Reductive elimination rhodium hydroformylation

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

Rate-determining step, hydroformylation, 163 Reactivity, enantiomers, 286 Recognition, enantiomers, 278 Reduction and oxidation, 5 Reductive coupling, dissolving metal, 288 Reductive elimination, 5, 111 Resolution. See Kinetic resolution Rhenium-carbene complexes, 288 Rhodium-catalyzed hydrogenation, 17, 352 amino acid synthesis, 18, 352 BINAP, 20... 

Another catalytic cycle studied by Matsubara, Morokuma, and coworkers [77] is the hydroformylation of olefin by an Rh(I) complex. Hydroformylation of olefin by the rhodium complex [78-80] is one of the most well known homogeneous catalytic reactions. Despite extensive studies made for this industrially worthwhile reaction [81, 82], the mechanism is still a point of issue. The active catalyst is considered to be RhH(CO)(PPh3)2, 47, as presented in Fig. 25. The most probable reaction cycle undergoes CO addition and phosphine dissociation to generate an active intermediate 41. The intramolecular ethylene insertion, CO insertion, H2 oxidative addition, and aldehyde reductive elimination are followed as shown with the surrounding dashed line. Authors have optimized the structures of nearly all the relevant transition states as well as the intermediates to determine the full potential-... 

This led in turn to the idea to also use combinations of different metals (e.g., Co/Rh, Co/Pt, Co/Fe, Co/Mo, Rh/Fe, Rh/Mn, Rh/Re, Rh/W, Rh/Mo) with the aim of creating synergy effects [2]. In the last decade, especially G arland and coworkers accumulated much evidence through spectroscopic measurements and density functional theory (DFT) calculations that in rhodium-catalyzed hydroformylation of non-isomerizable olefins (cyclopentene or 3,3-dimethylbut-l-ene), carbonyl complexes, which are less active in hydroformylation, such as HMn(CO)5 or FIRe(CO)5 [3], can support the reductive elimination of the aldehyde from... 

Preliminary mechanistic studies by Fernandez, Peris, and Crudden provided evidence that, under smooth hydroformylation conditions, NHCs remain ligated in the coordination sphere of rhodium [55, 68]. A similar conclusion was drawn by Scholten and Dupont for hydroformylation in ionic liquids [69]. A serious problem sometimes faced is the reductive elimination of carbene ligand from the metal as imidazohum salt under the effect of H2, as found in hydrogenation reactions [70]. 

The hydroformylation activity of rhodium(I) complexes bound to silica via silylated phosphines decreases as the sites become isolated and this suggests that a dinuclear reductive-elimination step occurs as in equation... 

Homogeneous catalysts play an important role in industry as well as in research laboratories. Established applications include, for example, polymerization processes with zirconocene and its derivatives, rhodium- or cobalt-catalyzed hydroformylation of olefins, and enantioselective isomerization catalysts for the preparation of menthol. In contrast to heterogeneous catalysts, more experimental studies of reaction mechanisms are available and the active species can be characterized experimentally in some cases. Most catalysts are based on transition metal compounds, for which electronic structures and properties are well studied theoretically. A substantial number of elementary reactions, such as reductive elimination, oxidative addition, alkene or carbonyl migratory insertion, etc., have been experimentally Studied in detail by means of isotopic, NMR, and IR studie.s, as well as theoretically. ... 

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