Alkenes catalysis, rhodium complexes

The presence of 4e as the predominant species during the catalysis is also in accord with the observed kinetic behavior of this catalyst with 1-octene and styrene as the substrates. The observation of this saturated acyl rhodium complex is in line with the positive dependence of the reaction rate on the hydrogen concentration and the zero order in alkene concentration. It was concluded previously that this saturated acyl complex is an unreactive resting state [18]. Before the final hydro-genolysis reaction step can occur, a CO molecule has to dissociate in order to form... 

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... 

Chauvin, Y., Mussmann, L., and Olivier, H., A novel class of versatile solvents for two-phase catalysis hydrogenation, isomerization, and hydroformylation of alkenes catalyzed by rhodium complexes in liquid 1,3-dialkylimidazolium salts, Angew. Chem. Int. Ed., 34, 2698-2700,1996. 

Homogeneous catalysis by transition metal complexes almost always involves processes in which product-catalyst separation and catalyst recycling are important issues. For years, researchers have worked to find effective ways to isolate metal-complex catalysts in phases separate from those containing the catalyst, usually by anchoring the metal complex to a solid surface. As summarized by Driessen-Holscher, it is now evident that the method that has met with most practical success in this direction involves the use of multiple liquid phases. For example, rhodium complexes with water-soluble sulfonated ligands are used to catalyze alkene hydroformyla-tion, and the aqueous-phase catalyst and the organic products are easily separated as insoluble liquid phases. 

Ammonia N-donor Ligands Asymmetric Synthesis by Homogeneous Catalysis Decarbonylation Catalysis Hydride Complexes of the Transition Metals Hydroboration Catalysis Hydrogenation Isomerization of Alkenes Hydrosilation Catalysis P-donor Ligands Rhodium Organometallic Chemistry. 

The carbonyl [Ru3(CO),2] is a good cocatalyst for the low pressure hydroformylation of internal alkenes using the classic rhodium phosphine [HRh(CO)(PPh3),] system in the presence of an excess of triphenylphosphine (P/Rh = 200) (22). Starting from a mixture of hex-2- and hex-3-ene, the addition of [Ru3(CO),2l (Rh/Ru = 1/1) increased both the reaction rate and the n/iso ratio of heptanals. More recently, Poilblanc and coworkers (23) have prepared a mixed ruthenium-rhodium complex formulated as [CIRh(/i-CO)(//-dppm)2Ru(CO)2] (dppm is Ph2PCH2PPh2). This species shows catalytic activity in the hydroformylation of pent-l-ene at 40 bar (H2/C0= 1/1) and 75°C. Conversion to hexanals was 90% in 24 hours and the linearity reached 70%. No further report has appeared to determine the role of the two metals in this catalysis. 

The metal complexes most often studied as polymer-bound catalysts have been Rh(I) complexes, such as analogues of Wilkinson s complex. The catalytic activity of a bound metal complex is nearly the same as that of the soluble analogue. Rhodium complexes are active for alkene hydrogenation, alkene hydroformylation, and, in the presence of CH3I cocatalyst, methanol carbonylation, etc. Polymer supports thus allow the chemistry of homogeneous catalysis to take place with the benefits of an insoluble, easily separated catalyst . ... 

The addition of a diazocarbonyl compound to an alkene with metal catalysis is an effective method for the formation of cyclopropanes, as discussed above. However, direct addition to aldehydes, ketones or imines is normally poor. Epoxide or aziridine formation can be promoted by trapping the carbene with a sulfide to give an intermediate sulfur ylide, which then adds to the aldehyde or imine. For example, addition of tetrahydrothiophene to the rhodium carbenoid generated from phenyldiazomethane gave the ylide 131, which adds to benzaldehyde to give the trans epoxide 132 in high yield (4.104). On formation of the epoxide, the sulfide is released and hence the sulfide (and the rhodium complex) can be used in substoichiometric amounts. 

The synthesis of aldehydes by hydroformylation of alkenes is an important industrial process discovered in 1938. The use of biphasic catalysis, which is a well-established method for the separation of the product and the recovery of the catalyst, was developed for this reaction in the Ruhrchemie-Rhdne-Poulenc process a water-soluble rhodium complex is used as catalyst - the reaction proceeding in water - and the organic layer is formed simply by the alkene reagent and the aldehyde products. Butanal is manufactured this way, but owing to the low solubility of longer olefins in water, this process is limited to C2-C5 olefins. To overcome this limitation, ionic liquids have been studied as a reaction solvent for this biphasic reaction. 

Catalysis by imprinted surfaces has been extended to transition metal catalyst hydrolysis and hydrogenation [70-73]. For the catalytic hydrogenation of alkenes, the dimeric and monomeric rhodium complexes were attached to silica surfaces as shown in Fig. 23. 

Notable is also the high selective hydrogenation of alkynes to alkenes achieved by a polysiloxane-bound (ether-phosphine) rhodium complex (Scheme 24-5) but not by the non-entrapped catalyst (Lindner, 1997). Likewise, remarkable is the fact that various entrapped alkene hydroformylation catalysis lead often to a much higher ratio of linear branched products than the homogeneous complex (see e.g., Lindner, 2000). 

The usual cyclopropanation reagent, diazomethane and diazoacetates, react smoothly with alkenes in the presence of palladium-catalyst to form cyclopropanes. However, it should be noted that rhodium complexes serve in most cases as superior catalyst. Cyclopropanes can be formed from alkenes (dienes) and nucleophiles using oxidative palladium catalysis (Scheme 5-175). In this case, copper(II) salts regenerate the palladium(II) complexes. The vinyl cyclopropanes produced undergo subsequent vinylcyclopropane-cyclopentane rearrangement to form bicycles. ... 

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