Rhodium complexes carbon-hydrogen bonds

The remote functionalization of carbon-hydrogen bonds by a-diazoketones according to the general eq. (1) is efficiently catalyzed by rhodium(II) complexes and yields cyclopentanones, lactams, and lactones, depending on the substituent Y [3, 4]. Typical reaction conditions are boiling methylene chloride or boiling... 

Although the first catalysts were copper-based, the insertion of metal-associated carbenes into carbon-hydrogen bonds has undergone a renaissance with the advent of rhodium(II) carboxylate catalysts [56]. Metal-catalyzed enan-tioselective C-H insertions of carbenes have not been studied in great detail. Most of the efficient enantioselective versions of this reaction involve chiral rhodium complexes and until recently, the use of chiral catalysts derived from metals other than copper and rhodium for the asymmetric C-H insertion of metal-associated carbenes are still unexplored. 

Janowicz AH, Periana RA, Buchanan JM, Kovac CA, Stryker JM, Wax MJ, Bergman RG (1984) Oxidative addition of soluble iridium and rhodium complexes to Carbon-Hydrogen bonds in methane and higher alkanes. Pure Appl Chem 56 13-23... 

Aoyama Y, Yoshida T, Sakurai K-i, Ogoshi H (1983) Activation of arene carbon-hydrogen bonds. Direct electrophilic aromatic metalation with a rhodium-porphyrin complex. J Chem Soc Chem Commun 478-479... 

Probing the mechanism of carbon-hydrogen bond activation by photochemically generated hydridotris-(pyrazolyl)borato carbonyl rhodium complexes New experimental and theoretical investigations... 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

The history of homogeneous hydrogenation with a transition metal catalyst really started in 1966 with the development of Wilkinson s catalyst (Figure 9.2). This rhodium complex was the first that allowed the controlled reduction of unsaturated carbon-carbon bonds under mild conditions [3]. 

Based on HRh(CO)2(L)2, the mechanism is initiated by the coordination of an alkene molecule, resulting in a sixfold-coordinated complex (A). The following rearrangement to an alkyl rhodium complex happens before a carbon monoxide is added to the complex in the next step and inserted in the rhodium alkyl bond (B). The oxidative addition of hydrogen (C) and the release of the aldehyde by reductive elimination reform the starting rhodium complex (D). 

Catalyst Description. The LPO catalyst is a triphenylphosphine modified carbonyl complex of rhodium. Triphenylphosphine, carbon monoxide, and hydrogen form labile bonds with rhodium. Exotic catalyst synthesis and complicated catalyst handling steps are avoided since the desired rhodium complex forms under reaction conditions. Early work showed that a variety of rhodium compounds might be charged initially to produce the catalyst. Final selection was made on the basis of high yield of the catalyst precursor from a commodity rhodium salt, low toxicity, and good stability to air, heat, light, and shock. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Enantioselective Hydrogenation. (R,5 )-CAMPHOS has been employed in combination with rhodium(I) to reduce alkene carbon-carbon double bonds. Thus, the Rh(I) complex formed from (R,5 )-CAMPHOS and [Rh(cyclooctene)2Cl]2 in toluene-EtOH-EtsN solution catalyzes the hydrogenation (1 atm H2, 20 °C) of atropic acid and of ct-acetamidocinnamic acid. The... 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

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