutions are stable upon storage [2]. The use of stock solutions is recommended because very small amounts of rhodium need to be applied when active catalysts are being studied. In batch reactions the formation of rhodium hydride species may be slow compared to catalysis. If so, one should pre-heat the catalyst system to allow the formation ofthe catalyst before adding the alkene. 

An interesting series of complexes with antitumour activity is that of the rhodium carboxylates, whose structure is a dimer containing bridging carboxylates ... 

The biological studies on the rhodium carboxylates have been reviewed and discussed [3, 9, 10]. A number of interesting points have emerged, emphasizing the diversity of metal-based complexes with antitumour activity, although for rhodium carboxylates the acute toxicity probably eliminates these complexes from further development. The results are summarized below ... 

These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

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 catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). 

Rhodium(II) carboxylate dimers and their carboxamide counterparts have been demonstrated to be exceptionally useful catalysts for carbene transfer processes involving diazocarbonyl substrates [1]. Doyle s seminal work identified Rh2(OAc)4 as the catalyst of choice for a variety of cyclopropanation, C-H insertion, and ylide rearrangement transformations using diazoketones or diazoesters [2]. Important contributions by Taber [3], Padwa [4], and Davies [5] further established the superior catalytic activity of dirho-dium catalysts and the excellent selectivity of rhodium-[Pg.417]

Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5 9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the previously documented behavior (J ) of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here. 

Monometallic ruthenium, bimetallic cobalt-ruthenium and rhodium-ruthenium catalysts coupled with iodide promoters have been recognized as the most active and selective systems for the hydrogenation steps of homologation processes (carbonylation + hydrogenation) of oxygenated substrates alcohols, ethers, esters and carboxylic acids (1,2). 

Rhodium catalysts modified with carboxylated phosphines 45 (Table 3 n=5, n=7)229 and phosphonium phosphines 103 (Table 5 n=2,3,6,10)255 form very active catalytic systems for the hydrogenation of olefins in aqueous/organic two phase systems. 

NaOH-HiOz) or to carboxylic acids (with w-chloroperbenzoic acid).-190 Double bonds can be hydroborated in the presence of triple bonds if the reagent is 9-BBN.191 On the other hand, dimesitylborane selectively hydroborates triple bonds in the presence of double bonds.192 Furthermore, it is often possible to hydroborate selectively one particular double bond of a nonconjugated diene.191 When the reagent is catecholborane, hydroboration is catalyzed by rhodium complexes, such as Wilkinson s catalyst.194 Enantioselective hydro-boration-oxidation has been achieved by the use of optically active rhodium complexes.195... 

Electron-withdrawing substituents generally increase diazo compounds stability toward decomposition. Dicarbonyl diazomethane, which bears two carbonyl groups flanking the diazomethane carbon, are more stable than diazo compounds with only one carbonyl substituent. In general, metal catalysed decomposition of dicarbonyl diazomethane requires higher temperature than does monocarbonyl substituted diazomethane. As indicated before, rhodium(II) carboxylates are the most active catalysts for diazo decomposition. With dicarbonyl diazomethane, the rhodium(II) carboxylate-promoted cyclopropanation process can also be carried out under ambient conditions to afford a high yield of products. 

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